igneous
Pegmatitic mica–feldspar rock
Granitic pegmatite, composed chiefly of potassium feldspar, quartz, and biotite mica
AI-generated identification · may be incorrectCoarse, interlocking crystals; creamy yellow feldspar with black, platy biotite and possible quartz. Feldspar hardness ~6, mica ~2.5–3; vitreous to pearly luster, distinct feldspar cleavage, and blocky crystal structure; apparent yellowing may reflect alteration or staining.
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Physical properties
Coarse, interlocking crystals; creamy yellow feldspar with black, platy biotite and possible quartz. Feldspar hardness ~6, mica ~2.5–3; vitreous to pearly luster, distinct feldspar cleavage, and blocky crystal structure; apparent yellowing may reflect alteration or staining.
Formation & geological history
Formed when water-rich granitic magma cooled slowly in a late-stage pegmatite vein, allowing unusually large crystals to grow, generally during Precambrian–Mesozoic intrusive events depending on locality.
Uses & applications
Used mainly as a teaching or collector specimen; pegmatites can be sources of feldspar, mica, quartz, lithium, beryllium, and rare gemstones, but this small piece has little industrial value.
Geological facts
The strong contrast between pale feldspar and black biotite is typical of granite-related pegmatites. Yellow feldspar can be altered to clay minerals or iron-stained rather than a distinct gemstone.
Field identification & locations
Look for very coarse crystals, feldspar’s two cleavage directions, mica’s tendency to split into flexible sheets, and quartz’s lack of cleavage. Common in exposed granitic terrains worldwide, including Maine, California, Brazil, and Madagascar.
