metamorphic
Slate
Foliated Metamorphic Rock (composed primarily of silica and alumina with iron and magnesium)
Hardness: 2.5-4 (Mohs); Color: Gray (most common), blue, purple, or green; Luster: Dull to satin; Crystal structure: Microcrystalline (not visible to naked eye); Cleavage: Perfect slaty cleavage; Specific gravity: 2.7–2.8
- Hardness
- 2
- Color
- Gray (most common), blue, purple, or green
- Luster
- Dull to satin
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Physical properties
Hardness: 2.5-4 (Mohs); Color: Gray (most common), blue, purple, or green; Luster: Dull to satin; Crystal structure: Microcrystalline (not visible to naked eye); Cleavage: Perfect slaty cleavage; Specific gravity: 2.7–2.8
Formation & geological history
Formed from the low-grade regional metamorphism of shale or mudstone under low heat and pressure conditions. This process causes clay minerals to realign into parallel planes, creating a foliated texture. It can range in age from the Precambrian to the Cenozoic era.
Uses & applications
Used extensively in the construction industry for roofing shingles, floor tiles, and billiard table tops due to its durability and water resistance. It is also used in landscaping and for traditional writing tablets (chalkboards).
Geological facts
Slate is famous for its 'slaty cleavage,' which allows it to be split into thin, flat sheets. This property is due to the microscopic alignment of chlorite and mica minerals within the rock. Famous deposits are found in Wales and the Appalachian Mountains.
Field identification & locations
Identify in the field by its distinct ability to split into thin, flat plates and its 'clink' sound when struck. It is commonly found in mountain belts and metamorphic terrains. Collectors should look for even thickness and clean breaks.
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