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, any of a group of widely distributed rock-forming minerals, magnesium-iron silicates, often with traces of calcium, aluminum, sodium, titanium, and other elements.
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(1) The most common rock-forming mineral of the group of monoclinic amphiboles (common hornblende). Its approximate chemical composition is Ca2Na[Mg, Fe]4[Al,-Fe][(Si, Al)4O11]2[OH, F]2, with substantial variations in the ratios of basic components and with various element impurities (Na <- K, Ba, Rb; Ca ← TR, Y, Sr, U, W, Mo; Al ← Ga; Mg ← Li; Fe ← Zn, Sc). Hornblende crystallizes in the monoclinic system. It forms long, prismatic needles, irregular grains, and fibrous aggregates of green, black, or brown color. It has a hardness of 5.5–6 on Mohs’ scale and a density of 3,100–3,300 kg/m3. Hornblende is characterized by two directions of well-developed cleavages intersecting at an angle of 124°.
Common hornblende is characteristic of intrusive rocks (granodiorites, syenites, diorites, hornblende gabbros), effusive rocks (andesites, basalts, porphyrites), and metamorphic (am-phibolites, hornblende gneisses) rocks. Common hornblende is a relatively stable mineral, but when acted upon by hydrothermal solutions it alters to chlorite, epidote, calcite, and quartz. In the process of weathering, it decomposes to form opal, carbonates, and various other minerals.
(2) The subgroup of calcium aluminiferous amphiboles that includes common hornblende, edenite, oligoclase, pargasite, and hastingsite.
REFERENCESDeer, W. A., R. A. Howie, and J. Zussmann. Porodoobrazuiushchie mineraly, vol 2. Moscow, 1965. (Translated from English.)
Lodochnikov, V. N. Glavneishie porodoobrazuiushchie mineraly, 5th ed. Moscow, 1974.
A. S. MARFUNIN