Amphibole - Crystallography, Crystal Chemistry and Provenance Potential
Amphiboles are common rock-forming minerals in the earth’s crust.
It forms in igneous and metamorphic rocks, but can be a relatively common detrital mineral for clastic sediments that have not undergone extensive diagenesis
The crystal chemistry of amphiboles is quite complex.
clinoamphiboles have seven distinct cation positions that can be occupied by cations of different charge and atomic radius
This is good because it has a wide stability range and responds to the local chemical environment well
Crystallographically, the amphibole structure is related to that of the pyroxenes, and consists of double chains of tetrahedra along the c axis, a pair of chains with apices facing each other forming a broad I-beam. Between the chains there are five other types of cation positions, designated M1 to M4 and A.
|The amphibole structure can be
considered as interleaved modules of pyroxene and mica structure. Much of
the crystal-chemical behavior of amphiboles can be considered in this light:
the M2 and M4 amphiboles sites are equivalent to the M1 and M2 sites,
respectively, of the pyroxene structure. The M1 and M3 amphibole sites are
like the octahedral sites in micas, and the A site resembles the mica
End members and site allocations
The amphibole formula unit has a large number of sites and great flexibility of substitution. The basic formula unit is
A0-1 M42 M133 M22 T8 O22(OH)2
where A is the large vacant or partly-filled site between the bases of chains, M13 represents the two M1 and one M3 (whose properties are very similar), and T are the tetrahedral sites.
The site preferences are as follows
Classification of the amphiboles
(in accordance with the International Mineralogical Association)
General classification of the amphiboles, exclusive of Mg-Fe-Li amphiboles
The following list contains the most commonly-used end members for monoclinic amphiboles. [V] is used to represent an A-site vacancy, Fe is Fe2+ unless expressly stated.
Because amphiboles show extensive solid solution, many aspects of their composition are controlled by the host rock chemistry. However, for hornblendes, there are a few useful grade-dependent trends:
Many of the substitutions will be controlled by equilibria with other phases in the assemblage, and some of these may be sufficiently T- or P-sensitive to be of practical use in thermobarometry.
A vector treatment for hornblendes, using tremolite as the additive component, and accounting for Ti, Mn and K substitutions, might use the following set of exchange components:
Note that other commonly-used end members can be expressed as linear combinations of these, e.g. pargasite = edenite + tschermak.
Possible crystal chemical signatures for provenance
Plutonic amphiboles - complex textural and
compositional ranges (Pe-Piper,
Unusual amphiboles diagnostic of source rocks
glaucophane - high P, moderate T
riebeckite or arvedsonite - alkaline igneous alteration
grunerite - metamorphosed ironstones
anthopyllite - metamorphosed ultramafic or unusual mafic
cummingtonite - medium grade metamorphosed mafic
kaersutite - volcanic phenocrysts