metamorphic
Slate
Slate (Metamorphic Rock)
Hardness: 2.5 to 4 on the Mohs scale. Color: Dark gray to black (can also be green, purple, or brown). Luster: Dull to satiny/sheen due to microscopic mica flakes. Texture: Very fine-grained with distinct slatey cleavage that allows it to split into thin, flat sheets.
- Hardness
- 2
- Color
- Dark gray to black (can also be green, purple, or brown)
- Luster
- Dull to satiny/sheen due to microscopic mica flakes
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Physical properties
Hardness: 2.5 to 4 on the Mohs scale. Color: Dark gray to black (can also be green, purple, or brown). Luster: Dull to satiny/sheen due to microscopic mica flakes. Texture: Very fine-grained with distinct slatey cleavage that allows it to split into thin, flat sheets.
Formation & geological history
Formed through low-grade regional metamorphism of shale or mudstone under low temperature and pressure conditions, causing fine-grained clay minerals to recrystallize into micas aligned perpendicular to the direction of compression.
Uses & applications
Widely used for building materials, roofing tiles, flooring, billiard table tops, blackboards, and decorative stone landscaping.
Geological facts
Slate has a distinct ring when tapped lightly, unlike shale which sounds dull. Because of its flat splitting planes, slate was historically used as writing slates in classrooms before modern paper and whiteboards became common.
Field identification & locations
Identified by its fine grain, dark grey-black color, dull luster, and tendancy to break along smooth, flat parallel cleavage planes. Found abundantly in regions with ancient mountain-building events such as Wales, Spain, Brazil, and the Appalachian Mountains in North America.
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