metamorphic
Dark gray metamorphic rock, likely slate or phyllite
Slate/phyllite; chiefly fine-grained mica, chlorite, quartz, and clay-derived minerals
AI-generated identification · may be incorrectDark gray, dull to slightly silky luster, very fine grained, with irregular platy cleavage and weak foliation visible on broken surfaces. Hardness is approximately Mohs 3–5; specific gravity about 2.7–2.8.
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Physical properties
Dark gray, dull to slightly silky luster, very fine grained, with irregular platy cleavage and weak foliation visible on broken surfaces. Hardness is approximately Mohs 3–5; specific gravity about 2.7–2.8.
Formation & geological history
Formed when shale or mudstone was compressed and weakly metamorphosed during mountain-building, commonly at low grade. Its exact geological age cannot be determined from the photograph.
Uses & applications
Slate is used for roofing, paving, tiles, and writing boards; attractive pieces are collected as teaching or display specimens. Phyllite has limited construction use because its foliation can cause splitting.
Geological facts
The specimen appears weathered and may also be a fine-grained schist or dark mudstone; a hand specimen alone cannot provide a definitive identification. A fresh surface and tests for cleavage, streak, and mineral grains would distinguish these possibilities.
Field identification & locations
Common in regional metamorphic belts worldwide, including Wales, the Appalachians, New England, and the Alps. Look for planar foliation, splitting into sheets, and a gray-to-black color; a glass-scratch test helps estimate hardness.
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