metamorphic
Dark gray foliated metamorphic rock, likely slate or phyllite
Slate/phyllite; commonly quartz, muscovite/illite, chlorite, and graphite or carbonaceous matter
AI-generated identification · may be incorrectFine-grained, dark gray, silky to dull luster with visible planar foliation; slate has hardness ~3–4 and excellent cleavage, while phyllite is slightly softer and more lustrous. The blurred image prevents confident separation from schist.
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Physical properties
Fine-grained, dark gray, silky to dull luster with visible planar foliation; slate has hardness ~3–4 and excellent cleavage, while phyllite is slightly softer and more lustrous. The blurred image prevents confident separation from schist.
Formation & geological history
Forms when shale or mudstone is regionally metamorphosed under low-grade pressure and heat, typically during mountain building; age depends on the source terrain and may range from Paleozoic to Precambrian.
Uses & applications
Used for roofing, floor and wall tiles, paving, blackboards, landscaping, and architectural cladding; attractive phyllite may be collected as a specimen.
Geological facts
Foliation records directed tectonic pressure, and slate commonly splits into thin, flat sheets. Phyllite represents a higher metamorphic grade than slate.
Field identification & locations
Look for strong, closely spaced planar splitting and a fine texture; a silky sheen suggests phyllite, whereas very smooth dull cleavage suggests slate. Common worldwide in metamorphic belts, including Wales, Vermont, Spain, and Brazil.
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