Amphibole: structure, chemistry, petrogenesis
Structure Amphiboles consist of double chains of silicate tetrahedra aligned along the c-axis of the unit cell.
with the individual chains linked by cations in octahedral (and occasionally greater) coordination. The structure of amphibole, with chain-linking cations, is shown below as projected along the c-axis of the orthorhombic unit-cell.
In amphibole the M1,M2 and M3 sites are octahedral and occupied by Mg2+ and F2+. While the M4 sites are typically occupied by Ca2+, and the A-sites may be occupied by Na+ or be vacant
Chemistry The basic formula for the amphibole group is:
where W represents the content of the
large A site which may be occupied by Na, Ca or K but is frequently vacant;
X (Ca, Na, K, Mg, Fe) is the content of the M4 site with either 6-fold
(octahedral) or 8-fold co-ordination; Y (Mg, Fe2+,
Fe3+, Al) is the content
of the octahedral M1, M2, M3 sites, and Z (Si, Al) the content of the tetrahedral
Amphiboles show extensive solid solution indicating the operation of numerous exchange vectors in this mineral group (in other words, amphiboles are pretty much compositional "garbage cans"). The composition of the Na-Ca-Al solid-solution series relating the blue amphiboles (e.g. glaucophane) , the aluminous amphiboles (hornblendes) and the sodic hornblendes (edenite, pargasite) to the key component tremolite can be described with just three exchange vectors : AlviAlivMgvi-1Siiv-1, Naviii Alvi Caviii-1Mgvi-1, and NaA Aliv Siiv-1vA-1 (where v stands for vacancy, and A is a large but variably coordinated site that is often vacant - or only partially occupied):
The amphibole group is divided into