Muscovite

Muscovite is the mineral that splits. Its structure is built of sheets held together weakly, so a specimen can be peeled into flakes thin enough to see through and flexible enough to bend without breaking — a combination almost nothing else offers.

The name records what it was used for. Large sheets from Russia were traded across Europe as a glazing material, and the mineral was called Muscovy glass long before it was called muscovite. It filled windows, and later stove and lantern doors, because unlike glass it does not shatter when heated unevenly.

Nothing else in a common rock peels into elastic transparent sheets. Split a flake and it stays flat and springs back when bent; the chemically similar chlorite splits too but its flakes are brittle and stay bent.

Muscovite is pale — silvery, colourless or faintly brown — where biotite, the other common mica, is dark brown to black. Both are soft enough to scratch with a fingernail on the cleavage face.

Ground mica is the form most of it takes now: a filler and extender in paint, joint compound, plastics and cosmetics, where its flat plates lie parallel and give both coverage and a pearly sheen. The shimmer in a great deal of makeup is mica.

Sheet muscovite remains valuable as an electrical insulator that tolerates heat, used in capacitors and in the insulation of heating elements. Its historic use as glazing has gone, but the property that made it work — transparency plus thermal shock resistance — still matters in furnace and stove windows.

Muscovite forms in granites and pegmatites, and abundantly in metamorphic rocks — schist is often largely mica, and its glittering, easily split character comes directly from the parallel alignment of muscovite and biotite plates.

It is chemically durable enough to survive weathering and transport, so flakes of muscovite are a common component of sandstone and of river and beach sand, where they catch the light conspicuously.