metamorphic
Almandine Garnet in Schist
Almandine, Fe3Al2(SiO4)3
Hardness: 6.5-7.5 (garnets) vs 3-4 (matrix). Color: deep reddish-brown to black crystals in a silvery-grey micaceous matrix. Luster: Vitreous to resinous crystals; pearly or metallic matrix. Crystal Structure: Isometric (dodecahedron). Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Luster
- Vitreous to resinous crystals
Identified More metamorphic →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6.5-7.5 (garnets) vs 3-4 (matrix). Color: deep reddish-brown to black crystals in a silvery-grey micaceous matrix. Luster: Vitreous to resinous crystals; pearly or metallic matrix. Crystal Structure: Isometric (dodecahedron). Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through regional metamorphism of clay-rich sedimentary rocks (shales) under high pressure and temperature. The garnets grow as porphyroblasts within the schist during the metamorphic process. Common in Phanerozoic orogenic belts.
Uses & applications
Industrial abrasives (sandblasting, waterjet cutting), gemstones (though usually only transparent varieties), and as an index mineral for geologists to determine metamorphic grade.
Geological facts
Garnets are one of the few minerals that grow in a perfectly geometric shape deep underground. The presence of garnet indicates the rock was subjected to 'amphibolite facies' metamorphic conditions.
Field identification & locations
Identify in the field by looking for distinct, hard, rounded 'balls' or dodecahedral crystals protruding from flaky, shiny, mica-rich layers. Frequently found in the Appalachian Mountains, Alps, and Himalayas. Collectors should look for well-defined crystal faces on the garnets.
More like this
