A Web Browser Flow Chart for the Classification of Igneous Rocks  

The IUGS Subcommission on the Systematics of Igneous Rocks (Woolley et al., 1996) writes
THE PROBLEMS OF CLASSIFICATION

Many schemes of classification for igneous rocks, such as that based on the total alkali versus silica method (TAS), have a major petrogenetic component, and ultimately all taxonomy of igneous rocks will incorporate genetic factors. For no other igneous rocks is the petrogenetic component of classification more important than for lamprophyres, lamproites and kimberlites. One criticism of the IUGS classification of rocks, discussed by Le Bas & Streckeisen (1991), is that it is deficient in a strong genetic component. This is essentially an historical artefact of the pragmatic approach originally adopted by Streckeisen to attain a consensus. Because of the plethora of classification systems that have been suggested and applied in the past, the Subcommission followed a consensual approach. The widespread adoption of the IUGS system would seem to have justified this approach for many igneous rocks. However, although much of the IUGS system undoubtedly has some petrogenetic significance, and is used in genetic discussion, for example the TAS system, purely descriptive terms may have to be applied where there is disagreement as to interrelationships of rock suites. This would seem to apply at present to the lamprophyres, for which difficulties remain in erecting a petrogenetic classification...
...Any classification should be capable of integration with the IUGS hierarchical system described by Le Maitre et al. (1989) and Le Bas & Streckeisen (1991, Fig. 8). In the published hierarchical system, lamprophyres, lamproites and kimberlites were combined under "lamprophyric rocks", but if this portmanteau term is to be abandoned, then it must be replaced by a logical hierarchy of classification for these three groups of rocks that integrates them with the melilitic and leucitic rocks...

In order to create a sustainable classification of igneous rocks which all geologists might use, an international body was set up by the IUGS: the IUGS Subcommission on the Systematics of Igneous Rocks. In the course of creating the classification, the Subcommission has established ten principles for its construction and for defining an appropriate nomenclature. The principles are: (mark the right cell with the mouse)

These principles and their rationale have not previously been enunciated. The classification separates and individually classifies the pyroclastic, carbonatitic, melititic, lamprophyric and charnockitic rocks before entering the main QAPF classification for plutonic and volcanic rocks which is based on the modal mineral proportions of quartz (Q), alkali feldspar (A) and plagioclase (P) or of alkali feldspar (A), plagioclase (P) and feldspathoids (F). Rocks with mafic content >90% have their own classification. If the mineral mode cannot be determined as is often the case for volcanic rocks, then a chemical classification of total alkalis versus silica (TAS) is used. 
The nomenclature for these classifiations necessitates only 297 rock names out of the about 1500 that exist (Le Bas & Streckeisen, 1991). 
    1. use descriptive attributes;
    2. use actual properties; 
    3. ensure suitability for all geologists; 
    4. use current terminology;
    5. define boundaries of rock species; 
    6. keep it simple to apply; 
    7. follow natural relations; 
    8. use modal mineralogy; 
    9. if mode not feasible, use chemistry;
    10. follow terminology of other IUGS advisory bodies. 


In the book "A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks" (Le Maitre et al. 1989), you will find the following chapter:

B.4. USING THE CLASSIFICATION 

One of the problems of classifying igneous rocks is that they cannot all be classified sensibly by using only one system. For example, the modal parameters required to adequately define a felsic rock, composed of quartz and feldspars, are very different from those required to define an ultramafic rock, consisting of olivine and pyroxenes. Similarly, lamprophyres have usually been classified as a separate group of rocks. Also modal classifications cannot be applied to rocks which contain glass or are too fine-grained to have their modes determined, so that other criteria, such as chemistry, have to be used in these cases.

As a result several classifications have to be presented, each of which is applicable to a certain group of rocks, e.g. pyroclastic rocks, lamprophyres, plutonic rocks, etc. This, however, means that one has to decide which of the classifications are appropriate for the rock in question. To do this in a consistent manner, so that different petrologists will arrive at the same answer, a hierarchy of classification had to be agreed upon. The basic principle involved in this was that the "special" rock types (e.g. lamprophyres, pyroclastic rocks, etc.) must be dealt with first so that anything that was not regarded as a "special" rock type would be classified in either the plutonic or volcanic classifications, which after all contain the vast majority of igneous rocks. The sequence that should be followed is shown diagrammatically on the chart supplied with the book and is as follows:

Has the rock pyroclastic features? [NO]  YES =>  Use pyroclastic rock classification 
Carbonates > 50 %? [NO]  YES =>  Use carbonatite classification 
see classification for melilitic, kalsilitic, leucitic rocks and kimberlites, lamproites and lamprophyres 
[NO] down 
=> =>  Flow chart for melilitic, kalsilitic, leucitic rocks... and lamprophyres  
Is it charnockitic? [NO] down  YES =>  Use charnockite classification 
Is it plutonic? YES => 
[NO] down 
M < 90 %? YES => 
[NO] => 
Use plutonic QAPF 
Use ultramafic classification 
Is it volcanic? YES =>  Mode possible? YES => 
[NO] down 
Use volcanic QAPF 
  Is it high-Mg? 
[NO] downYES => 
Use high-Mg classification 
If you get to this point, either the rock is not igneous or you have made a serious mistake.  _____=>____ 
<= [NO] 
Use TAS. If it falls in fields F or U1, use norm ne v. norm ab classification * 

* The "norm ne versus norm ab classification" is that given by Le Bas (1989).

B.5. PYROCLASTIC ROCKS AND TEPHRA 

This classification was prepared by a small working group of the IUGS Subcommission, chaired by Rolf Schmid, after circulating and analysing the results of six questionnaires sent to more than 150 geologists throughout the world (Schmid, 1981).
It should be used only if the rock is considered to have had a pyroclastic origin i.e. was formed by disruption as a direct result of volcanic action. It specifically excludes rocks formed by the autobrecciation of lava flows, because the lava flow itself is the direct result of volcanic action, not its brecciaton.
The nomenclature and classification is purely descriptive and thus can easily be applied by non-specialists. By defining the term "pyroclast" in a broad sense (see section B.5.1), the classification can be applied to air fall, flow and surge deposits as well as to lahars, subsurface and vent deposits (e.g. hyaloclastites, intrusion and extrusion breccias, tuff dykes, diatremes, etc.). The terms used in the classification solely describe the granulometric state of the rocks or deposits. Combined with other terms, however, compositional or genetic information may be included.
The grain size limits used for subdividing pyroclasts and pyroclastic deposits should be regarded as provisional until there is international agreement on the granulometric divisions of sedimentary rocks. When indicating the grain size of a single pyroclast or the middle grain size of an assemblage of pyroclasts the general terms "mean diameter" and "average pyroclast size" are used, without defining them explicitly, as grain size can be expressed in several ways. It is up to the user of this nomenclature to specifiy the method by which grain size was measured in those cases where it seems necessary to do so.

B.5.1. Pyroclasts 

Pyroclasts are defined as fragments generated by disruption as a direct result of volcanic action. Note that this excludes particles formed by autobrecciataion of lava flows, because the flow itself is the direct result of volcanic action, not its brecciation.
The fragments may be individual crystals, crystal fragments, glass fragments or rock fragments. Their shapes acquired during disruption or during subsequent transport to the primary deposit must not have been altered by later redepositional processes. If they have the fragments are called "reworked pyroclasts", or "epiclasts" if their pyroclastic origin is uncertain. The various types of pyroclasts are mainly distinguished by their size (see Table B.1):-
Bombs - pyroclasts whose mean diameter exceeds 64mm and a shape or surface (e.g. bread-crust surface), which indicates that they were in a wholly or partly molten condition during their formation and subsequent transport.
Blocks - pyroclasts whose mean diameter exceeds 64mm and whose angular to subangular shape indicates that they were solid during their formation.
Lapilli - pyroclasts of any shape with an mean diameter of 64mm to 2mm.
Ash grains - pyroclasts with an mean diameter of less than 2mm. They may be further divided into coarse ash grains (2mm to 1/16mm) and fine ash grains (less than 1/16mm). The fine ash grains may also be called dust grains. 

B.5.2. Pyroclastic Deposits 

Pyroclastic deposits are defined as an assemblage of pyroclasts which may be unconsolidated or consolidated. They must contain more than 75% by volume of pyroclasts, the remaining materials generally being of epiclastic, organic, chemical sedimentary, or authigenic origin. When they are predominantly consolidated they may be called pyroclastic rocks and when predominantly unconsolidated they may be called tephra. The following terms are applicable to unimodal and well-sorted pyroclastic rocks (Table B.1):-
Agglomerate - a pyroclastic deposit whose average pyroclast size exceeds 64mm and in which rounded pyroclasts predominate.
Pyroclastic breccia-a pyroclastic rock whose average pyroclast size exceeds 64mm and in which angular pyroclasts predominate.
Lapilli tuff - a pyroclastic rock whose average pyroclast size is 64mm to 2mm. Tuff or ash tuff - a pyroclastic rock whose average pyroclast size is less than 2mm. lt may be further divided into coarse (ash) tuff (2mm to 1/16mm) and fine (ash) tuff (less than 1/16mm). The fine ash tuff may also be called dust tuff. Tuffs and ashes may be further qualified by their fragmental composition as shown in Fig. B.1, i.e a lithic tuff would contain a predominance of rock fragments, a vitric tuff would contain a predominance of pumice and glass fragments, and a crystal tuff would contain a predominance of crystal fragments.


