Hornblende Article

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Hornblende
Apatite-(CaOH)-Hornblende-38354.jpg
Hornblende crystal (dark green) about 35 mm long, with apatite (white)
General
CategorySilicate mineral
Formula
(repeating unit)
Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2
Crystal system Monoclinic unknown space group
Identification
ColorBlack to dark green or brown
Crystal habitHexagonal/granular
CleavageImperfect at 56° and 124°
FractureUneven
Mohs scale hardness5–6
LusterVitreous to dull
StreakPale gray, gray-white, [1] [2] white, colorless [3]
Specific gravity2.9
PleochroismStrong

Hornblende is a complex inosilicate series of minerals (ferrohornblende – magnesiohornblende). [4] 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:

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:

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. [5]

See also

References

  • 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
  • Scandinavian mineral gallery retrieved 06/21/05