hornblende

n.

A green to black amphibolic mineral, CaNa(Mg,Fe)₄(Al,Fe,Ti)₃Si₆O₂₂(OH,F)₂, formed in the late stages of cooling in igneous rock.

[German : Horn, horn (from Middle High German, from Old High German) + Blende, blende; see blende.]

The name that was traditionally assigned to common calcic amphiboles of metamorphic and igneous rocks. However, a nomenclature scheme for amphiboles was introduced in 1997 in which the names now carry strict compositional restrictions. Magnesiohornblende (contains magnesium) and ferrohornblende (contains iron) are monoclinic amphiboles with end-member compositions Ca₂(Mg₄Al)(Si₇Al)O₂₂(OH)₂ and Ca₂(Fe²⁺₄Al)(Si₇Al)O₂₂(OH)₂, respectively (Ca = calcium, MG = magnesium, Al = aluminum, Si = silicon, O = oxygen, Fe = iron, OH = hydroxyl). Most natural compositions differ significantly from these ideal end members. Significant deviations from these compositions are denoted by the addition and replacement of prefixes and adjectival modifiers characteristic of the compositions involved. Thus fluorohornblende (contains fluorine, F) has the end-member composition Ca₂(Mg₄Al)(Si₇Al)-O₂₂F₂, in which all of the OH in hornblende has been replaced by F. When used to denote an amphibole of known chemical composition, the term hornblende is never used without a prefix or adjectival modifier. The unmodified term hornblende specifically refers to a calcic amphibole identified by physical or optical properties without characterization of the chemical composition.

Hornblende is a common rock-forming mineral in medium- and high-grade metamorphic rocks, particularly those of mafic and ultramafic composition. In mafic rocks, it first appears in the upper part of the low grade by a chemical reaction involving the disappearance of actinolite, a nonaluminous calcic amphibole. This change is extremely noticeable in thin sections, very pale-green actinolite giving way to blue-green hornblende. With prograde metamorphism, the composition of the hornblende gradually changes in a highly complex manner that is a function of temperature, pressure, oxygen fugacity (a measure of the activity of oxygen), and the chemical composition of the rock. This causes a gradual color change from blue-green through various shades of green to olive green and brown. At the middle of the high grade, hornblende becomes unstable and breaks down to form pyroxene (plus other minerals). The prominence of hornblende in medium-grade metabasic rocks has led to these rocks being called amphibolites. See also Pyroxene.

Hornblende is commonly found as a minor phase in a wide variety of igneous rocks. Magnesium-rich hornblendes do occur as primary phases in basic and ultrabasic rocks, but this is not common. Igneous amphiboles are most abundant in calcic-alkaline diorites, granodiorites, and granites, becoming more iron-rich with increasing acidity of the host rock. This compositional trend is also characterized by a progressive increase in the alkali content of the amphibole, and hornblende grades into hastingsite, riebeckite, and arfvedsonite in granitic rocks. See also Granite; Granodiorite; Igneous rocks.

Due to its complex structure and chemistry, hornblende contains much information on its formation. Its behavior is understood reasonably well, and hornblende is of considerable use in interpreting the geological history of the rocks in which it occurs. See also Amphibolite; Metamorphism.

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CaNa(Mg,Fe)
Monoclinic -- prismatic

Environment

Like tremolite and actinolite, a mineral of metamorphic and igneous rocks, often pseudomorphously replacing pyroxene (uralite).

Crystal description

Commonly crystallized, short to long prismatic, in calcite and/or quartz. Often several inches (1 dm) in length. Also solid schistose crystalline aggregates.

Physical properties

Rarely pale, even white or yellow as microcrystalline, occasionally green (edenite), bluish green (pargasite), to black. Luster glassy; hardness 5-6; specific gravity 3.0-3.4; fracture subconchoidal to uneven; cleavage prismatic. Microcrystalline form may be transparent to translucent on splinter edges, mostly opaque black.

Composition

With several interchangeable elements, the hornblendes have been subdivided into a whole series of aluminous amphiboles (some authorities even throw in nonaluminous tremolite and actinolite) with numerous ideal -- but possibly nonexistent -- end-members, difficult even for the professional to name. The collector can and should usually only make a good guess on identity and regard hornblende as almost a group name. (Alkaline calcium, sodium, magnesium, iron, aluminum silicate; with about 15% Al ₂ O ₃ , and 40% SiO ₂ ).

Tests

Fusible with some difficulty to a black glass. Gives water in a closed tube.

Distinguishing characteristics

Identified as an amphibole by its sharper cleavage angles of 56° and 124° in contrast to the squarer cross section of pyroxenes. The individuals are recognized by their color; any readily cleavable black amphibole is probably hornblende. Tourmaline lacks the cleavage and splintery fracture, though readily breaking unevenly.

Occurrence

Hornblende is a common dark igneous or plutonic rock constituent. It sometimes forms schistose masses known as amphibolite or hornblende schists that are made up of black, thin, parallel crystals. Amphibole also commonly forms by pseudomorphic alteration of pyroxenes in the late stages of the cooling of igneous rock, when they may be bathed in hot water. Fibrous amphibole grains are commonly seen in pyroxene-suggestive shapes in thin sections of rock. Large -- and stubby for hornblende -- crystals were found at Franklin, New Jersey, embedded in calcite. Similar large crystals are found in St. Lawrence Co., New York, and Renfrew Co., Ontario, where in carbonatite pegmatities they attain giant dimensions. Tiny yellow crystals may be seen in Vesuvius bombs and in Murcia, Spain, volcanics.

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IN BRIEF: A dark mineral in certain rocks.

The geologist was surprised to see hornblende in that rock because it was not naturally occurring in that area.

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