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:

W X2 Y5 Z8 O22 (OH)2

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 sites.
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):

(note that two further dimensional to the amphibole compositional space are implied by the FeMg-1, K Na-1 exchange vectors - making a 5-d compositional space!)
Note that the precise form of the exchange vectors that can be used to describe the compositional space is not unique, and can easily be modified as shown below.
In this figure the new axis NaA Mgvi Alvi-1vA-1 is equivalent to NaA Aliv Siiv-1vA-1 minus AlviAlivMgvi-1Siiv-1 (i.e., "edenite exchange"- "tschermak's exchange"). This figure also shows that the region of compositional space accessible by such exchange vectors is limited.

The amphibole group is divided into the orthoamphiboles
anthophyllite   Mg7Si8O22(OH)2
gedrite AlAlMg-1Si-1  
  FeMg-1  NaFe6Al3Si7O22(OH)2
and the clinoamphiboles, the main minerals of which are
tremolite   Ca2Mg5Si8O22(OH)2
actinolite FeMg-1 Ca2Fe5Si8O22(OH)2
cummingtonite MgCa-1 Mg7Si8O22(OH)2
grunerite FeMg-1 Fe7Si8O22(OH)2
hornblende AlAlMg-1Si-1 Ca2Mg4Al2Si7O22(OH)2
  NaAlSi-1 NaCa2Mg5Al3Si6O22(OH)2
  FeMg-1 NaCa2Fe5Al3Si6O22(OH)2
glaucophane NaAlMg-1Ca-1 Na2Mg3Al2Si8O22(OH)2