metamorphic
Banded amphibolite (hornblende schist)
Amphibolite, chiefly hornblende + plagioclase feldspar, commonly with biotite and quartz
AI-generated identification · may be incorrectTypically dark green-black to gray, with a coarse, weakly foliated or banded texture and vitreous to dull luster; Mohs hardness about 5–6, no prominent cleavage as a rock, and specific gravity roughly 2.9–3.3.
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Physical properties
Typically dark green-black to gray, with a coarse, weakly foliated or banded texture and vitreous to dull luster; Mohs hardness about 5–6, no prominent cleavage as a rock, and specific gravity roughly 2.9–3.3.
Formation & geological history
Formed when basaltic or gabbroic protoliths undergo medium- to high-grade regional metamorphism, commonly during mountain building; ages range from Precambrian to recent tectonic belts.
Uses & applications
Used as crushed aggregate, road stone, riprap, and occasionally dimension stone; attractive foliated pieces are collected but are rarely used as gemstones.
Geological facts
Its dark amphibole-rich bands and lighter feldspar-rich streaks reflect metamorphic mineral segregation and deformation.
Field identification & locations
Identify it by its heavy, dark, crystalline appearance, elongated amphibole grains, and planar banding; common in exposed metamorphic terrains worldwide, including the Appalachians, Canadian Shield, and Precambrian shields.
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