metamorphic

Biotite-rich schist (mica schist)

Mica schist, chiefly quartz + biotite mica; approximate biotite formula K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂

AI-generated identification · may be incorrect

Dark gray-brown, sparkly, foliated material with abundant reflective mica flakes and finer quartz/feldspar grains. Biotite hardness is about 2.5–3; quartz is 7; schist typically has a micaceous sheen, irregular cleavage along foliation, and specific gravity around 2.7–3.0.

Identified September 6, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when shale or another clay-rich sediment is buried and subjected to moderate-to-high heat and directed pressure, typically during mountain building. Its exact age cannot be determined from the photograph; rocks of this type around Montréal commonly belong to Precambrian Grenville-age metamorphic terranes.

Uses & applications

Used mainly as decorative stone, landscaping material, and collector specimens; strongly foliated pieces have limited construction value because they split easily.

Geological facts

The glitter comes from aligned biotite flakes, producing schistosity. Some grains may be pyrite or graphite, but image evidence is insufficient to confirm them.

Field identification & locations

Likely plausible near 45.5°N, 73.6°W, where metamorphic rocks and glacially transported stones are common. Look for visible mica sheets, foliation, and splitting; a glassy quartz streak and nonmagnetic behavior support this identification.