metamorphic
Pyritic Gneiss (with Garnet)
Gneiss containing Pyrite (FeS2) and Almandine Garnet (Fe3Al2(SiO4)3)
Hardness: 6-7.5 (garnet/silicates), 6-6.5 (pyrite). Color: Brassy yellow flakes, grey-white matrix, reddish garnets. Luster: Metallic to vitreous. Structure: Foliated with granular crystals. Cleavage: Indistinct.
- Hardness
- 6-7
- Color
- Brassy yellow flakes, grey-white matrix, reddish garnets
- Luster
- Metallic to vitreous
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Physical properties
Hardness: 6-7.5 (garnet/silicates), 6-6.5 (pyrite). Color: Brassy yellow flakes, grey-white matrix, reddish garnets. Luster: Metallic to vitreous. Structure: Foliated with granular crystals. Cleavage: Indistinct.
Formation & geological history
Formed through high-grade regional metamorphism of clay-rich sedimentary rocks or felsic igneous rocks under high pressure and temperature. The pyrite indicates a sulfur-rich environment during formation.
Uses & applications
Primarily used as a collector specimen or for geological study; the host rock (gneiss) is used in construction and landscaping.
Geological facts
This specimen shows 'Fools Gold' (pyrite) embedded in a metamorphic matrix. The presence of garnets suggests the rock reached significant depths within the Earth's crust during its formation.
Field identification & locations
Identify by looking for distinct bands or metamorphic foliation, the heavy weight of pyrite, and the presence of small, red vitreous garnet crystals. Commonly found in Precambrian shield areas.
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