metamorphic
Gray slate
Slate; fine-grained foliated metamorphic rock composed mainly of mica and quartz
AI-generated identification · may be incorrectTypically dark gray to blue-gray, dull to slightly silky, with excellent slaty cleavage and thin planar splitting; hardness about 3–4 Mohs, fine-grained and low porosity. The specimen appears weathered and may include minor pyrite or other accessory minerals.
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Physical properties
Typically dark gray to blue-gray, dull to slightly silky, with excellent slaty cleavage and thin planar splitting; hardness about 3–4 Mohs, fine-grained and low porosity. The specimen appears weathered and may include minor pyrite or other accessory minerals.
Formation & geological history
Forms when shale or mudstone undergoes low-grade regional metamorphism, generally during mountain-building; its exact geological age depends on the source formation, commonly Paleozoic or older. Compression recrystallizes clay minerals into aligned mica, producing cleavage.
Uses & applications
Used for roofing, floor and wall tiles, paving, landscaping, writing tablets, and decorative slabs; attractive specimens have modest collecting value.
Geological facts
Slate is distinguished from shale by its metamorphic cleavage: it splits along newly formed, usually planar surfaces rather than merely bedding planes. It can preserve fossils or original sedimentary layering in low-grade examples.
Field identification & locations
Common in metamorphic belts worldwide, including Wales, Vermont, Pennsylvania, Spain, and Brazil. Identify it by its fine texture, planar splitting, gray color, and inability to scratch glass; value is typically $2–$15 for a common hand specimen, or $20–$75 for an attractive labeled specimen.
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