Table B.1. Classification and nomenclature of pyroclasts and well-sorted pyroclastic deposits based on clast size (after Schmid, 1981, Table 1). 

Clast size in mm 

Pyroclast

Pyroclastic deposit 

Mainly unconsolidated tephra  Mainly consolidated pyroclastic rock 
> 64  bomb, block  agglomerate bed of blocks or bomb, block tephra  agglomerate pyroclastic breccia 
64 to 2  lapillus  layer, bed of lapilli or lapilli tephra  lapilli tuff 
2 to 1/16  coarse ash grain  coarse ash  coarse (ash) tuff 
< 1/16  fine ash grain  fine ash (dust)  fine (ash) tuff 

  Fig. B. 1. Classification and nomenclature of tuffs and ashes based on their fragmental composition (after Schmid, 1981, Fig. 1). 

Any of these terms for pyroclastic deposits may also be further qualified by the use of any other suitable prefix, e.g. air-fall tuff, flow tuff, basaltic lapilli tuff, lacustrine tuff, rhyolitic ash, vent agglomerate, etc. The terms may also be replaced by purely genetic terms, such as hyaloclastite or base-surge deposit, whenever it seems appropriate to do so. 


Polymodal or poorly-sorted pyroclastic deposits in which there is more than one dominant size fraction of clasts, should be named using the appropriate combination of terms given in Table B.1. Some examples could be:
bulletash lapilli tuff - where lapilli > ash lapilli
bulletash tuff - where ash > lapilli
bulletbed of lapilli and ash, lapilli ash tephra - where ash > lapilli

B.5.3. Mixed Pyroclastic-Epiclastic Deposits 

For rocks which contain both pyroclastic and normal classic (epiclastic) material the Subcommission suggested that the general term tuffites can be used within the limits given in Table B.2. Tuffites may be further divided according to their average grain size by the addition of the term "tuffaceous" to the normal sedimentary term e.g. tuffaceous sandstone.

Table B.2. Terms to be used for mixed pyroclastic-epiclastic rocks (after Schmid, 1981, Table 2). 

Average  clast size in mm. 

Pyroclastic 

Tuffites  (mixed pyroclastic-epiclastic) 

Epiclastic (volcanic and/or nonvolcanic) 

> 64  Agglomerate, pyroclastic breccia  Tuffaceous conglomerate, tuffaceous breccia  Conglomerate, breccia 
64 - 2  Lapilli tuff     
2 - 1/16  coarse  Tuffaceous sandstone  Sandstone 
1/16 - 1/256  fine  Tuffaceous siltstone  Siltstone 
< 1/256    Tuffaceous mudstone, shale  Mudstone, shale 
Amount pyroclastic material  100% to 75%  75% to 25%  25% to 0% 

Pyroclastic  Flows -hazards 



  B.6. CARBONATITES 

This classification should be used only if the rock contains more than 50% modal carbonates (Streckeisen, 1978 and 1979). Carbonatites may be either plutonic or volcanic in origin. Mineralogically the following classes of carbonatites may be distinguished. 
* Calcite-carbonatite - where the main carbonate is calcite. If the rock is coarse- grained it may be called sovite; if medium to fine-grained, alvikite 
* Dolomite-carbonatite-where the main carbonate is dolomite. This may also be called beforsite. 
* Ferrocarbonatite - where the main carbonate is iron-rich. 
* Natrocarbonatite - essentially composed of sodium, potassium, and calcium carbonates. At present this unusual rock type is only found at Oldoinyo Lengai volcano in Tanzania. 

Fig. B.2. Chemical classification of carbonatites using wt% oxides (Woolley & Kempe, 1989).


Qualifications, such as dolomite-bearing, may be used to emphasise the presence of a minor constituent (<10%). Similarly, igneous rocks containing less than 10% of carbonate may be called calcite bearing ijolite, dolomite-bearing peridotite, etc.
Igneous rocks with between 10% and 50% carbonate minerals may be called calcitic ijolite or carbonatitic ijolite, etc.
Note that the terms leucocratic and melanocratic should not be applied when describing carbonatites, as all primary carbonate minerals fall into group M.
If the carbonatite is too fine-grained for an accurate mode to be determined or if the carbonates are complex Ca-Mg-Fe solid solutions then the chemical classification shown in Fig. B.2 can be used.



This flow chart for the melilitic, kalsilitic and leucitic rocks and the kimberlites, lamproites and lamprophyres is entered after the "carbonates > 50%" box and exits to the "charnockitic" box. The symbols used follow those of Kretz (1983) where possible.

Melilite > 10 % [NO]   YES =>  use melilitic rock classification 
contains kalsilite [NO]  YES =>  use kalsilitic rock classification 
normative larnite [NO]  

YES

leucite or minor intrusion with only mafic phenocrysts [NO]  

YES

K2O/Na2O > 3.0, molar K2O/Al2O3 > 0.8 and peralkaline 
NO  

YES use lamproite classification 



  Classification of melilitic rocks 

The melilite-bearing rock classification is used for rocks with >10% modal melilite. Triangular plots are presented for plutonic (melilitolite) (Fig. 1) and volcanic (melilitite) rocks, and Table I is intended for melilitic rocks containing kalsilite. 
If the mode cannot be determined, then one should apply the total alkalis versus silica (TAS) chemical classification, as follows: 
(a) The rock should plot in the foidite field. 
(b) If the rock does not contain kalsilite but has larnite in the noryn, then one should apply Figure 3. 
(c) If normative larnite is greater than 10% and K2O is less than Na2O (wt%), then it is a melilitite or olivine melilitite. 
(d) If K2O is greater than Na2O and K2O exceeds 2 wt%, then it is a potassic melilitite or potassic olivine melilitite. The latter can be termed a katungite, which mineralogically is a kalsilite - leucite - olivine meli]itite. 
(e) If normative larnite is less than 10%, then the rock is a melilite nephelinite or a melilite leucitite according to the nature of the dominant feldspathoid mineral. 

Fig. 1. Classification of the plutonic (melilitolite) melilitic rocks with modal melilite >10% (Le Maitre et al. 1989 Fig. B.3) and of the volcanic (melilitite) melilitic rocks with modal melilite >10%.

TABLE 1. SUGGESTED NOMENCLATURE FOR KAMAFUGITIC ROCKS

Rock name  Mineral Assemblage 
Mafurite  Olivine-pyroxene kalsitifite 
Katungite  Kalsilite-leucite-olivine melilitite 
Venanzite  Kalsilite-phlogopite-olivine-leucite melilitite 
Coppaclite  Kalsilite-phlogopite melilitite 


THE PROBLEMS OF CLASSIFICATION

Melilitic rocks 

The main problems here concern volcanic melilitic rocks. In Le Maitre et al. (1989), the coarser-grained melilitolite is classified on the basis of modal proportions, but a completely satisfactory classification for the finer-grained rocks was not attained. A definition based on rock chemistry is desirable, but unfortunately these rocks cannot be distinguished adequately from other volcanic rocks in the TAS system. However, the presence of melilite in more than trace modal amounts results in the formation of larnite (or calcium orthosilicate) in the CIPW norm, and this can be used as a potential discriminant. Although larnite may appear in the norm of some melilite-free nepbelinitic rocks containing clinopyroxene rich in the Tschermaks component (H.S. Yoder, Jr., pers. comm.), we find that the latter is typically expressed in the norm as anorthite.
A further problem remains, that there is a continuous series from melilitite, through melilite nephelinite to nephelinite. Investigation of this problem indicates that a reasonably clear discriminant between melilitite and melilite nephelinite is normative larnite. In Figure 3, samples of melilitite and nephelinite are plotted in terms of normative larnite versus normative nepheline. The best division appears to be at 10% larnite.
When classifying the melilitic rocks, the following should be taken into consideration:
(a) The present classification for melilitic rocks in Le Maitre et al. (1989, p. 12) is based on the presence of modal melilite exceeding 10 vol.% in either plutonic (melilitolite) or volcanic (melilitite) occurrences, in combination with M > 90%.(b) In the IUGS scheme (see flow chart accompanying Le Maitre et al. 1989), their identification is made after excluding the lamprophyres but before entering QAPF. It is now considered preferable to identify melilitic rocks before lamproites, kimberlites and lamprophyres.
(c) Even in fine-grained rocks, melilite can usually be identified in thin section where it occurs in essential proportions, i.e., >10 vol.%. This assumes the rock is not altered; if it is, melilite is usually carbonated.
(d) Some fine-grained melilitic rocks are strongly potassic, e.g. katungite, the potassic character usually being reflected in the presence of modal leueite or kalsilite (or both).
(e) Melilitite is characterized by the presence of melilite and perovskite and contains less than 38 wt% SiO2 and greater than 13 wt% CaO.



Classification of kalsilitic rocks 

The principal minerals of the kalsilitic rocks include clinopyroxene, kalsilite, leucite, melilite, olivine and phlogopite (Table 2). These rocks cannot be called pyroxenite because that term is reserved for plutonic rocks. The rock types mafurite and katungite, together with the closely associated leucitic rock ugandite (which is excluded from Table 1, as it does not contain kalsilite and is more logically classified as an olivine leucitite), constitute the kamafugitic series of Sahama (1974). From the point of view of the IUGS system of classification, the presence of essential melilite or leucite (or both) indicates that either the classification that deals with melilitic or leucitic rocks should be applied. However, the presence of kalsilite and leucite is considered petrogenetically so distinctive and important that the accepted term, kamafugite, should be retained for this consanguineous series of rocks. Table 1 indicates their nomenclature as a function of mineral assemblage.
Plutonic kalsilitic rocks of the Aldan and North Baikal petrological provinces of Russia, which are not kamafugitic, may be distinguished by the prefix "kalsilite". Thus, synnyrite becomes kalsilite syenite, and yakutite becomes kalsilite-biotite pyroxenite.

