metamorphic
Metamorphic rock with large feldspar crystals (likely porphyroblastic gneiss)
Feldspar-porphyroblastic gneiss; likely quartz and feldspar with mica/amphibole, identification from image only
AI-generated identification · may be incorrectCoarse, irregular foliation with prominent blocky gray-green crystals in a tan-brown matrix. Feldspar is typically Mohs 6 with vitreous to dull luster and two cleavages near 90°; quartz and mica may also be present. Exact mineral composition cannot be confirmed visually.
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Physical properties
Coarse, irregular foliation with prominent blocky gray-green crystals in a tan-brown matrix. Feldspar is typically Mohs 6 with vitreous to dull luster and two cleavages near 90°; quartz and mica may also be present. Exact mineral composition cannot be confirmed visually.
Formation & geological history
Formed when a pre-existing rock underwent high-temperature, high-pressure metamorphism; large crystals grew during recrystallization. Its age and precise formation environment cannot be determined from the photograph.
Uses & applications
Mostly of collector interest; gneiss can be used as dimension stone, paving, aggregate, or landscaping material when sound and suitable.
Geological facts
The conspicuous large crystals may be feldspar porphyroblasts, which grew larger than the surrounding minerals during metamorphism.
Field identification & locations
Look for coarse mineral grains and compositional banding or foliation; feldspar commonly shows blocky shape and cleavage. The specimen’s weathered surface obscures diagnostic details, so a fresh surface or mineral tests would improve identification.
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