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phlogopite A member of the mica group of minerals, phlogopite is the Mg-rich variety of biotite K2(Mg,Fe2+)6 [Si3AlO10]2(OH,F)4 with Mg/Fe more than 2/1; sp. gr. 2.76–2.90; hardness 2.0–2.5; subtransparent; pale brown; small, tabular crystals; occurs in metamorphosed, impure, magnesian limestones by reaction between dolomite and K-feldspar or muscovite, in kimberlites, and in some leucite-bearing rocks.