metamorphic
Almandine Garnet in Matrix
Almandine Garnet (Fe3Al2(SiO4)3)
Hardness: 7-7.5 (garnet) 5-6 (matrix). Color: Dark reddish-brown to black garnet in a grey or tan schist/micaceous matrix. Luster: Vitreous to resinous. Crystal structure: Isometric (dodecahedral). Cleavage: None, sub-conchoidal fracture.
- Hardness
- 7-7
- Luster
- Vitreous to resinous
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Physical properties
Hardness: 7-7.5 (garnet) 5-6 (matrix). Color: Dark reddish-brown to black garnet in a grey or tan schist/micaceous matrix. Luster: Vitreous to resinous. Crystal structure: Isometric (dodecahedral). Cleavage: None, sub-conchoidal fracture.
Formation & geological history
Formed through regional metamorphism of clay-rich sedimentary rocks (pelites) such as shale. Under high heat and pressure, aluminum-rich minerals recrystallize into garnet crystals. They are commonly found in schists and gneisses.
Uses & applications
Abrasives (industrial garnet sand), gemstone for jewelry (if transparent), and as a common index mineral for geological study. This specific specimen is of collector/educational value.
Geological facts
Garnets are known as 'index minerals' because geologists use them to determine the temperature and pressure conditions the rock experienced during metamorphism. Almandine is the most common member of the garnet group.
Field identification & locations
Identify in the field by looking for '12-sided' equant crystals (dodecahedrons) protruding from foliated metamorphic rocks. Found globally, with notable deposits in New York (Gore Mountain), India, and Brazil.
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