Английская Википедия:Hornblende

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Hornblende is a complex inosilicate series of minerals. 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 minerals are common in igneous and metamorphic rocks.

The general formula is Шаблон:Chem2.

Physical properties

Hornblende has a hardness of 5–6, a specific gravity of 3.0 to 3.6, and is typically an opaque green, dark green, brown, or black color. It tends to form slender prismatic to bladed crystals, diamond-shaped in cross-section, or is present as irregular grains or fibrous masses.

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°.

Hornblende is an inosilicate (chain silicate) mineral, built around double chains of silica tetrahedra. These chains extend the length of the crystal and are bonded to their neighbors by additional metal ions to form the complete crystal structure.Шаблон:Sfn

Compositional variances

Hornblende is part of the calcium-amphibole group of amphibole minerals.Шаблон:Sfn It is highly variable in composition, and includes at least five solid solution series:

• Magnesiohornblende-ferrohornblende, Шаблон:Chem2
• Tschermakite-ferrotschermakite, Шаблон:Chem2
• Edenite-ferroedenite, Шаблон:Chem2
• Pargasite-ferropargasite, Шаблон:Chem2
• Magnesiohastingstite-hastingsite, Шаблон:Chem2

In addition, titanium, manganese, or chromium can substitute for some of the cations and oxygen, fluorine, or chlorine for some of the hydroxide (OH). The different chemical types are almost impossible to distinguish even by optical or X-ray methods, and detailed chemical analysis using an electron microprobe is required.Шаблон:Sfn

There is a solid solution series between hornblende and the closely related amphibole minerals, tremolite-actinolite, at elevated temperature. A miscibility gap exists at lower temperatures, and, as a result, hornblende often contains exsolution lamellae of grunerite.

Occurrence

Файл:Horndio.jpg

Hornblende diorite from the Henry Mountains, Utah, US

Hornblende is a common constituent of many igneous and metamorphic rocks such as granite, syenite, diorite, gabbro, basalt, andesite, gneiss, and schist. It crystallizes in preference to pyroxene minerals from cooler magma that is richer in silica and water.Шаблон:Sfn

It is the principal mineral of amphibolites, which form during medium- to high-grade metamorphism of mafic to intermediate igneous rock (igneous rocks with relative low silica content) in the presence of pore water. Much of the pore water comes from the breakdown of micas or other hydrous minerals. However, hornblende itself breaks down at very high temperatures.Шаблон:Sfn Hornblende alters easily to chlorite, biotite, or other mafic minerals.Шаблон:Sfn

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.

Oxyhornblende is a variety in which most of the iron has been oxidized to the ferric state, Шаблон:Chem2. Charge balance is preserved by the substitution of oxygen ions for hydroxide. Oxyhornblende is also typically enriched in titanium. It is found almost exclusively in volcanic rock and is sometimes called basaltic hornblende.Шаблон:Sfn

Etymology

The word hornblende is derived from German Шаблон:Lang ('horn') and Шаблон:Lang ('deceive'), in allusion to its similar appearance to metal-bearing ore minerals.

See also

• Шаблон:Annotated link