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The Mica Minerals


The mica minerals are distinguished by their perfect basal cleavage, which means that they are easily split into thin, often transparent sheets. Two micas, biotite and muscovite, are so common that they are considered rock-forming minerals. The rest are relatively uncommon, but phlogopite is the most likely of these to be seen in the field. Rock shops overwhelmingly favor the colorful fuchsite and lepidolite.

The general formula for the mica minerals is XY2-3[(Si,Al)4O10](OH,F)2, where X = K,Na,Ca and Y = Mg,Fe,Li,Al. Their molecular makeup consists of double sheets of strongly joined silica units (SiO4) that sandwich between them a sheet of hydroxyl (OH) plus Y cations. The X cations lie between these sandwiches and bind them loosely.

Along with talc, chlorite, serpentine and the clay minerals, the micas are classified as phyllosilicate minerals, "phyllo-" meaning "leaf." Not only do the micas split into sheets, but the sheets are also flexible.

Images 1-11 of 11
Black micaBiotiteThe painter's sea-greenCeladoniteChromian muscoviteFuchsiteMakes marine rocks greenGlauconite
Lithium micaLepidoliteA brittle calcium micaMargariteWhite micaMuscoviteA low-Al neighbor of muscovitePhengite (Mariposite)
Brown micaPhlogopiteShiny silky micaSericiteDouble-iron phyllosilicateStilpnomelane
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