Photoglossary of volcanic terms

Glossary of Igneous Textural Terms

Crystallinity

Holocrystalline	Consisting entirely of crystals (default term, not commonly used).
Hypocrystalline	Containing both crystals and glass. Hypocrystalline texture: plagioclase, biotite and magnetite are set in glass
Holohyaline, Vitric	Consisting entirely of glass.

Grain Size

Aphanitic	Most minerals are too fine-grained to see with the naked eye.
Phaneritic	The minerals are coarse enough to see with the naked eye.
Cryptocrystalline	Too fine-grained to distinguish the minerals microscopically.
Fine grained	Average crystal diameter is less than 1 mm.
Medium grained	Average crystal diameter is 1-5 mm.
Coarse grained	Average crystal diameter is greater than 5 mm.
Pegmatitic	Very coarse grained.
Aplitic	Fine- to medium-grained xenomorphic and equigranular (looks sugary).
Equigranular	Grains are all approximately the same size.
Inequigranular	Grains vary considerably in size.

Porphyritic Textures

Porphyritic	Approximately bimodal size distribution (usually requires a great difference).
Megaporphyritic	Porphyritic texture that can be seen in hand specimen (rarely used).
Microporphyritic	Porphyritic texture that is visible only under the microscope.
Phyric (-phyric)	An adjective (or suffix) referring to porphyritic texture.
Phenocryst	Large crystal set in a fine matrix.
Microphenocryst	Microscopic crystals that are still larger than the remainder of the groundmass.
Megacryst	An unusually large crystal, either a phenocryst or a xenocryst.
Poikilitic	The host phenocryst (oikocryst) contains many inclusions of other minerals.
Oikocryst	The host phenocryst in poikilitic texture.
Groundmass	The glassy or finer grained element in the porphyritic texture (matrix, mesostasis).
Cumulophyric	Phenocrysts of the same or different minerals occur in clusters (Figure 3-11a).
Glomeroporphyritic	Synonymous with cumulophyric (used by some to specify that only one mineral is involved).
Hiatial porphyritic	There is a pronounced difference in size between the phyric and groundmass phases (the default term: rarely used).
Seriate	There is a continuous gradation in size.
Aphyric	Non-porphyritic (no phenocrysts).

Form of Individual Grains

Euhedral (idiomorphic)	Completely bounded by crystal faces.
Subhedral	Crystal faces only partially developed. (subidiomorphic)
Anhedral (allotriomorphic)	Crystal faces are entirely absent.
Crystal habits: equant, prismatic, columnar, acicular, fibrous, tabular, platy, micaceous, lath-shaped, etc. (see any mineralogy text).
Corroded (embayed)	Subhedral or anhedral forms produced by partial melting (resorption) of phenocrysts by the melt.

Forms of Grains in the Rock as a Whole

Panidiomorphic	Most of the grains are euhedral (rare).
Hypidiomorphic	Consisting predominantly of subhedral grains (common in many granitic rocks).
Allotriomorphic	Most of the grains are anhedral. Common.
Sutured	Characterized by articulation along highly irregular inter-penetrating boundaries. Common in recrystallized deformed rocks.
Mosaic	A texture of polygonal equigranular crystals.

Intergrowths

Host (Oikocryst)	The large mineral that includes others in poikilitic texture.
Guest	The included mineral in poikilitic texture.
Poikilitic	One large crystal contains several small discrete crystals of another mineral. Refers to growth phenomena, not exsolution or replacement.
Graphic	An intergrowth in which the guest shows angular wedge-like forms. Usually occurs with quartz in microcline (Figure 3-9b).
Micrographic	Graphic texture visible only under the microscope.
Granophyric	A texture of certain porphyritic rocks in which the groundmass minerals (usually quartz and alkali feldspar) penetrate each other as feathery irregular intergrowths. Resembles micrographic texture, but is more irregular.
Exsolution lamellae	Lamellar bands of a phase exsolved from a host phase (Figure 3-9a).
Perthitic	Irregular veins, patches, lenses, etc., of sodic plagioclase in an alkali feldspar host. Usually results from exsolution (Figure 3-18a).
Antiperthitic	Exsolution lamellae of alkali feldspar in a plagioclase host. Usually much thinner than perthite.
Symplectite	A replacement texture in which a mineral is replaced by an intergrowth of one or more minerals.
Myrmekite	A secondary texture consisting of irregular "wormy" blebs or rods of quartz in a plagioclase host adjacent to alkali feldspar grains (Figure 3-21).
Spherulitic	A radial intergrowth of fibrous minerals. Commonly alkali feldspar and quartz in devitrified silicic volcanics.
Axiolitic	Similar to spherulitic, but the fibers occur in a layer and are oriented normal to its walls.
Variolitic	Spherulitic structures consisting of divergent plagioclase fibers in certain basalts.
Coalescent	Anhedral textures developed by simultaneous growth of two mineral grains in contact.

