metamorphic
Slate
Slate; predominantly fine-grained phyllosilicates (muscovite/illite, chlorite) with quartz and feldspar
AI-generated identification · may be incorrectDark gray, very fine-grained, dull to slightly silky luster, low-grade metamorphic foliation, and excellent slaty cleavage causing thin, planar splitting; hardness about 3–4 Mohs, specific gravity roughly 2.7–2.8.
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Physical properties
Dark gray, very fine-grained, dull to slightly silky luster, low-grade metamorphic foliation, and excellent slaty cleavage causing thin, planar splitting; hardness about 3–4 Mohs, specific gravity roughly 2.7–2.8.
Formation & geological history
Formed when shale or mudstone underwent low-grade regional metamorphism, typically during mountain building; its exact geological age cannot be determined from the photograph and may range from Precambrian to recent tectonic belts.
Uses & applications
Used for roofing, floor and wall tiles, paving, billiard tables, chalkboards, and landscaping. Attractive foliated pieces are also sold as cabinet or teaching specimens, though this example appears common-grade.
Geological facts
Slate is the lowest-grade metamorphic equivalent of shale and commonly preserves faint sedimentary layering or fossils from its precursor. Its cleavage is produced by the alignment of microscopic mica and clay minerals.
Field identification & locations
Likely slate based on its dark color, fine texture, and planar cleavage, but dark shale or phyllite are possible without testing. Common sources include Wales, Spain, Portugal, Brazil, India, and the eastern United States; examine cleavage, streak, and texture with a hand lens.
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