metamorphic
Almandine Garnet in Schist Matrix
Almandine - Fe3Al2(SiO4)3
Hardness: 6.5-7.5 (Mohs scale); Color: Deep reddish-brown to dark red crystals in a silvery-grey matrix; Luster: Vitreous to resinous (crystals), Silky or pearly (matrix); Crystal structure: Isometric (dodecahedral or trapezohedral); Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Luster
- Vitreous to resinous (crystals), Silky or pearly (matrix)
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Physical properties
Hardness: 6.5-7.5 (Mohs scale); Color: Deep reddish-brown to dark red crystals in a silvery-grey matrix; Luster: Vitreous to resinous (crystals), Silky or pearly (matrix); Crystal structure: Isometric (dodecahedral or trapezohedral); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed during regional metamorphism of clay-rich sedimentary rocks (shales or mudstones) under high pressure and temperature conditions. Typically found in mica schists and gneisses dating from the Proterozoic to Phanerozoic eras.
Uses & applications
Abrasives (industrial waterjet cutting and sandpaper), gemstones when of high clarity, and highly valued by mineral collectors for the contrast between the dark crystals and silvery matrix.
Geological facts
Garnets are known as 'indicator minerals' used by geologists to determine the temperature and pressure history of the rock's formation. Almandine is the most common member of the garnet group.
Field identification & locations
Identify in the field by looking for distinct, hard, rounded or multi-faceted dark red 'balls' embedded in flaky, micaceous rock. Common in Appalachian Mountains (USA), the Alps, and Scandinavia. Use a hand lens to check for dodecahedral crystal shapes.
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