TABLE 2. MINERAL ASSEMBLAGES OF KALSILITIC ROCKS

  Phl  Cpx  Lct  Kal  Met  Ol  Gls 
Mafurite    X  
Katungite   
Venanzite   
Coppaelite     

Symbols: Phi: phlogopite, Cpx: clinopyroxene, Lct: leucite, Mel: melilite Ol: olivine,
Gls: glass. x: present, -: absent. After Mitchell & Bergman (1991, Table 2.3).

THE PROBLEMS OF CLASSIFICATION

Kalsilitic rocks 

Kalsilitic rocks have not previously been considered by the Subcommission. These fall into two groups: the karnafugitic series of Sahama (1974), and the kalsilite bearing syenites and pyroxenites, e.g., synnyrite and yakutite, occurring in the Aldan and North Baikal petrological provinces of Russia (Kogarko et al. 1995, Kostyuk et al. 1990). Some of the kamafugitic rocks contain leucite or melilite (or both) and nlight be considered feldspathoidal or melilitic rocks. However, the presence of kalsilite is considered so important that it requires assignment of these rocks to a special group.



No leucite, ol > 35 % + mtc/phl/carb/srp/di  [NO]   YES =>  use kimberlite classification 
Ti-phl + lct + ol + richt/di/sa/wade/pride [NO]   YES =>  use lamproite classification 
leucite [NO]   YES =>  use leucitic rocks classification 
mica/amph  use lamprophyres classification 
NO   

THE PROBLEMS OF CLASSIFICATION

Foiditic and leucitic rocks 

The problems posed by these rocks are restricted to the fine-grained members. The coarser-grained rocks of equivalent composition containing nepheline or leucite (or both) and, despite heteromorphism in some cases (Yoder 1986), can be satisfactorily classified using the QAPF and other mineralogical systems (Le Maitre et al. 1989).
The boundaries between the feldspathoidal field and the basanite-tephrite, phonotephrite, tephriphonolite and phonolite fields in the TAS system are not wholly satisfactory, as they do not provide an acceptable boundary for the nephelinitic and leucitic rocks. The problem with regard to the leucitic rocks is illustrated by Figure 4.
It is evident that leucitic rocks cannot be distinguished chemically on the TAS diagram. However, as leucite is, with few exceptions, a phenocryst phase, or forms small but identifiable crystals, a modal system should be feasible. This approach was not adopted by Le Maitre et al. (1989). The distinction between nephelinite and basanite has been considered by Le Bas (1989).

TABLE 4. MINERRALOGY OF THE PRINCIPAL GROUPS OF LEUCITIC ROCKS

Rock 

Cpx 

Lct 

Pl 

Sa 

Ol 

Leucitite 

> 10% 

Tephritic leucitite 

Phonolitic leucitite 

Leucite tephrite 

< 10% 

Leucite basanite 

> 10% 

Leucite phonolite 

Symbols: Cpx: clinopyroxene, Lct: leucite, Pl: plagioclase, Sa: sanidine (products of its exsolution), Ol: olivine. x: present, -: absent, All these rocks may contain some nepheline.



THE PROBLEMS OF CLASSIFICATION

Kimberlites 

The Subcommission considered it inappropriate to re-investigate the nomenclature and definition of kimberlites in detail, because this has been done extensively in the last few years by the many specialists of kimberlites (Skinner & Clement 1979, Dawson 1980, Clement et al. 1984, Mitchell 1986). Nevertheless, a clear definition of kimberlite should be formulated, particularly for purposes of distinguishing these rocks frorn olivine lamproite, and for placing kimberlites in the hierarchical classification system. Currently, the classification of kimberlite is undergoing revision, and the nomenclature advanced by Mitchell (1994b) has not yet been fully explored.

Classification of kimberlites 

Kimberlites are currently divided into Group I and Group II (Smith et al. 1985, Skinner 1989). Group-I kimberlites correspond to archetypal rocks from Kimberley, South Africa, which were formerly termed "basaltic kimberlites" by Wagner (1914). Group-II kimberlites correspond to the micaceous or lamprophyric kimberlites of Wagner (1914).
Petrologists actively studying kimberlites have concluded that there are significant petrological differences between the two groups, although opinion is divided as to the extent of the revisions required to their nomenclature. Some wish to retain the status quo (Skinner 1989), whereas others (Mitchell & Bergman 1991, Mitchell 1994b) believe that the terminology should be completely revised (see below). Regardless, the working group is unanimous in agreeing that a single definition cannot be used to describe both rock types. Because of the mineralogical complexity of the rocks, a simple succinct definition cannot be devised. Following a concept originally developed by Dawson (1980), the rocks may be recognized as containing a characteristic assemblage of minerals.
The following characterization of Group-I kimberlites is after Mitchell (1995), which is based essentially on that of Mitchell (1986, 1994b), and evolved from earlier "definitions" given by Clement et al. (1984) and Mitchell (1979).
Group-I kimberlites consist of volatile-rich (dominantly CO2) potassic ultrabasic rocks commonly exhibiting a distinctive inequigranular texture resulting from the presence of macrocrysts (a general term for large crystals, typically 0.5-10 mm in diameter) and, in some cases, megacrysts (larger crystals, typically 1-20 cm) set in a fine-grained matrix. The assemblage of macrocrysts and megacrysts. at least some of which are xenocrystic, includes anhedral crystals of olivine, magnesian ilmenite, pyrope, diopside (in some cases subcalcic), phlogopite, enstatite and Ti-poor chromite. Olivine macrocrysts are a characteristic and dominant constituent in all but fractionated kimberlites. The matrix contains a second generation of primary euhedral to subhedral olivine, which occurs together with one or more of the following primary minerals: monticellite, phlogopite, perovskite, spinel (magnesian ulvospinel - magnesiochromite - ulvospinel magnetite solid solutions), apatite, carbonate and serpentine. Many kimberlites contain a late-stage poikilitic mica belonging to the barian phlogopite kinoshitalite series. Nickeliferous sulfides and rutile are common accessory minerals. The replacement of earlier-formed olivine, phlogopite, monticellite and apatite by deuteric serpentine and calcite is common. Evolved members of the Group may be poor in, or devoid of macrocrysts, and composed essentially of second-generation olivine, calcite, serpentine and magnetite, together with minor phlogopite, apatite and perovskite.
It is evident that kimberlites are complex hybrid rocks in which the problem of distinguishing the primary constituents from the entrained xenocrysts precludes simple definition. The above characterization attempts to recognize that the composition and mineralogy of kimberlites are not entirely derived from a parent magma, and the nongenetic terms macrocryst and megacryst are used to describe minerals of cryptogenic, i.e., unknown, origin. Macrocrysts include forsteritic olivine, chromian pyrope, almandine-pyrope, chromian diopside, magnesian ilmenite and phlogopite crystals, that are now generally believed to originate by the disaggregation of mantle-derived lherzolite, harzburgite, eclogite and metasomatized peridotite xenoliths. In most cases, diamond, which is excluded from the above "definition", belongs to this suite of minerals but is much less common. Megacrysts are dominated by magnesian ilmenite, titanian pyrope, diopside, olivine and enstatite that have relatively Cr-poor compositions (<2 wt% Cr2O3). The origin of the megacrysts is still being debated (e.g., Mitchell 1986), and some petrologists believe that they may be cognate. Both of these suites of minerals are included in the characterization because of their common presence in kimberlites.
It can be debated whether reference to these characteristic constituents should be removed from the "definition" of kimberlite, Strictly, minerals that are known to be xenocrysts should not be included in a petrological definition, as they have not crystallized from the parental magma. Smaller grains of both the macrocryst- and megacryst-suite minerals also occur, but may be easily distinguished on the basis of their compositions. In this respect, it is important to distinguish pseudoprimary groundmass diopside from macrocrystic or megacrystic clinopyroxene. Group-I kimberlites do not usually contain the former except as a product of crystallization induced by the assimilation of siliceous xenoliths (Scott Smith et al. 1983). The primary nature of groundmass serpophitic serpentine was originally recognized by Mitchell & Putnis (1988).
Recent studies (Smith et al. 1985, Skinner 1989, Mitchell 1994b, 1995, Tainton & Browning 1991) have demonstrated that Group I and Group II kimberlites are mineralogically different and petrogenetically separate rock-types. A definition of Group-II kimberlites has not yet been agreed upon, as they have been insufficiently studied. Mitchell (1994b, 1995) has suggested that these rocks are not kimberlitic at all, and should be termed "orangeite", in recognition of their distinct character and unique occurrence in southern Africa. Wagner (1928) previously suggested that the rocks which he initially termed micaceous kimberlite (Wagner 1914) be renamed "orangite" (sic). The following characterization of the rocks currently described as Group-II kimberlites or micaceous kimberlites follows that of Mitchell (1995).
Group-II kimberlites (or orangeites) belong to a clan of ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O), characterized by phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite". Rounded macrocrysts of olivine and euhedral primary crystals of olivine are common, but are not invariably major constituents. Characteristic primary phases in the groundmass include: diopside, commonly zoned to, and mantled by, titanian aegirine; spinels ranging in composition from Mg-bearing chromite to Ti-bearing magnetite; Sr- and REE-rich perovskite; Sr-rich apatite; REE-rich phosphates (monazite, daqingshanite); potassian barian titanates belonging to the hollandite group; potassium triskaidecatitanates (K2Ti13O17); Nb-bearing rutile and Mn-bearing ilmenite. These are set in a mesostasis that may contain calcite, dolomite, ancylite and other rare-earth carbonates, witherite, norsethite and serpentine. Evolved members of the group contain groundmass sanidine and potassium richterite. Zirconium silicates (wadeite, zircon, kimzeyitic garnet, Ca-Zr-silicate) may occur as late-stage groundmass minerals. Barite is a common deuteric secondary mineral.
Note that these rocks have a greater mineralogical affinity to lamproites than to Group-I kimberlites. However, there are significant differences in the compositions and overall assemblage of minerals, as detailed above, that permit their discrimination from lamproites (Mitchell 1994b, 1995).