Textures of Mafic Igneous Rocks

Ophitic	Large pyroxene grains enclose small random plagioclase laths (Figure 3-8).
Subophitic	The plagioclase laths are larger and only partially enclosed by the pyroxene.
Nesophitic	Plagioclase is larger yet and the pyroxenes are interstitial.
Intergranular	Small discrete grains of pyroxene, olivine, etc., fill the interstices in a random network of larger plagioclase laths (Figure 3-15).
Intersertal	Glass, cryptocrystalline material, or alteration products occupy the spaces between plagioclase laths.
Hyalo-ophitic	An intersertal texture in which a larger amount of glass is present and less pyroxene.
Hyalopilitic	Still more glass is present and plagioclase occurs only as tiny random microlites.
Diktytaxitic	Texture of certain basalts containing abundant angular interstitial gas cavities between the plagioclase laths.
Cumulate	Interstitial growth of a mineral between earlier ones which are all in contact and give the distinct impression t they accumulated at the bottom of a magma chamber (Figure 3-14).
Orthocumulate	Cumulate texture with other minerals occupying the interstitial areas (Figure 3-14b).
Adcumulate	Cumulate texture in which the early cumulate minerals grow to fill the pore space (Figure 3-14c).
Mesocumulate	Intermediate between ortho- and adcumulate.

Replacement Textures

Pseudomorph	One or more minerals replace another, retaining the form of the original mineral.
Symplectite	A replacement texture in which a mineral is replaced by an intergrowth of one or more minerals. Other forms include replacement of rims, cores, or patches of one mineral by another. Common processes include:
Uralitization	Replacement of pyroxene by amphibole (Figure 3-21a).
Saussuritization	Replacement of plagioclase by epidote.
Biotitization	Replacement of pyroxene or amphibole by biotite.
Chloritization	Replacement of any mafic mineral by chlorite (Figure 3-20b).
Sericitization	Replacement of feldspar or feldspathoids by fine white micas.

Miscellaneous Terms

Interstitial	One mineral fills the interstices between earlier crystallized grains (Figure 3-7).
Crystallites	Minute, inchoate crystals in earliest stages of formation. They are isotropic and cannot be identified under the microscope.
Microlites	Tiny needle- or lath-like crystals of which at least some properties are microscopically determinable.
Felty	Consisting of random microlites (Figure 3-13b).
Pilotaxitic	Like felty.
Trachytic	Consisting of (feldspar) microlites aligned due to flow (Figure 3-12a).
Embayed	Having embayments due to reaction with the melt (resorption) (Figure 3-2).
Skeletal	Crystals which grew as, or have been corroded to, a skeletal framework with a high proportion of internal voids (Figure 3-4).
Sieve	Crystals are filled with channelways (appearing as holes) due to resorption (Figure 3-11a).
Epitactic	Oriented nucleation of one mineral on another of a different kind.
Rapakivi	Overgrowths of plagioclase on alkali feldspar.
Vesicular	Contains gas bubbles.
Scoriaceous	Highly vesicular.
Pumiceous	Frothy vesicular structure characteristic of pumice.
Diktytaxitic	Containing vesicles into which microphenocrysts protrude.
Miarolitic	Gas cavities present in certain plutonic rocks into which euhedral minerals protrude.
Pipe vesicles	Tube-like elongate vesicles that result from rising gases.
Vesicular pipes	Cylindrical bodies highly charged with vesicles.
Amygdaloidal	Vesicles completely or partially filled with secondary minerals.
Lithophysae	Large ovoid structures representing gas bubbles in devitrified rhyolitic glass.
Flow	A parallel structure developed as the result of flow.
Foliation	Planar parallelism.
Banding	Alternating planar layers.
Lineation	Linear parallelism.
Xenolith	An inclusion of country rock.
Xenocryst	A single-crystal foreign inclusion.
Perlitic	Concentric fracture pattern resulting from contraction of some volcanic glasses upon cooling. Pyroclastic
Comprised of fragments.
Ocelli	Ovoid blobs created by liquid immiscibility, mingled magmas, or filled vesicles.
Orbicules	Ovoid masses of radiating crystals, commonly concentrically banded, found in some granites.
Spinifex	Centimeter-scale sub-parallel to dendritic growth of olivine crystals in some quenched ultramafics.

Pyroclastic Terms

See Section 2.5.

Pyroclastic Glass textures

Pele's tears	Glassy lapilli.
Pele's hair	Hair-like strands of glass.
Fiamme	Compressed pumice fragments in a tuff.