metamorphic
Biotite-rich gneiss (probable)
Foliated quartz–feldspar–biotite gneiss; composition commonly includes quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectHardness about 6–7 overall; brown, gray, and rusty colors with a granular to subtly banded texture and vitreous-to-pearly luster. Quartz and feldspar form interlocking crystals, while biotite creates dark platy layers; cleavage is strong in mica but poor in the rock overall.
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Physical properties
Hardness about 6–7 overall; brown, gray, and rusty colors with a granular to subtly banded texture and vitreous-to-pearly luster. Quartz and feldspar form interlocking crystals, while biotite creates dark platy layers; cleavage is strong in mica but poor in the rock overall.
Formation & geological history
Formed when granite or related sedimentary protoliths underwent intense heat and pressure, producing recrystallization and foliation, commonly during Precambrian mountain-building events. The specimen may also show surface iron-oxide weathering.
Uses & applications
Used locally as dimension stone, facing stone, paving, and aggregate; attractive polished pieces are collected, though this example appears mainly of educational or decorative value.
Geological facts
Gneiss commonly preserves alternating mineral bands and can resemble granite, but its minerals are commonly aligned or segregated into layers.
Field identification & locations
Look for visible quartz/feldspar grains and wavy dark mica-rich banding; a glassy streak suggests quartz, while mica flakes split into thin sheets. Common in ancient continental shields and metamorphic mountain belts worldwide.
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