Annite

General Information:
 Chemical Formula:
KFe++3AlSi3O10(OH,F)2
 Composition:
(Molecular Weight = 512.88 gm)
Potassium 7.62 % K 
Aluminum 5.26 % Al
Iron 32.67 % Fe
Silicon 16.43 % Si
Hydrogen 0.29 % H 
Oxygen 35.87 % O 
Fluorine 1.85 % F 
 Empirical Formula:
KFe2+3AlSi3O10(OH)1.5F0.5
Physical Properties:
 Cleavage:
[???] Perfect
 Color:
reddish brown or black.
 Density:
3.17
 Diaphaniety:
Transparent to Subopaque
 Hardness:
2.5-3 - Finger Nail-Calcite
 Luster:
Sub Metallic
 Streak:
brownish white
More details...

ANNITE