Two hornblende samples
|Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2|
Unknown space group
|Cleavage||Imperfect at 56 and 124 degrees|
|Mohs scale hardness||5–6|
|Luster||Vitreous to dull|
|Streak||Pale gray, gray-white|
Hornblende is a complex inosilicate series of minerals (ferrohornblende – magnesiohornblende). It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole.
The general formula can be given as (Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2.
Some metals vary in their occurrence and magnitude:
- Manganese and titanium are often present.
- Sodium and potassium are often present and fluorine often substitutes for the hydroxyl in the crystalline structure.
It is the principal mineral of amphibolites. Very dark brown to black hornblendes that contain titanium are ordinarily called basaltic hornblende, from the fact that they are usually a constituent of basalt and related rocks. Hornblende alters easily to chlorite and epidote.
Other minerals in the hornblende series include: