Chemistry:Hornblende

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Short description: Complex inosilicate series of minerals
Hornblende
Apatite-(CaOH)-Hornblende-38354.jpg
Hornblende crystal (dark green) about 35 mm long, with apatite (white)
General
CategorySilicate mineral
Formula
(repeating unit)
Ca
2
(Mg,Fe,Al)
5
(Al,Si)
8
O
22
(OH)
2
Crystal systemMonoclinic
Space groupC2/m
Identification
ColorBlack to dark green or brown
Crystal habitHexagonal/granular
CleavageImperfect at 56° and 124°
FractureUneven
Mohs scale hardness5–6
|re|er}}Vitreous to dull
StreakPale gray, gray-white,[1][2] white, colorless[3]
Specific gravity2.9
PleochroismStrong
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)
8
O
22
(OH,F)
2
.

Compositional variances

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

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]

See also

References

  1. http://www.mindat.org/min-1519.html Mindat Ferrohornblende
  2. http://www.mindat.org/min-2524.html Mindat Magnesiohornblende
  3. "Hornblende Mineral | Uses and Properties". https://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. https://pubs.geoscienceworld.org/msa/ammin/article-abstract/74/7-8/764/42278/Effects-of-oxidation-dehydrogenation-in?redirectedFrom=fulltext. Retrieved 30 December 2020. 
  5. http://www.mindat.org/min-8789.html Mindat
  6. 6.0 6.1 Handbook of Mineralogy
  7. 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

de:Amphibolgruppe#Calcium-Amphibole