Hornblende

Hornblende
Hornblende
	Hornblende crystal (dark green) about 35 mm long, with apatite (white)
General
Category	Silicate mineral
Formula; (repeating unit)	Ca2(Mg,Fe,Al)5(Al,Si)8O22(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)₈O₂₂(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 planes of cleavage intersect at 56° and 124° angles. Hornblende is most often confused with the pyroxene series and biotite mica, which are also dark minerals found in granite and charnockite. Pyroxenes differ in their cleavage planes, which intersect at 87° and 93°.[7]

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 German Horn ('horn') and blende ('deceive'), in allusion to its similar 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
"Hornblende Mineral | Uses and Properties".
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
Pough, Frederick H. (1976). A Field Guide To Rocks and Minerals (4 ed.). Boston: Houghton Mifflin. p. 249.

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] "Hornblende Mineral | Uses and Properties".

[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

[7] Pough, Frederick H. (1976). A Field Guide To Rocks and Minerals (4 ed.). Boston: Houghton Mifflin. p. 249.