 

Kimberlites 

Orangeites 

Lamproites 

Minettes 

Ultramafic lamprophyres 

Olivine Macrocrysts 
Phenocrysts 

 

 

rare 

__ 

rare 

 

 

 

rare 

 

Mica Macrocrysts- 
phenocrysts 
Groundmass 

 

, phlogopite 

, phlogopite to 

 

 

phlogopite 

 

Ti-phlogopite 

phlogopite 

phlogopite 

phlogopite kinoshitalite

tetraferriphlogopite 

, Ti-tetraferriphlogopite

, Al-biotite 

, Al-biotite 

Spinels  abundant, Mg- chromite to Mg-ulvospinel  rare, Mg-chromite to Ti-magnetite  rare, Mg-chromite to Ti-magnetite 

, Mg-chromite to Ti-magnetite

, Mg-chromite to Ti-magnetite

Monticellite 

 

__ 

__ 

__ 

 

Diopside 

__ 

, Al- + Ti-poor 

, Al- + Ti-poor 

, Al- + Ti-rich 

, Al- + Ti-rich 

Perovskite 

, Sr- + REE-poor 

rare, Sr- + REE-rich 

rare, Sr- + REE-rich 

__ 

, Sr- + REE-poor 

Apatite 

, Sr- + REE-poor 

abundant, Sr- + REE-rich 

, Sr- + REE- rich 

, Sr- + REE-poor 

, Sr- + REE-poor 

Calcite 

abundant 

 

__ 

 

 

Sanidine 

__ 

rare groundmass 

, phenocrysts + groundmass

abundant, groundmass 

__ 

K-richterite 

__ 

rare groundmass 

, phenocrysts + groundmass

__ 

__ 

K-Ba-titanates 

very rare 

 

 

__ 

__ 

Zr-silicates 

very rare 

 

 

very rare 

__ 

Mn-ilmenite 

rare 

 

very rare 

 

rare 

Leucite 

__ 

rare pseudomorphs 

phenocrysts 

__ 

__ 

= common, ___ = absent
Mineralogical comparison of kimberlites, orangeites, lamproites, minettes and ultramafic lampropyres (after Mitchell 1995, Table 2; slightly modified)



Classification of lamproites 

The classification system of lamproites described by Mitchell & Bergman (1991) is recommended; it involves mineralogical and geochemical criteria, as follows:
Lamproites are characterized by the presence of widely varying amounts (5-90 vol.%) of the following primary phases:

  1. titanian (2-10 wt% TiO2), aluminum-poor (5-12 wt% Al2O3) phenocrystic phlogopite
  2. titanian (5-10 wt% TiO2) groundmass poikilitic "tetraferriphlogopite"
  3. titanian (3-5 wt% TiO2) potassium (4-6 wt% K2O) richterite
  4. forsteritic olivine
  5. aluminum-poor (<l wt% Al2O3), sodium-poor (<l wt% Na2O) diopside
  6. nonstoichiometric iron-rich (1-4 wt% Fe2O3) leucite, and
  7. iron-rich sanidine (typically 1-5 wt% Fe2O3)).

The presence of all the above phases is not required in order to classify a rock as a lamproite. Any one mineral may be dominant, and this, together with the two or three other major minerals present, suffices to determine the petrographic name.
Minor and common accessory phases include priderite, wadeite, apatite, perovskite, magnesiochromite, titanian magnesiochromite, and magnesian titaniferous magnetite; less commonly, but characteristically, jeppeite, armalcolite, shcherbakovite, ilmenite and enstatite also are present.
The presence of the following minerals precludes a rock from being classified as a lamproite: primary plagioclase, melilite, monticellite, kalsilite, nepheline, Na-rich alkali feldspar, sodalite, nosean, hauyne, melanite, schorlomite or kimzeyite.
Lamproites conform to the following chemical characteristics:

  1. molar K2O/Na2O > 3, i.e., ultrapotassic
  2. molar K2O/Al2O3> 6.8 and commonly > 1
  3. molar K2O + Na2O/ Al2O3 typically > 1 i.e., peralkaline
  4. typically <10 wt% each of FeO and CaO, TiO2 1-7 wt%, >2000 and commonly >5000 ppm Ba, >500 ppm Zr, >1000 ppm Sr and >200 ppm La.

The subdivision of the lamproites should follow the scheme of Mitchell & Bergman (1991), in which the historical terminology is discarded in favor of compound names based on the predominance of phlogopite, richterite, olivine, diopside, sanidine and leucite, as given in Table 3. It should be noted that the term "madupitic" in Table 3 indicates that the rock contains poikilitic groundmass phlogopite, as opposed to phlogopite lamproite, in which phlogopite occurs as phenocrysts.
The complex compositional and mineralogical criteria required to define lamproites result from the diverse conditions involved in their genesis, compared with those of rocks that can be readily classified using the IUGS system. The main petrogenetic factors contributing to the complexity of composition and mineralogy of lamproites are the variable nature of their metasomatized source-regions in the mantle, depth and extent of partial melting, coupled with their common extensive differentiation.

TABLE 3. NOMENCLATURE OF LAMPROITES

Historical name  Revised name 
Wyomingite  diopside-leucite-phlogopite lamproite 
Orendite  diopside-sanidine-phiogopite lamproite 
Madupite  diopside madupitic lamproite 
Cedricite  diopside-leucite lamproite 
Mamilite  leucite-richterite lamproite 
Wolgidite  diopside-leucite-richterite madupitic lamproite 
Fitzroyite  leucite-phlogopite lamproite 
Verite  hyalo-olivine-diopside-phlogopite lamproite 
Juinillite  olivine-diopside-richterite madupitic lamproite 
Fortunite  hyalo-enstatite-phlogopite lamproite 
Cancalite  enstatite-sanidine-phlogopite lamproite 

after Mitchell & Bergman (1991)

THE PROBLEMS OF CLASSIFICATION

Lamproites 

Lamproites have always been a difficult group of rocks in terms of their identification and r nomenclature. Recent interest in them has been prompted by the discovery of economically viable diamond-bearing varieties, which has led to a detailed re-examination of the group and complete revision of the nomenclature ( Scott Smith & Skinner 1983, Mitchell & Bergman 1991). These revisions have resulted in the reclassification as lamproites of some rocks previously regarded as kimberlites (e.g., Prairie Creek, Arkansas, and Majhgawan, India).
It is only in the last few decades that lamproites have been considered to crystallize from a distinct type of magma. Formerly, the presence of leucite, the similarity of some olivine-bearing lamproite to kimberlite, and the presence of some "lamprophyric" characteristics, e.g., abundant phenocrysts of phlogopite, led not only to a plethora of names, but to an ill-defined place in petrological taxonomy. The problem was further exacerbated by their geochemical characteristics, with the magma chemistry stabilizing a large number of unusual minerals (K-Ba titanates and silicates, K-Zr silicates), some of which can be used as discriminants for these rocks. For most examples of lamproite, the presence of these minerals reflects derivation from a source enriched in incompatible and large-ion-lithophile elements; this enrichment distinguishes lamproitic rocks from Group-I kimberlites and lamprophyres. A full discussion of the nomenclature of lamproites, their relationships to other potassic and ultrapotassic rocks, in both mineralogical and chemical terms, together with a suggested revised nomenclature, is found in Mitchell & Bergman (1991). The Subcommission essentially accepted the detailed work of Mitchell & Bergman (1991) and Scott Smith & Skinner (1983), and needed only to integrate these rocks into the IUGS hierarchical system.



Classification of lamprophyres 

Lamprophyres are mesocratic to melanocratic igneous rocks, usually hypabyssal, with a panidiomorphic texture and abundant mafic phenocrysts of dark mica or amphibole (or both) with or without pyroxene, with or without olivine, set in a matrix of the same minerals, and with feldspar (usually alkali feldspar) restricted to the groundmass. For the general classification of these rocks, see Le Maitre et al. (1989), and for detailed descriptions, see Rock (1991).
The Subcommission no longer endorses the terms "lamprophyric rocks", or "lamprophyre clan", as used by Le Maitre et al. (1989) and Rock (1991) to encompass lamprophyres, lamproites and kimberlites, because lamproites and kimberlites are best considered independently of lamprophyres.

