metamorphic
Foliated amphibolite (probable hornblende schist)
Amphibolite; chiefly hornblende + plagioclase, with minor biotite/quartz
AI-generated identification · may be incorrectDark gray, coarse-grained rock with conspicuous pale mineral patches and weak foliation; crystals have a subhedral granular structure, hornblende shows vitreous luster and hardness about 5–6, while plagioclase is about 6–6.5.…
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Physical properties
Dark gray, coarse-grained rock with conspicuous pale mineral patches and weak foliation; crystals have a subhedral granular structure, hornblende shows vitreous luster and hardness about 5–6, while plagioclase is about 6–6.5. Cleavage is generally poor to moderate overall, though amphibole crystals have two prominent cleavages.
Formation & geological history
Formed when basaltic or gabbroic protoliths underwent medium- to high-grade regional metamorphism, commonly during mountain building. It may range from Precambrian to Phanerozoic in age; the photograph alone cannot establish age.
Uses & applications
Used as crushed stone, road aggregate, riprap, and occasionally dimension stone; attractive polished pieces may be collected, but it is rarely a jewelry material.
Geological facts
The white spots are likely plagioclase-rich domains or quartz, contrasting with dark amphibole-rich bands. Amphibolite commonly preserves evidence of ancient oceanic crust or mafic volcanic rocks transformed under amphibolite-facies conditions.
Field identification & locations
Look for dark green-black amphibole, white plagioclase, toughness, and foliation; a streak, hardness test, and thin-section examination would confirm it. Common in metamorphic belts worldwide, including the Appalachians, Scandinavia, and Precambrian shields.
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