metamorphic
Dark metamorphic rock, likely slate or phyllite
Foliated metapelite; commonly mica-rich quartz–clay mineral assemblage
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly silky luster, with visible foliation and slab-like fracture. Approximate hardness 3–5 Mohs; composition likely quartz, muscovite/illite, chlorite, and graphite or iron oxides; exact identification requires testing.
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Physical properties
Dark gray to black, fine-grained, dull to weakly silky luster, with visible foliation and slab-like fracture. Approximate hardness 3–5 Mohs; composition likely quartz, muscovite/illite, chlorite, and graphite or iron oxides; exact identification requires testing.
Formation & geological history
Formed when shale or mudstone underwent low-grade regional metamorphism during mountain building, typically from Paleozoic or older sedimentary protoliths. The layered texture reflects aligned sheet silicate minerals under directed pressure.
Uses & applications
Slate is used for roofing, flooring, tiles, billiard tables, and writing slabs; phyllite is mainly a decorative or teaching specimen. Common rough pieces generally have little commercial value.
Geological facts
Slate splits into thin, relatively flat sheets, whereas phyllite has a more pronounced silky sheen and wavy foliation. The specimen’s irregular, strongly crumpled surface may indicate phyllite or weathered schist rather than true slate.
Field identification & locations
Common in metamorphic belts worldwide, including Wales, the northeastern United States, Appalachia, and parts of Europe and Asia. Examine foliation, test for sheet splitting, and use a streak, hardness, and acid test; photo-based identification is provisional.
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