THE PROBLEMS OF CLASSIFICATION

Lamprophyres 

The lamprophyres are a complex group of rocks that have mineralogical similarities to some kimberlites and lamproites. Lamprophyres are difficult to classify unambiguously using existing criteria. They are not amenable to classification according to modal proportions, such as the system QAPF, nor compositional discrimination diagrams, such as TAS (Le Maitre et al. 1989). It seems unlikely that a simple taxonomic system will be found unless appropriate genetic criteria are applied, that is, unless the classification takes into account the genesis of the rocks.
The term "lamprophyre" was introduced by von Gumbel in 1874 for a group of dark rocks that form minor intrusions, contain phenocrystal brown mica and hornblende, but lack feldspar phenocrysts. Following its introduction, the term was used by Rosenbusch (1877) to encompass a wide variety of hypabyssal rocks containing ferromagnesian phenocrysts, e.g.. minette, kersantite, camptonite and vogesite. Eventually, spessartite, monchiquite and alnoite also were included in the group. Thus, the group became a repository for any mafic-phenocryst-rich rock that was difficult to categorize. Subsequently, Middlemost (1986) and Rock (1986, 1991) extended the definition further to include kimberlites, lamproites and even rocks containing feldspar and leucite phenocrysts. Reviews of the history of lamprophyre nomenclature can be found in Rock (1991), Mitchell & Bergman (1991) and Mitchell (1994a).
Rock (1991) used "lamprophyre" synonymously with "lamprophyre clan", a term that included lamprophyres, lamproites and kimberlites. To many petrologists working on lamproites and kimberlites, "lamprophyre" is an inappropriate general term. Consequently, the working group and Subcommission overwhelmingly rejected the use of the term "lamprophyre clan" to encompass lamprophyres (sensu Rosenbusch), kimberlites and lamproites.
Mitchell (1994a) discussed at some length the inadequacies of the concept of the "lamprophyre clan" and proposed adoption of the term "lamprophyre facies" to convey the concept that some members of a petrological clan crystallized under different conditions than other members of the clan. Mitchell's approach to the problem is determined by a conviction that systems of classification should be genetic in character.
The working groups found it impossible to devise a definition of "lamprophyric rocks" that is not so broad as to be almost petrologically meaningless. For the lamprophyres (sensu Rosenbusch or Rock), the working group could not draft a satisfactory definition, in part because Rock (1991) included a number of rock types that differ mineralogically from the generally accepted characteristics of lamprophyres. However, of greater significance is the realization that members of the lamprophyre group have distinctly different origins, and thus it is unwise to describe or group together rocks that are genetically different.
Among the rocks included in the lamprophyre group by Rock (1991) are alnoite and polzeilite. These contain more than 10 vol.% melilite, and thus are now considered as varieties of melilitic rocks. Similarly, the abundant carbonate in aillikite suggests that it may be considered a variety of silicocarbonatite.
Minette is a biotite-rich lamprophyre. However, certain mica-rich rocks that usually occur in minor intrusions should not be called minette because the mica is phlogopite (commonly titaniferous), and the rock is alkaline. Such rocks might be better termed "alkali minette" rather than "glimmerite" or "phlogopitite", as it is the alkalinity rather than the Fe/Mg ratio that is their characteristic feature.
The widespread usage of the term Iamprophyre, in English, French and German petrological literature is in marked contrast to its infrequency in the extensive Russian literature. There the term is usually reserved for alkaline rocks. Specific rock types, such as kersantite, are considered as varieties of diorite. Similarly, camptonite and alnoite are regarded as variants of gabbro and melilitite, respectively (Andreeva et al. 1985, Kononova 1984). This approach to lamprophyre nomenclature is similar to the facies concept proposed by Mitchell (1994a).

Lamprophyre nomenclature (after Mitchell, 1995)

Existing classifications and definitions of lamprophyres (Rock, 1991; Le Maitre et al., 1989) are unsatisfactory from a genetic viewpoint, as rocks of diverse parentage are grouped together (Mitchell, 1994b). Consequently, it is difficult to devise a succinct definition for this modally diverse group of polygenetic rocks (Woolley et al., 1996). Mitchell (1994b) has suggested that rocks belonging to the lamprophyre facies are characterized by the presence of phenocrysts of mica and/or amphibole together with lesser clinopyroxene and/or melilite set in a groundmass which may consist (either singly or in various combinations) of plagioclase, alkali feldspar, feldspathoids, carbonate, monticellite, melilite, mica, amphibole, pyroxene, perovskite, Fe-Ti oxides and glass. Lamprophyres include such diverse rocks as minettes, aillikites and alnoites, for which genetic classifications have not yet been devized. Although these lamprophyric rocks cannot be misclassified with respect to each other, they do raise some problems regarding the identification of potentially-diamondiferous rocks, as there are macroscopic and petrographic similarities between minettes and some lamproites or between aillikites and some kimberlites and orangeites. However, application of the mineralogical compositional criteria listed in Table 2 permits, with ease, discrimination between. these genetically-diverse rocks.



Table B.4. Special terms used for charnockitic rocks (from Streckeisen, 1974, p.355).

QAPF field  General term  Special term 
hypersthene alkali feldspar granite  alkali feldspar chamockite 
hypersthene granite  chamockite (3b farsundite) 
hypersthene granodiorite  opdalite or charno-enderbite 
hypersthene tonalite  enderbite 
hypersthene alkali feldspar syenite 
hypersthene syenite 
hypersthene monzonite  mangerite 
monzonorite (hypersthene monzodiorite)  jotunite 
10  norite (hypersthene diorite), anorthosite (M<10) 


B.9. CHARNOCKITIC ROCKS 

This classificafion should be used only if the rock is considered to belong to the charnockitic suite of rocks which is characterised by the presence of hypersthene (or fayalite plus quartz) and, in many of the rocks, perthite, mesoperthite or antiperthite (Streckeisen, 1974, 1976). They are often associated with norites and anorthosites and are closely linked with Precambrian terranes.
Although many show signs of metamorphic overprinting, such as deformation and recrystallisation, they conform to the group of "igneous and igneous-looking rocks" and have, therefore, been included in the classification scheme.
The classification is based on the QAP triangle, i.e. the upper half of the QAPF double triangle (Fig. B.4), and the general and special names that may be applied to the fields are given in Table B.4.
However, as one of the characteristic of charnockites is the presence of various types of perthite, this raises the problem of how to distribute the perthites between A and P. The Subcommission has, therefore, recommended that the perthitic feldspars should be distributed between A and P in the following way:
* Perthite - assign to A as the major component is alkali feldspar.
* Mesoperthite - assign equally between A and P as the amounts of the alkali feldspar and plagioclase (usually oligoclase or andesine) components are roughly the same.
* Antiperthite - assign to P as the major component is andesine with minor albite as the alkali feldspar phase.
To distinguish those charnockite rocks that contain mesoperthite it is further suggested that the prefix m-, being short for mesoperthite, could be used e.g. m-charnockite. 



B.10. PLUTONIC ROCKS 

QAPF diagram  This classification should be used only if the rock is considered to be plutonic i.e. it is assumed to have formed at considerable depth and has a relatively coarse-grained depth and has a relatively coarse-grained texture in which the individual crystals can easily be seen with the naked eye. There is, of course, a gradation between plutonic rocks and volcanic rocks and the Subcommission suggests that, if there is any uncertainty as to which classification to use, the plutonic root name should be given and prefixed with the term "micro". For example, microsyenite could be used for a rock that was considered to have formed at considerable depth even if many of the individual crystals could not be seen with the naked eye. The classification is based on modal parameters and is divided into three parts: 
Fig. B.4. Classification and nomenclature of plutonic rocks according to their modal mineral contents using theQAPF diagram (based on Streckeisen, 1976, Fig. 1a).The corners of the double triangle are Q = quartz, A = alkali feldspar, P = plagioclase and F = feldpathoid. However, for more definitions refer to section B.2. This diagram must not be used for rocks in which mafic mineral content, M, is greater than 90%.  (a) if M is less than 90% the rock is classified according to its felsic minerals, using the now familiar QAPF diagram (Fig. B.4), often simply referred to as the QAPF classification or the QAPF double triangle (section B.10.1). 
(b) if M is greater or equal to 90%, the rock is ultramafic and is classified according to its mafic minerals (Fig. B.8), as shown in section B. 10.2. 
(c) if a mineral mode is not yet available, the "field" classification (Fig. B.9) of section B.10.3 may be provisionally used. 
  B.10.1. QAPF Classification (M<90%) 

The modal classification of plutonic rocks is based on the QAPF diagram and was the first to be completed and recommended by the IUGS Subcommission (Streckeisen 1973 and 1976). The diagram is based on the fundamental work of many earlier petrologists, which is excellently summarised by Streckeisen (1967). The root names for the classification and the field numbers are given in Fig, B.4 and B.5, respectively. 
To use the classification the modal amounts of Q, A, P, and F must be known and recalculated so that their sum is 100%. For example, a rock with Q = 10%, A = 30%, P = 20%, and M = 40% would give recalculated values of Q, A, and P as follows: 
Q = 100 x 10/60 = 16.7 
A = 100 x 30/60 = 50.0 
P = 100 x 20/60 = 33.3 
Fig. B.5. Field numbers of the QAPF diagram (based on Streckeisen, 1976, Fig. 1a). Fields 6* to 10* are slightly oversaturated variants of fields 6 to 10, respectively, while 6' to 10' are slightly undersaturated variants. The field number 16 is allocated for rocks in which the detailed the mafic mineral content, M, is greater than 90%. 
Although at this stage the rock can then be plotted directly into the triangular diagram, if all that is required is to name the rock it is easier to determine the plagioclase ratio=100x P/(A+ P), as the nonhorizontal divisions in the QAPF diagram are lines of constant plagioclase ratio. The field into which the rock falls can then easily be determined by inspection.  How is the plagioclase ratio for the example rock and into which QAPF field falls the rock? 


