metamorphic
Foliated slate or phyllite
Fine-grained metamorphosed mudstone; typically quartz, mica, chlorite, and clay-derived minerals
AI-generated identification · may be incorrectGray-green, fine-grained rock with pronounced planar layering and weak silky sheen, consistent with slaty cleavage or phyllitic foliation. Hardness is approximately 3–5.5 Mohs; specific gravity about 2.7–2.8; cleavage/parting allows it to split into thin slabs.
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Physical properties
Gray-green, fine-grained rock with pronounced planar layering and weak silky sheen, consistent with slaty cleavage or phyllitic foliation. Hardness is approximately 3–5.5 Mohs; specific gravity about 2.7–2.8; cleavage/parting allows it to split into thin slabs.
Formation & geological history
Formed when shale or mudstone was subjected to low-grade regional metamorphism during mountain building, commonly hundreds of millions of years ago depending on the source terrane. Increasing metamorphic grade can transition slate into phyllite and then schist.
Uses & applications
Used for roofing, flooring, paving, landscaping, and decorative stone when it splits cleanly; rough hand specimens have modest collecting value.
Geological facts
The layered appearance reflects aligned platy minerals rather than ordinary sedimentary bedding alone. A dull surface suggests slate, while a noticeable silky mica sheen favors phyllite.
Field identification & locations
Common in metamorphic belts worldwide, including Wales, Vermont, the Appalachian Mountains, and the Alps. Inspect for persistent cleavage, fine grain, and mica sheen; a streak or hardness test can help distinguish it from shale.
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