hornblende

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hornblende:

see amphiboleamphibole
, 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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Hornblende

 

(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.

REFERENCES

Deer, 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

hornblende

[′hȯrn‚blend]
(mineralogy)
A general name given to the monoclinic calcium amphiboles that form an extensive solid-solution series between the various metals in the generalized formula (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2.

hornblende

a black or greenish-black mineral of the amphibole group, found in igneous and metamorphic rocks. Composition: calcium magnesium iron sodium aluminium aluminosilicate. General formula: (Ca,Na)2.3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2
References in periodicals archive ?
This deformation phase is defined by actinolite, chlorite and muscovite in the greenschists (Figures 3a y 3b), by hornblende in the hornblende schists (Figure 3c) and the amphibolites (Figure 3e), by chlorite, muscovite and graphite in quartz-muscovite-chlorite schists or mica schists (Figures 3h and 30, and by hornblende and anthophyllite in the anthophyllite schists (Figure 3j).
Microsopically, this deformation phase also was recorded in the hornblende schists.
Thereby, in the Arquia Group type section, the greenschists change in a transitional form to hornblende schists westward, and the amphibolites appear further to the west.
This is supportd by the mineralogy of greenschist facies that is found among the mineral assemblages of the amphibolite and epidote-amphibolite facies, such as chlorite + actinolite + epidote minerals + quartz (mineralogy of greenschist facies), between hornblende + epidote minerals + quartz + albite [+ or -] garnet (mineralogy of epidote-amphibolite facies) in the hornblende schists (Figures 4d), and the chlorite + muscovite + epidote minerals + plagioclase (albite) + quartz (mineralogy of greenschist facies) between the hornblende + garnet + plagioclase [+ or -] quartz (amphibolite facies) in the garnet amphibolites (Figure 3f).
This assemblage was observed in hornblende schists and garnet amphibolites with the Grt+Hbl+Ep+Pl+Qtz association (sample Arq54; Figure 1; Table 2), whereas the amphibole is represented in the Arquia Group rocks by hornblende.