metamorphic

Biotite-rich schist (mica schist)

Mica schist, chiefly biotite (K(Mg,Fe)₃AlSi₃O₁₀(OH,F)₂), with possible quartz and feldspar

AI-generated identification · may be incorrect

Dark brown-black, strongly glittering, foliated surface with visible platy mica grains. Biotite has Mohs hardness about 2.5–3 and perfect basal cleavage; the rock’s hardness varies with its other minerals. Specific gravity is typically around 2.7–3.0.

Identified September 26, 2026More metamorphic →

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Physical properties

Formation & geological history

Forms when clay-rich sedimentary or igneous protoliths undergo regional metamorphism, aligning mica into schistosity. The image alone cannot establish its age; schists occur in rocks spanning much of geological time.

Uses & applications

Mostly a collector or teaching specimen; mica schist is sometimes used as decorative stone or aggregate, but foliation can make it split and weather readily.

Geological facts

The glitter comes from light reflecting off numerous mica flakes. The coarse, visibly aligned minerals are characteristic of schistosity.

Field identification & locations

Look for sparkly, platy minerals arranged in a layered or wavy fabric; biotite flakes are dark and split into thin sheets. Hong Kong’s region includes varied volcanic and metamorphic rocks, so this is plausible, though the photo cannot confirm provenance or exact mineral makeup.