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Phlogopite is an aluminosilicate mineral of potassium and magnesium, and a member of the mica group. Phlogopite usually ranges in colour from yellow to brown to reddish brown. Its colour can be helpful in distinguishing it from other mica minerals. Phlogopite and muscovite are the only two mica minerals that are used commercially. Phlogopite is most commonly found in metamorphic rocks. An ideal condition for phlogopite formation is when a dolomitic limestone or a magnesium-rich limestone, with some clay content, is subjected to hydrothermal metamorphism. The result is typically one of the following: scattered flakes and small crystals of phlogopite throughout the rock; concentrations of phlogopite in parts of the limestone where clay was abundant; or, concentrations of phlogopite in metamorphosed shale along the margins of the limestone. Phlogopite schist can form through this same process. Phlogopite is also found in igneous rocks. These include ultramafic rocks such as peridotite, kimberlite, lamproite, and serpentinite. Phlogopite is also found in high-alumina basalts. Almost all sheet and block mica is found in pegmatites - "".

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