metamorphic
Banded Slate
Slate (Metamorphosed Siltstone/Mudstone)
Hardness: 2.5-4 Mohs; Color: Dark grey to black with distinct light-colored vertical mineral bands; Luster: Dull to sub-vitreous; Structure: Foliated; Cleavage: Excellent slaty cleavage; SG: 2.7-2.8
- Hardness
- 2
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 2.5-4 Mohs; Color: Dark grey to black with distinct light-colored vertical mineral bands; Luster: Dull to sub-vitreous; Structure: Foliated; Cleavage: Excellent slaty cleavage; SG: 2.7-2.8
Formation & geological history
Formed through the regional metamorphism of shale or mudstone under low pressure and temperature. The banding represents original sedimentary layers or mineral-filled fractures that survived the metamorphic process.
Uses & applications
Commonly used for roofing tiles, flooring, laboratory bench tops, billiard tables, and as high-quality decorative 'river rocks' for landscaping or gardens.
Geological facts
Slate is a low-grade metamorphic rock. The 'bands' or stripes often highlight the rock's sedimentary heritage, showing the original bedding planes that existed before the rock was subjected to heat and pressure.
Field identification & locations
Identify in the field by its ability to split into thin, flat sheets (slaty cleavage) and its fine-grained texture. It will not fizz with acid unless it contains calcite veins. Frequently found in mountainous regions or riverbeds downstream from metamorphic belts.
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