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Magnesiohornblende
Chemical Formula	Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2
Species	Silicates
Crystal System	Monoclinic
Mohs Scale	5-6
Specific Gravity	2.9
Color	Black/dark green
Streak	Pale gray, gray-white[1][2]
Luster	Vitreous
Refractive Index	n = 1.616 - 1.680 n = 1.626 - 1.695 n = 1.636 - 1.700
Cleavage	Perfecton
Fracture	Uneven
Crystal Habit：Hexagonal/granular

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.

Hornblende is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an aluminium-iron-magnesium silicate, and an iron-magnesium silicate.

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 the minerals augite and biotite mica, both of 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

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.