Hornblende

Hornblende
	Hornblende crystal (dark green) about 35 mm long, with apatite (white)
General
Category	Silicate mineral
Formula; (repeating unit)	Ca; 2(Mg,Fe,Al); 5(Al,Si); 8O; 22(OH); 2
Crystal system	Monoclinic
Space group	C2/m
Identification
Color	Black to dark green or brown
Crystal habit	Hexagonal/granular
Cleavage	Imperfect at 56° and 124°
Fracture	Uneven
Mohs scale hardness	5–6
Luster	Vitreous to dull
Streak	Pale gray, gray-white,[1][2] white, colorless[3]
Specific gravity	2.9
Pleochroism	Strong
References	[4]

Hornblende is a complex inosilicate series of minerals (ferrohornblende – magnesiohornblende).[5] 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 is (Ca,Na)
2-3(Mg,Fe,Al)
5(Al,Si)
8O
22(OH,F)
2.

Compositional variances

Some metals vary in their occurrence and magnitude:[6]

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 is named edenite from its locality in Edenville, Orange County, New York.

Other minerals in the hornblende series include:

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.[6]

References

http://www.mindat.org/min-1519.html Mindat Ferrohornblende
http://www.mindat.org/min-2524.html Mindat Magnesiohornblende
https://geology.com/minerals/hornblende.shtml
Phillips, M.W.; Draheim, J.E.; Popp, R.K.; Clowe, C.A.; Pinkerton, A.A. (1989). "Effects of oxidation-dehydrogenation in tschermakitic hornblende". American Mineralogist. 74: 764–773. Retrieved 30 December 2020.
http://www.mindat.org/min-8789.html Mindat
Handbook of Mineralogy

Additional reading

Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, pp. 416–17, ISBN 0-471-80580-7

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[1] ttp://www.mindat.org/min-1519.html Mindat Ferrohornblende

[2] ttp://www.mindat.org/min-2524.html Mindat Magnesiohornblende

[3] ttps://geology.com/minerals/hornblende.shtml

[4] Phillips, M.W.; Draheim, J.E.; Popp, R.K.; Clowe, C.A.; Pinkerton, A.A. (1989). "Effects of oxidation-dehydrogenation in tschermakitic hornblende". American Mineralogist. 74: 764–773. Retrieved 30 December 2020.

[5] ttp://www.mindat.org/min-8789.html Mindat

[HBM-6] Handbook of Mineralogy