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.