Hornblende

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.

Compositional variances 
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.

Physical properties 
Hornblende has a hardness of 5–6, a specific gravity of 2.9–3.4 and is typically an opaque green, greenish-brown, brown or black color.

Its cleavage angles are at 56 and 124 degrees. It is most often confused with various pyroxene minerals and biotite mica, which are black and can be found in granite and in charnockite.

Occurrence
Hornblende is a common constituent of many igneous and metamorphic rocks such as granite, syenite, diorite, gabbro, basalt, andesite, gneiss, and schist.

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.

A rare variety of hornblende contains less than 5% of iron oxide, is gray to white in color, and named edenite, from its locality in Edenville, Orange County, New York.

Other minerals in the hornblende series include:
 pargasite
 hastingsite
 tschermakite
 edenite

Etymology
The word hornblende is derived from the German horn and blenden, to 'deceive' in allusion to its similarity in appearance to metal-bearing ore minerals.

See also

References

 Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, pp. 416–17, 
 Scandinavian mineral gallery retrieved 06/21/05
Category:Inosilicates
Category:Calcium minerals
Category:Iron minerals
Category:Magnesium minerals
Category:Aluminium minerals
Category:Monoclinic minerals

de:Amphibolgruppe#Calcium-Amphibole