metamorphic
Gneiss with Iron Staining and Sulfide Mineralization
Metamorphic Gneiss / Amphibolite with Iron/Sulfide Oxidation
Hardness: 6-7 on Mohs scale. Color: Grayish-black matrix with reddish-brown rust staining, yellowish-green lichen/epidote, and metallic iridescent undertones. Luster: Sub-metallic to dull, vitreous on quartz/feldspar grains. Cleavage: Poorly developed along foliation planes.…
- Hardness
- 6-7 on Mohs scale
- Luster
- Sub-metallic to dull, vitreous on quartz/feldspar grains
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Grayish-black matrix with reddish-brown rust staining, yellowish-green lichen/epidote, and metallic iridescent undertones. Luster: Sub-metallic to dull, vitreous on quartz/feldspar grains. Cleavage: Poorly developed along foliation planes. Texture: Medium- to coarse-grained metamorphic foliation with a weathered alteration crust.
Formation & geological history
Formed under high temperature and high pressure regional metamorphism deep within the Earth's crust, often derived from granitic or sedimentary protoliths. The rusty surface features indicate subsequent chemical weathering and oxidation of iron-bearing minerals like pyrite or biotite.
Uses & applications
Metamorphic rocks like gneiss are commonly crushed for construction aggregate, used as dimension stone for building facades, landscaping, and studied by geologists to map regional tectonic history.
Geological facts
The dark reddish-brown weathering rind is known as a limonite/gossan stain, caused by the atmospheric breakdown of iron-rich minerals. Specimens showing iridescent oxides or sulfide traces are often collected as field samples by prospectors as indicator stones.
Field identification & locations
Field identification is key via its banded or crystalline metamorphic texture, dense weight, and distinctive reddish-brown iron rust rinds. Commonly found in exposed Precambrian shield regions, mountain belts, and glacial erratics worldwide. Collector value for raw field specimens is typically minimal.
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