Amphibole -  Crystallography, Crystal Chemistry and Provenance Potential

Amphiboles are common rock-forming minerals in the earth’s crust.


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 interlayer site.

 

image source: http://www.gfz-potsdam.de/pb4/pg1/projects/A_PropofGeomat/a11amphi.html

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.



Variation of amphibole composition with grade

 

 

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