igneous
Biotite-rich granite or granodiorite fragment
Coarse-grained felsic intrusive rock; mainly quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectHardness about 6–7 overall (biotite 2.5–3); brown-black, gray, and tan granular appearance with vitreous quartz/feldspar and shiny platy mica. Interlocking crystalline texture; cleavage is prominent in biotite but absent in quartz.
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Physical properties
Hardness about 6–7 overall (biotite 2.5–3); brown-black, gray, and tan granular appearance with vitreous quartz/feldspar and shiny platy mica. Interlocking crystalline texture; cleavage is prominent in biotite but absent in quartz.
Formation & geological history
Formed by slow crystallization of silica-rich magma deep within continental crust, typically during Precambrian to Phanerozoic plutonic activity. The exact age cannot be determined from the photograph.
Uses & applications
Used as dimension stone, countertops, paving, aggregate, and decorative stone; small fragments are common geological specimens. Collector value is generally low unless unusually attractive or locality-specific.
Geological facts
The visible dark flakes are likely biotite, while lighter grains are feldspar and quartz. Brown coloration may reflect weathering or iron-oxide staining.
Field identification & locations
Identify it by its coarse interlocking grains, abundant light quartz/feldspar, and reflective dark mica flakes; it should not fizz with dilute hydrochloric acid. Common worldwide in continental mountain belts and exposed plutons.