Recalculate a rock with A = 50%, P= 5%, F = 30%, and M = 15%, give the Plagioclase ratio and the number of the QAPF field and, therefore, the term of the rock.  to see the answer mark the cell on the right with your mouse  A rock with A = 50%, P= 5%, F = 30%, and M = 15% would recalculate as follows: A 100 x 50 / 85 = 58.8 P 100 x 05 / 85 = 05.9 F 100 x 30 / 85 = 35.3 
Plagioclase ratio = 9 

-input
result

  This rock falls into QAPF field 11 and would therefore, be called a foid syenite. Furthermore, if the major foid in the rock is nepheline, it would be called a nepheline syenite. 

B. 10.1.1. Details of Fields 

Field 2 - rocks in the field of alkali feldspar granite have been called alkali granite by many authors. The Subcommission, however, recommends that the term alkali granite be restricted to those rocks that contain alkali amphiboles and/ or pyroxenes. The term alaskite may be used for a light-coloured (M = < 10) alkali feldspar granite.
Field 3 - the term granite has been used in many senses; in most English and American textbooks it has been restricted to subfield 3a, whereas subfield 3b has contained terms such as adamellite and quartz monzonite. In the European literature, however, granite has been used to cover both subfields, a view adopted by the Subcommission. The Subcommission has also recommended that the term adamellite should no longer be used, as it has been given several meanings, and does not even occur in the Adamello massif as commonly defined (Streckeisen, 1976). Although the term quartz monzonite has also been used with several meanings, the Subcommission decided to retain the term in its original sense, i.e. for rocks in field 8*.
Field 4 - the most widespread rocks in this field are granodiorites, commonly containing oligoclase, more rarely andesine. Itseems advisable to add the condition that the average An content of the plagioclase should he less than 50% in order to distinguish the common granodiorites from the rare granogabbro in which the An content of the plagioclase is greater than 50%.
Field 5 - the root name tonalite should be used whether hornblende is present or not. Trondhjemite and plagiogranite (of the USSR) may be used for a light-coloured (M = < 10) tonalite.
Fields 6 and 7-these fields contain the root names alkali feldspar syenite and syenite, respectively.
Field 8 - the root name is monzonite; many so-called "syenites" fall into this field.
Field 9 - the two root names in this field, monzodiorite and monzogabbro, are separated according to the average composition of their plagicoclase; if An is less than 50% the rock is a monzodiorite; if An is greater than 50% the rock is a monzogabbro, and may be further subdivided, if required, as shown below. The terms syenodiorite and syenogabbro may be used as comprehensive names for rocks between syenite and diorite/gabbro i.e. for monzonites (field 8) and monzodiorite/ monzogabbro, respectively.
Field 10 - the three root names in this field, diorite, gabbro, and anorthosite, are separated according to the average composition of their plagioclase and the colour index; if M is less than 10% the rock is an anorthosite; if An is less than 50% the rock is a diorite; if An is greater than 50% the rock is a gabbro and may be further subdivided, if required, as shown below. Either of the two synonymous terms dolerite or diabase may be used for medium-grained gabbros rather than the term microgabbro, if required.
Gabbroic rocks - the gabbros (sensu lato) of QAPF field 10, may be further subdivided according to the relative abundances of their orthopyroxene, clinopyroxene, olivine, and hornblende as shown in Fig. B.6. Some of the special terms used are: -
Gabbro (sensu stricto) = plagioclase + clinopyroxene.
Norite = plagioclase + orthopyroxene.
Troctolite = plagioclase + olivine.
Gabbronorite = plagioclase with almost equal amounts of clinopyroxcne and orthopyroxene.
Orthopyroxene gabbro = plagioclase + clinopyroxene with minor amounts of orthopyroxene.
Clinopyroxene norite = plagioclase + orthopyroxene with minor amounts of clinopyroxene.
Hornblende gabbro=plagioclase+ hornblende with pyroxene < 5%.
Field 11 - although foid syenite is the root name, the most abundant foid present should be used in the name, e.g. nepheline syenite, sodalite syenite, etc. This remark also applies to fields 12 to 15.
Field 12 - the root name foid monzodiorite may be replaced by the synonym foid plagisyenite. Wherever possible, replace the term foid with the name of the most abundant feldspathoid. Miaskite, which contains oligoclase, may also be used.
Field 13 - the two root names in this field, foid monzodiorite and foid monzogabbro, are separated according to the average composition of their plagioclase, as for rocks in field 9; if An is less than 50% the rock is a foid monzodiorite; if An is greater than 50% the rock is a foid menzogabbro. Wherever possible, replace the term foid with the name of the most abundant feldspathoid. The term essexite may be applied to nepheline monzodiorite or nepheline monzogabbro.
Field 14 - the two root names in this field, foid diorite and foid gabbro, are separated according to the average composition of their plagioclase, as for rocks in field 9; if An is less than 50% the rock is a foid diorite; if An is greater than 50% the rock is a foid gabbro. Wherever possible, replace the term foid with the name of the most abundant feldspathoid. Two special terms may continue to be used, theralite for nepheline gabbro and teschenite for analcime gabbro.
Field 15 - this field contains rocks in which the light-coloured minerals are almost entirely foids and is given the root name foidolite to distinguish it from the volcanic equivalent which is called foidite. As these rocks are rather rare the field has not been subdivided, except to note that the most abundant foid should appear in the name, e.g. nephelinolite, etc.

  Fig. B.6. Classification and nomenclature of gabbroic rocks based on the proportions of plagioclase (Plag), pyroxene (Px), olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), and hornblende (Hbl) (after Streckeisen, 1976, Fig. 3). 
Rocks falling in the shaded areas of the triangular diagrams may be further subdivided according to the diagram within the shaded rectangle. 



B.10.2 Ultramafic Rocks (M > 90 %) 

  The plutonic ultramafic rocks are classified according to their content of mafic minerals, which consist of olivine, orthopyroxene, clinopyroxene, hornblende, sometimes biotite, and various but usually small amounts of garnet and spinel. Two diagrams are recommended by the subcommission (Streckeisen, 1973, 1976), one for the rocks consisting essentially of olivine, orthopyroxene, and clinopyroxene, and the other for rocks containing hornblende, pyroxenes, and olivine (Fig. B.8). 
Peridotites are distinguished from pyroxenites by containing more than 40 % olivine. This value, rather than 50 % was chosen because many lherzolites contain up to 60 % pyroxene. 

The peridotites are subdived into dunite (or olivinite if the spinel mineral is magnetite), harzburgite, lherzolite and wehrlite

The pyroxenites are further subdivided into orthopyroxenite (e.g. bronzitite), websterite, and clinopyroxenite (e.g. diallagite). 


Ulramafic rocks containig garnet ot spinel should be qualified in the following manner. If garnet or spinel is less than 50 % use garnet peridotite, chromite bearing dunite, etc. If garnet or spinel is greater than 5 % use garnet peridotite, chromite dunite, etc. 

Fig. B.6 Classification and nomenclature of ultramafic rocks based on the proportion of olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), pyroxene (Px), and hornblende (Hbl) (after Streckeisen 1973, Figs. 2a and 2b).



B. 11.1 QAPF Classification (M < 90 %) 

This classification should be used only if the rock is considered to be volcanic and if a mineral mode can be determined (Streckeisen, 1978 and 1979). The root names for the classification are given in Figure Fig. B.10. The numbers of the fields are the same as those for plutonic rock classification.

  Fig. B.10. Classification and nomenclature of volcanic rocks according to their modal mineral contents using the QAPF diagram (based on Streckeisen, 1978, Fig. 1). The corners of the double triangle are Q=quartz, A=alkalifeldspar, P=plagioclasc and F=feldspathoid. However, for more detailed definitions refer to section B.2. 

B 11.1.1 Details of Fields 

Field 2 - the root name alkali feldspar rhyolite, corresponds with alkali feldspar granite. The term alkali rhyolite (=peralkaline rhyolitic) can be used when the rock contains alkali pyroxene and/or amphibole. The name rhyolite may be replaced by the synonym liparite. Fields 3a and 3b - in an analogous manner to the granites, rhyolite (liparite) covers both fields 3a and 3b. The term rhyodacite, which has been used ambiguously for rocks of fields 3b and 4, can be used for transitional rocks between rhyolite and dacite without attributing it to a distinct field. 

Fields 4 and 5 - rocks in both these fields are covered by the root name dacite in the broad sense. Volcanic rocks of field 5, to which terms such as "plagidacite" and "quartz andesite" have been applied, are frequently also described as dacite, which is the recommended name.
Fields 6, 7, 8 - rocks with root names alkali feldspar trachyte, trachyte or latite, which contain no modal foids but do contain nepheline in the norm, may be qualified with "ne-normative" to indicate that they would fall in subfields 6'-8', respectively. Alkali trachyte (=peralkaline trachyte) may be used for trachytes containing alkali pyroxcne and/or amphibole.
Fields 9 and 10 - these fields contain the large majority of volcanic rocks, including basalt and andesite, which are tentatively separated using colour index, at a Iiinit of 40 wt% or 35 vol%, and 52% SiO2 as shown in Fig. B. 1 1. A plagioclase composition (at a limit of An<50) is less suitable for the distinction between basalt and andesite, because many andesites commonly contain "phenocrysts" of labradorite or bytownite. Although this may seem rather unsatisfactory, it is unlikely that many of these rocks will be classified using the QAPF diagram, as the modes of most basalts and andesites are difficult to accurately determine and so that the TAS classification will have to be used.

