Minerals composed of double chain SiO4 tetrahedra linked at the vertices. The distance between sheets is defined
as 
. Amphiboles have the general formula
or
where A may exist as a vacancy. Like pyroxenes, they may be either orthorhombic or monoclinic
("calcic"). Also like pyroxenes, they may be represented by I-beams, although
amphibole I-beams are "fat" because they represent two chains instead of one. The A site is the
space between stacked I-beams which, because of its large size, may accommodate cations. A diagnostic
property for differentiating amphiboles from pyroxenes is that amphiboles cleave at 56°, while
pyroxenes cleave at 93°.
The following table gives common amphiboles, where M indicates a monoclinic mineral and O an orthoclinic.
| Amphibole |
M/O |
Formula |
| actinolite |
M |
Ca2(Mg,Fe)5Si8O22(OH)2 |
| edenite |
M |
NaCa2(Mg)5AlSi7O22(OH)2 |
| hornblende |
M |
(Ca,Na2,K2) (M+2,
M+3)5Si6(Al,Si)2O22(OH,F)2 |
| glaucophane |
M |
Na2(Mg3Al2)Si8O22(OH)2 |
| riebeckite |
M |
Na2(Fe Fe )Si8O22(OH)2 |
| tremolite |
M |
Ca2(Mg)5Si8O22(OH)2 |
| anthophyllite |
O |
Mg2(Mg)5Si8O22(OH)2 |
| grunerite |
O |
Fe (FeFe )Si8O22(OH)2 |
