metamorphic
Pyritic Gneiss
Metamorphic Gneiss with Pyrite inclusions
Hardness: 6-7 (Mohs); Color: Mottled brown, gray, and brassy gold; Luster: Earthy to sub-metallic; Crystal structure: Foliated/granular; Cleavage: Poor; Specific gravity: 2.7-3.0.
- Hardness
- 6-7 (Mohs)
- Color
- Mottled brown, gray, and brassy gold
- Luster
- Earthy to sub-metallic
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Physical properties
Hardness: 6-7 (Mohs); Color: Mottled brown, gray, and brassy gold; Luster: Earthy to sub-metallic; Crystal structure: Foliated/granular; Cleavage: Poor; Specific gravity: 2.7-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense pressure and temperature. The iron sulfides likely formed in a reducing environment prior to or during metamorphism.
Uses & applications
Used primarily as crushed stone for road construction and landscaping. Specimens with visible pyrite are popular for educational purposes and amateur mineral collecting.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens dating back over 4 billion years. The presence of pyrite can lead to rapid weathering into iron oxides (limonite/rust).
Field identification & locations
Identify by distinct banding or foliation and the presence of small, metallic glints or rusty oxidation spots. Commonly found in metamorphic terrains such as the Appalachian or Blue Ridge Mountains.
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