metamorphic

Mica-rich schist (likely biotite schist)

Biotite–quartz–feldspar schist; mica is chiefly biotite, K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂

AI-generated identification · may be incorrect

Strongly foliated, flaky texture with abundant dark bronze-to-black mica plates and scattered gray quartz/feldspar. Mica has Mohs hardness 2.5–3, perfect basal cleavage, vitreous to pearly luster; quartz is ~7 and feldspar ~6.

Identified September 14, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when shale or another clay-rich sediment was subjected to regional metamorphism, aligning mica into schistosity; commonly Paleozoic to Mesozoic in age, depending on the source terrane.

Uses & applications

Used mainly as a decorative or educational specimen; crushed schist may be used locally as fill or landscaping stone, but its pronounced foliation limits structural construction use.

Geological facts

The reflective flakes can resemble metallic ore, but the platy cleavage and flexible mica sheets are diagnostic. Rusty brown areas may represent weathered iron-bearing biotite or minor sulfides.

Field identification & locations

The image favors biotite schist rather than a single mineral. Around 38.7°N, 121.3°W, Sierra Nevada-derived metamorphic rocks and transported cobbles are plausible, though exact locality context is needed; test for flaky cleavage and a streak that is not metallic.