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phlogopite
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Phlogopite mica from Warwick, N.Y.
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Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.
(Bed) Rocks and (Flint) Stones
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Also called:
brown mica
Related Topics:
mica
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Phlogopite mica from Warwick, N.Y.
Phlogopite mica from Warwick, N.Y.

phlogopite

mineral
Also known as: brown mica
Written and fact-checked by
Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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History

phlogopite, basic aluminosilicate of potassium, magnesium, and iron that is a member of the common mica group. Varieties that contain only small amounts of iron are economically important as electrical insulators. Phlogopite occurs typically as a metamorphic product (e.g., in crystalline metamorphosed limestones) and also in ultramafic igneous rocks. Phlogopite forms a chemical substitution series with biotite, from which it is arbitrarily distinguished by a magnesium-to-iron ratio greater than 2:1. For detailed physical properties, see mica (table).

The name phlogopite also denotes an iron-free compound regarded as making up a large part of the mineral. Its chemical composition is K2Mg6(Si6Al2O20)(OH)4.

This article was most recently revised and updated by John P. Rafferty.