Mica

Mica is not one mineral but an entire family of hydrous potassium-aluminum sheet silicates, prized for a perfect basal cleavage that splits crystals into thin, elastic, glass-clear flakes flashing with a characteristic pearly luster — the trait that gave the group its name, from the Latin "micare", to glisten. The family is divided into common micas with monovalent interlayer cations, such as muscovite and paragonite, and brittle micas with divalent cations, such as margarite and clintonite. Of roughly two dozen recognized species, collectors focus mainly on six: pale, colorless-to-greenish muscovite; dark, iron-bearing biotite; magnesium-rich, honey-brown phlogopite; lithium-bearing, lilac-to-pink lepidolite; silvery sodium mica paragonite; and chromium-green fuchsite, a muscovite variety. Muscovite crystallizes chiefly in granites, gneisses, schists and complex granite pegmatites, biotite in granites, gabbros, syenites and pegmatites, phlogopite in metamorphosed dolomitic marbles and kimberlites, and lepidolite almost exclusively in lithium-rich pegmatites alongside beryl, spodumene, topaz and tourmaline.

Classic type localities read like a world tour of pegmatite mining. In Brazil's Minas Gerais, the pegmatites of Araçuaí yield superb violet lepidolite, while Linópolis and Divino das Laranjeiras produce fine muscovite books, often with colorful tourmaline. India, the world's largest producer, centers on Bihar and the Nellore district, with muscovite books showing striking associations with quartz and tourmaline. Madagascar's pegmatites supply muscovite, phlogopite and lepidolite together with spectacular associated species, while Russia's Ilmen Mountains and Kola Peninsula have long furnished large sheet muscovite once prized for windows and industrial insulators. Phlogopite specimens from Franklin, New Jersey, occur as lustrous barrel-shaped crystals embedded in white marble, and biotite from Mount Somma and Vesuvius, Italy, forms sharp, glassy black crystals long sought by classic European collections. Norway's Brevik and Arendal, the Ilmen Mountains of the Southern Urals, and Ontario's Bancroft and Wakefield districts round out the historic biotite and phlogopite localities, while South Africa is notable for chromium fuchsite in verdite.

Collectors value mica specimens for their sheer diversity of habit and color — from single, cleavable "books" many centimeters across to barrel-shaped phlogopite crystals and delicate rosettes of lepidolite — and for the way each locality imprints its own combination of color, crystal form and mineral association, whether Franklin's phlogopite-in-marble, Vesuvian biotite, or Brazilian lepidolite-tourmaline pairings.