  Fig. B. 11. Division of QAPF field 9 and 10 rocks into basalt and andesite, using colour index and SiO2 content (after Streckeisen, 1978, Fig. 2) 

Field 11 - the root name phonolite is used in the sense of Rosenbusch for rocks consisting essentially of alkali feldspar, any feldspathoid and mafic minerals. The nature of the predominant foid should be added to the root name, e.g. leucite phonolite, analcime phonolite, leucite nepheline phonolite (with nepheline > leucite), etc. Phonolites containing nepheline and/or haüyne as the main foids are commonly described simply as "phonolite".
Field 12 - the root name for these rather rare rocks is tephritic phonolite. Although it was originally suggested that the term tephriphonolite was a synonym (Streckeisen, 1978), it is probably better to reserve this term for the root name of TAS field U3, to indicate that the name has been given chemically and may not be identical to those of QAPF field 12.
Field 13 - this field contains the root names phonolitic basanite and phonolitic tephrite which are separated on the amount of olivine in the CIPW norm. If normative olivine is greater than 10% the rock is called a phonolitic basanite; if less than 10% it is a phonolitic tephrite. Although it was originally suggested that the term phonotephrite was a synonym of phonolitic tephrite (Streckeisen, 1978), it is probably better to reserve this term for the root name of TAS field U2, to indicate that the name has been given chemically and may not be identical to those of QAPF field 13. There is no conflict with the term phonobasanite being used as a synonym for phonolitic basanite as the term is not used in TAS.
Field 14 - this field contains the root names basanite and tephrite which are separated on the amount of olivine in the CIPW norm. If normative olivine is greater than 10% the rock is called a basanite; if less than 10% it is a tephrite. The nature of the dominant foid should be indicated in the name, e.g. nepheline basanite, leucite tephrite, etc.
Field 15 - the general root name of this field is foidite, but as these rocks occur relatively frequently the field has been subdivided into 15a. 15b and 15c.
Field 15a - the root name is phonolitic foidite but wherever possible, more specific terms, such as phonolitic nephelinite, etc. should be used. Alternatively, the term alkali feldspar foidite could be used as the root name, which would give specific terms such as sanidine nephelinite, etc.
Field 15b - the root names are tephritic foidite or basanitic foidite and are separated according to olivine content as in field 14. Wherever possible, more specific terms, such as tephritic leucitite, basanitic nephelinite, etc. should be used.
Field 15e - the root name is foidite and should be distinguished by the nxne of the predominant foid, e.g. nephelinite, leucitite, analcimite, etc.
Field 16 - the root name is ultramafitite, which should be qualified by indicating the predominant mafic minerals.



B.11.2. The TAS Classification 

This classification should be used only if the rock is considered to be volcanic and if a mineral mode cannot be determined, due either to the presence of glass or to the fine-grained nature of the rock and a chemical analysis of the rock is available.
The main part of the classification is based on the total alkali silica (TAS) diagram. The root names for the classification and the field symbols are given in Fig. B.13 and B.14, respectively (Le Bas et al. 1986). The classification is easy to use as all that is initially required are the values of NA2O+K2O and SiO2 although, if the analysis falls in certain fields, additional calculations, such as the CIPW norm, must be performed in order to arrive at the correct root name.
The TAS classification was originally constructed with the more common rock types in mind using the following principles:-
(a) each field was chosen to accord as closely as possible with the current usage of the root name with the help of data from 24,000 analyses of named fresh volcanic rocks from the CLAIR and PETROS databases (Le Maitre, 1982).
(b) fresh rocks were taken to be those in which H2O+ < 2% and CO2 < 0.5%.
(c) each analysis was recalculated to 100% on an H2O and CO2 free basis.
(d) wherever possible, the boundaries were located to minimise overlap between adjacent fields.
(e) the vertical SiO2 boundaries between the fields of basalt, basaltic andesite, andesite, and dacite, were chosen to be those in common use.
(f) the boundary between the S (for silica Saturated) fields and the U (for silica Undersaturated) fields was chosen to be roughly parallel with the empirically determined contours of 10% normative F in QAPF.
(g) the boundary between the S fields and O (for silica Oversaturated) fields was chosen where there was a density minimum between volcanic rock series that were alkaline and those that were calc-alkaline.
(h) the boundaries between fields SI-S2-S3-T were all made parallel to a pronounced edge found in the distribution of analyses of rocks that had been called trachyte from CLAIR and PETROS.
(i) similarly, the boundaries between fields U1-U2-U3-Ph were also drawn parallel to each other. They are not at rightangles to the line separating fields S from U.
However, after the TAS classification was published the Subcoinmission considererd whether or not it was possible to include some of the olivine- and pyroxene-rich ("high-Mg") volcanic rocks e.g. picrites, komatiites, meimechites, and boninites into the scheme. After lengthy discussions this has been done, but only by using some additional parameters not used in TAS. As a result these rocks must be considered first, as they are not the "normal" type of volcanic rocks for which the TAS classification was originally designed.
It is important to note that the TAS diagram published by Le Maitre (1984) has been slightly modified in Le Bas< I> et al. (1986) in the light of further information. Similarly, the TAS classification presented here has been slightly improved in the light of additional information in two ways:-
(a) by "opening" the boundary between fields U1 and F and making it a dashed line. This now allows nephelinites and leucitites to be better classified as they can fall in both fields U1 and F.
(b) by the inclusion of some of the "high Mg" volcanic rocks such as picrites, komadites, meimechites, and boninites which has been done after further lengthy discussions. However, as they are not the "normal" type of volcanic rocks for which the TAS classification was originally designed, they must be extracted first, in line with the general logic of the classification scheme.
It must also be stressed that die TAS classification is purely descriptive, and that no genetic significance is implied. Furthermore, analyses of rocks that are weathered, altered, metasomatised, metamorphosed or have undergone crystal accumulation should be used with caution, as spurious results may be obtained. As a general rule it is, therefore, suggested that only analyses with H2O+ < 2% and with CO2 < 0.5 % should be used, except if the rock is a picrite, komatiite, meimechite or boninite, when this restriction is withdrawn. Note that the application of TAS to altered rocks is discussed by Sabine et al. (1985) who found that many low grade metavolcanic rocks could be satisfactorily classified.

B.11.2.1. Using TAS 
Before using the classification the following procedures must be adopted:-
(a) all analyses must be recalculated to 100% on an H 2O and CO2 free basis
(b) if a CIPW norm has to be calculated, in order to determine the correct root name, the amounts of FeO and Fe2O3 should be left as determined. If only total iron has been determined, then it is up to the user to justify the method used for partitioning the iron between FeO and Fe 2O 3. One method that can be used to get an estimate of what the FeO and Fe2O3would have been, had they been determined, is that of Le Maitre (1976). Remember, it is the feeling of the Subcommission that rocks should be named according to what they are, and not according to what they might have been.
The analysis must then be checked to see if it is a "high-Mg" volcanic rock, i.e. a picrite, komatiite, meimechite or boninite. This is done as follows (see Fig. B. 12):

  1. Boninite - SiO2 > 53%, MgO > 8%, and TiO 2 < O.5%
  2. Picritic rocks - SiO2 < 53%, Na2O+K2O < 2.O%, and MgO > 18%. These are divided into:-
  3. bulletPicrite - Na2O+K2O > 1 %
    bulletKomatiite - Na2O+K2O < 1% and TiO 2 < 1%
    bulletMeimechite - Na2O+K2O < 1% and TiO2 > 1%

Note that the Subcommission recommends that the term picritic rocks can be used to include the rock names picrite, komatiite, and meimechite.

  Fig. B.12. Classification and nomenclature of "high-Mg" volcanic rocks (picrite, komatiite, meimechite and boninite ) using TAS together with wt% MgO and TiO2. The thick stippled lines indicate the location of TAS fields. 
If the rock is not one of these four ultramafic types it is then classified using the total alkali silica (TAS) diagram as shown in Fig. B.13. Certain of the fields can then be subdivided further as described below. 


 

Fig. B.13. Chemical classification and nomenclature of volcanic rocks using the total alkali versus silica (TAS) diagram (after Le Bas et al., 1986, Fig. 2). Rocks falling in the shaded areas may be further subdivided as shown in the table underneath the diagram. The line between the foidite field and the basanite-tephrite field is dashed to indicate that further criteria must be used to separate these types. Abbreviations: q = normative quartz; ol = normative olivine. 
 


B. 11.2.2. Details of Fields 

Field B - the root name basalt may be divided into alkali basalt and subalkali basalt according to the state of silica saturation. If the analysis contains normative nepheline, the rock may be called an alkali basalt; if the analysis contains no normative nepheline it may be called a subalkali basalt. It should be noted that in the USSR alkali basalt and subalkali basalt (as defined above) were formerly called subalkali basalt and basalt, respectively; however, it was agreed that in the USSR these two rock types should now be called midalkali basalt and subalkali basalt in order to conform more closely to the IUGS recommendation. Further subdivision was not considered desirable at this stage due to the current state of flux and lack of consistency in many of the specialist names used for basalts.

