The same mineral sparkles in lipstick, fills drywall seams and insulates radio capacitors
Mica peels apart into sheets thinner than a tenth of a millimetre that still block electricity and shrug off heat. That odd combination puts it in capacitors, car paint, eye shadow, toothpaste and the paste used to smooth drywall. One crystal found in an Ontario mine weighed about 330 tonnes.
Micas are a family of 37 silicate minerals defined by perfect basal cleavage: a crystal splits cleanly into thin, flexible, elastic plates. The reason lies in their layered architecture. Each layer is a sandwich of two sheets of silica tetrahedra tightly bonded to a central sheet of octahedra, and those sandwiches are held to one another only by loosely attached ions of potassium, sodium or calcium. Pull along the weak interlayer bonds and the crystal leafs apart like a book. Calcium-bearing varieties are classed as brittle micas.
Mica is common in granites, schists and pegmatites, and pegmatites yield the big crystals, known as books, sometimes several feet across. The largest known is a phlogopite from the Lacey Mine in Ontario measuring 10 by 4.3 by 4.3 metres. In 2010 world production of scrap and flake mica was about 350,000 tonnes, led by Russia, while India produced most sheet mica. Top-grade sheet can fetch more than 2,000 dollars a kilogram. In Madagascar and India some mica is dug by hand in poor conditions, including with child labour.
Electrically it is superb. Split as thin as 0.025 millimetres, it still insulates, resists breakdown, loses little energy as heat and stays stable to 500 degrees; muscovite goes into high-frequency capacitors, and phlogopite tolerates about 900 degrees. It also replaced asbestos in some brake linings and clutch plates.
Most ground mica ends up in humbler places. In 2008, joint compound for drywall used 54 percent of US dry-ground mica, where it smooths the paste and resists cracking; paint took 22 percent and drilling muds 15 percent, sealing porous rock in boreholes. Coated with titanium dioxide, mica flakes give pearlescent car paints their colour-shifting gleam, and in cosmetics its reflective sparkle appears in blush, mascara and nail polish.
Source: Mica
