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.
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