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Hornblende

Amphibole Hornblende
General
Category Igneous, metamorphic
Chemical formula Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2
Identification
Color black/dark green
Crystal habit hexagonal/granular
Crystal system monoclinic
Cleavage imperfect at 56 and 124 degrees
Fracture uneven
Mohs scale hardness 5-6
Luster vitreous to dull
Streak brown-grey
Specific gravity 2.9
Pleochroism strong

Hornblende is a complex inosilicate series of minerals (ferrohornblende - magnesiohornblende).[1] Hornblende 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. It is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an aluminium-iron-magnesium silicate, and an iron-magnesium silicate. Manganese, titanium, and sodium are sometimes present. Fluorine often substitutes for the hydroxyl in the structure. The general formula can be given as (Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O 22(OH,F)2. 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.

Contents

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 variety of hornblende that contains less than 5% of iron oxides is gray to white in color and named edenite, from its locality in Edenville, New York. Other minerals in the hornblende series include: pargasite, hastingsite and tschermakite.

Etymology

The word hornblende is derived from the German horn and blenden, to 'blind' or 'dazzle'. The term blende is often used to refer to a brilliant non-metallic luster, for example, zincblende and pitchblende, a lustrous form of uraninite.

See also

References

  • Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, p 416-7, ISBN 0-471-80580-7
  • Mindat retrieved 06/21/05
  • Scandinavian mineral gallery retrieved 06/21/05
  • Table Info retrieved 2/27/07
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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

HORNBLENDE, an important member of the amphibole group of rock-forming minerals. The name is an old one of German origin, and was used for any dark-coloured prismatic cr y stals from which metals could not be extracted. It is now applied to the dark-coloured aluminous members of the monoclinic amphiboles, occupying in this group the same position that augite occupies in the pyroxene group. The monoclinic crystals are prismatic in habit with a six-sided cross-section; the angle between the prismfaces (M), parallel to which there are perfect cleavages, is 55° 49' The colour (green, brown or black) and the specific gravity (3.0-3.3) vary with the amount of iron present. The pleochroism is always strong, and the angle of optical extinction on the plane of symmetry (x in the figure) varies from o° to 37°. The chemical composition is expressed by mixtures in varying proportions of the molecules Ca(Mg,Fe)3(S103)4, (Mg,Fe) (AI,Fe) 2 SiO 6 and NaAI(S103)2 Numerous varieties have been distinguished by special names: edenite, from Edenville in New York, is a pale-coloured aluminous amphibole containing little iron; pargasite, from Pargas near Abo in Finland, a green or bluish-green variety; common hornblende includes the greenish-black and black kinds containing more iron. The dark.-coloured porphyritic crystals of basalts are known as basaltic hornblende.

Hornblende occurs as an essential constituent of many kinds 1 Buffon, as was his manner, enlarges on the cruel injustice done to these birds by Nature in encumbering them with this deformity, which he declares must hinder .them from getting their food with ease. The only corroboration his perverted view receives is afforded by the observed fact that hornbills, in captivity at any rate, never have any fat about them.

2 In The Malay Archipelago (i. 213), Wallace describes a nestling hornbill (B. bicornis) which he obtained as "a most curious object, as large as a pigeon, but without a particle of plumage on any part of it. It was exceedingly plump and soft, and with a semi-transparent skin, so that it looked more like a bag of jelly, with head and feet stuck on, than like a real bird." of igneous rocks, such as hornblende-granite, syenite, diorite, hornblende-andesite, basalt, &c.; and in many crystalline schists, for example, amphibolite and hornblende-schist which are composed almost entirely of this mineral. Well-crystallized specimens are met with at many localities, for example: brilliant black crystals (syntagmatite) with augite and mica in the sanidine bombs of Monte Somma, Vesuvius; large crystals at Arendal in Norway, and at several places in the state of New York; isolated crystals from the basalts of Bohemia. (L. J. S.)


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Simple English

[[File:|thumb|right|220px|Amphibole hornblende]]

Hornblende is a complex series of silicate minerals.[1]

Hornblende 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.

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

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. It is most often confused with the minerals augite and biotite mica, both of which are black and can be found in granite.

Contents

Occurrence

Hornblende is a common constituent of many igneous and metamorphic rocks such as granite, diorite, gabbro, basalt, andesite, gneiss, and schist. It is the principal mineral of amphibolites.

Very dark brown to black hornblendes that contain titanium may be called basaltic hornblende, from the fact that they are usually a constituent of basalt and related rocks.

Etymology

The word hornblende is derived from the German horn and blenden, to 'blind' or 'dazzle'. The term blende is often used to refer to a brilliant non-metallic luster, for example, zincblende and pitchblende, the ore of uranium.

See also

References


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