  Fig. B. 14. Field symbols of the total alkali versus silica (TAS) diagram (after Le Bas et al., 1986, Fig. 1). The pairs of numbers are the coordinates of the intersections of the lines. 

Fields B, O1, O2, O3, R - the root names basalt (if SiO2 is greater than 48%), basaltic andesite, andesite, dacite, and rhyolite may be qualified using the terms low-K, medium-K, and high-K as shown in Fig. B.15. This is in accord with the concept developed by Peccerillo & Taylor (1976), but the lines have been s!ightly modified and simplified. It must be stressed that the term high-K is not synonymous with potassic, as high-K rocks can have more Na2O than K2O. 
  Fig. B.15. Division of basalts (with SiO2 > 48%), basaltic andesites, andesites, dacites and rhyolites in low-K, mediwn-K and high-K types. Note that high-K is NOT synonymous with potassic. The thick stippled lines indicate the equivalent position of some of the fields in the TAS diagram. 

 
Fig.B.16. Separation of trachytcs and rhyolites into comenditic and pantelleritic types using the Al2O3 versus total iron as FeO diagram (after Macdonald, 1974).  
Field R - the root name rhyolite may be further subdivided into peralkaline rhyolite, if the peralkaline index, molecular (Na2O + K2O)/Al2O3 is greater than 1. 
Field T - this field contains the root names trachyte and trachydacite which are separated by the amount of CIPW normative Q in Q+an+ab+or (i.e. the normative equivalent of Q and QAPF). If the value is less than 20% the rock is called trachyte; if greater than 20% it is a trachydacite. Trachytes may be further subdivided into peralkaline trachytes, if the peralkaline index is greater than 1. 

Peralkaline rhyolites and trachytes - the subcommission has considered it useful to further subdivide these rocks into comenditic rhyolite (= comendite), comenditic trachyte, pantelleritic rhyolite (= pantellerite), and pantelleritic trachyte according to the method of Macdonald (1974) which is based on the relative amounts of Al2O3 versus total iron as FeO as shown in Fig. B.16.
Field Sl - the root name trachybasalt maybe divided into hawaiite and potassic trachybasalt according to the relative amounts of Na2O and K2O If Na2O - 2 is greater than K2O the rock is considered to be "sodic" and is called hawaiite; if Na2O - 2 is less than K2O the rock is considered to be "potassic" and is called potassic trachybasalt.
Field S2 - using the same criteria as for field S 1, the root name basaltic trachyandesite may be divided into mugearite ("sodic") and shoshonite ("potassic").
Field S3 - using the same criteria as for field S1, the root name trachyandesite may be divided into benmoreite ("sodic") and latite ("potassic").
Field Ul and F - field U1 contains the root names basanite and tephrite while field F contains foidite, of which the two main varieties are nephelinite and leucitite. Unlike earlier versions of TAS this boundary is now dashed as, although most named basanites and tephrites do fall in field Ul, it has been found that nephelinites and leucitites fall in both fields U1 and F. This means that additional parameters will have to be used to effectively separate the various rock types. This is a problem the Subcommission is currently working on and hopes to come up with a solution in the near future. Until this matter is resolved these names should be assigned with caution.

Nephelinitic and Basanitic Rocks

by M. J. LE BAS (on behalf of the IUGS Subcommission on the Systematics of Igneous Rocks) Department of Geology, University of Leicester, Leicester LE1 7RH, UK (Received 5 May 1988; revised typescript accepted 19 January 1989)
ABSTRACT

Basanites (which mineralogically should be characterized by an abundance of plagioclase and only a little feldspathoid) and nephelinitic rocks (which normally lack modal plagioclase but should contain nepheline) are not easily distinguished in the field, and both are commonly confused with basalts. Since the mineralogical distinction sometimes cannot be applied, as in the case when no modal felsic minerals are apparent in the groundmass, a chemical distinction between nephelinitic and basanitic rocks becomes desirable. The best solution is provided by the use of the CIPW norm with basanites recognized as having > 5 % normative ab and < 20% normative ne, melanephelinites as having < 5% normative ab and <20% normative ne, and nephelinites as having >20% normative ne. It is proposed that melanephelinites defined in this manner can be further divided and that rocks formerly termed olivine nephelinite should now be called olivine melanephelinite, and that the pyroxene-phyric, olivine-poor melanephelinites should now be termed pyroxene melanephelinite.

  Fig. 5. CIPW normative (ne + lc) vs. ab plot of the nephelinitic rocks (excluding melilite-bearing ones which all have zero normative ab and basanites, showing the best discriminatory boundaries between the fields. The letters with subscripts, e.g., B2 are the average compositions listed in Table 6. The average olivine melanephelinite compositions O3 and O4, coincide with O1 and these, together with many data points with ab=0, are not shown owing to congestion. At zero normative ab, many of the olivine and the pyroxene melanephelinites have normative (ne + lc) values up to 35% as a result of large lc values in the norm. 
 
Fig. 6. CIPW normative ne vs. ab plot of the nephelinitic rocks and basanitcs similar to Fig. 5, showing the best discriminatory boundaries between the fields. The letters with subscripts, e.g., B2 are average compositions listed in Table 6. Not all melanephelinites that plot in the vicinity of O3 and O4 are shown, owing to congestion. 



References 

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Appendix 

Table 1. Mineral Symbols (Kretz, 1983)

Acm acmite  Act actinolite  Agt aegirine-augite  Ak akermanite 
Ab albite  Aln allanite  Alm almandine  Anl analcite 
Ant anatase  And andalusite  Adr andradite  Anh anhydrite 
Ank ankerite  Ann annite  An anorthite  Atg antigorite 
Ath anthophyllite  Ap apatite  Apo apophyllite  Arg aragonite 
Arf arfvedsonite  Apy arsenopyrite  Aug augite  Ax axinite 
Brt barite  Brl beryl  Bt biotite  Bhm boehmite 
Bn bornite  Brk brookite  Brc brucite  Bat bustamite 
Cam Ca clinoamphibole  Cpx Ca clinopyroxene  Cal calcite  Ccn cancrinite 
Crn carnegieite  Cat cassiterite  Cls celestite  Cbz chabazite 
Cc chalcocite  Ccp chalcopyrite  Chl chlorite  Cld chloritoid 
Chn chondrodite  Chr chromite  Ccl chrysocolia  Ctl chrysotile 
Cen clinoenstatite  Cfs clinoferrosilite  Chu clinohumite  Czo clinozoisite 
Crd cordierite  Crn corundum  Cv covellite  Crs cristoballite 
Cum cummingtonite  Dsp diaspore  Dg diginite  Di diopside 
Dol dolomite  Drv dravite  Eck eckermannite  Ed edenite 
Elb elbaite  En enstatite (ortho)  Ep epidote  Fst fassite 
Fa fayalite  Fac ferroactinolite  Fed ferroedenite  Fa ferrosilite (ortho) 
Fts ferrotschermakite  Fl fluorite  Fo forsterite  Gn galena 
Grt garnet  Ged gedrite  Gh gehlenite  Gbs gibbsite 
Glt glauconite  Gln glaucophane  Gt geothite  Gr graphite 
Grs grossularite  Cru grunerite  Gp gypsum  Hl halite 
Hs hastingsite  Hyn haüne  Hd hedenhergite  Hem hematite 
He hercynite  Hul heulandite  Hbl hornblende  Hu humite 
Ill illite  Ilm ilmenite  Jd jadeite  Jh johannsenite 
Krs kaersutite  Kls kalsilite  Kln kaolinite  Ktp kataphorite 
Kfs K feldspar  Krn kornerupine  Ky kyanite  Lmt laumontite 
Lws lawsonite  Lpd lepidolite  Lct leucite  Lm limonite 
Lz lizardite  Lo loellingite  Mgh maghemite  Mkt magnesiokatophorite 
Mrb magnesioriebeckite  Mgs magnesite  Mag magnetite  Mrg margarite 
Mel melilite  Mc microcline  Mo molybdenite  Mnz monazite 
Mtc monticellite  Mnt montmorillonite  Mul mullite  Ms muscovite 
Ntr natrolite  Ne nepheline  Nrb norbergite  Nsn nosean 
Ol olivine  Omp omphacite  Oam orthoamphibole  Or orthoclase 
Opx orthopyroxene  Pg paragonite  Prg pargasite  Pet pectolite 
Pn pentlandite  Per periclase  Prv perovskite  Phl phlogopite 
Pgt pigeonite  Pl plagioclase  Prh prehnite  Pen protoenstatite 
Pmp pumpellyite  Py pyrite  Prp pyrope  Prl pyrophyllite 
Po pyrrhotite  Qtz quartz  Rbk riebeckite  Rds rhodochrosite 
Rdn rhodonite  Rt rutile  Sa sanidine  Spr sapphirine 
Scp scapolite  Srl schorl  Srp serpentine  Sd siderite 
Sil sillimanite  Sdl sodalite  Spa spessartine  Sp sphalerite 
Spn sphene  Spl spinel  Spd spodumene  St staurolite 
Stb stilbite  Stp stilpnomelane  Str strontianite  Tlc talc 
Tmp thompson-te  Ttn titanite  Toz topaz  Tur tourmaline 
Tr tremolite  Trd tridymite  Tro troilite  Ts tschermakite 
Usp ulvospinel  Vrm vermiculite  Ves vesuvianite  Wth witherite 
Wo wollastonite  Wus wustite  Zrn zircon  Zo zoisite 



I. Thum , thum97@bigfoot.com , Fax +49  6374  70864
Appendix

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