Phlogopite

Phlogopite is a magnesium-rich mica mineral in the mica group of phyllosilicates. It is a potassium aluminum magnesium silicate with hydroxide, commonly written in a simplified form as KMg3AlSi3O10(OH)2, with iron substituting for magnesium in natural samples. Phlogopite is typically brown to olive-green and occurs as tabular, plate-like crystals with the characteristic perfect basal cleavage of micas. It has a Mohs hardness of about 2 to 3 and a specific gravity around 3.0 to 3.2. The mineral crystallizes in the monoclinic system and displays a pearly luster on cleavage surfaces.

Occurrence and formation

Phlogopite forms in metamorphic and some igneous rocks. It is common in ultramafic and magnesium-rich rocks such as serpentinite, peridotite, and pyroxenites, and appears in contact metamorphic zones and mica-rich schists derived from magnesium-bearing limestones or dolostones. It can also occur in pegmatites associated with granitoid bodies. Its presence is often indicative of high-temperature, magnesium-rich metamorphism and it commonly co-occurs with olivine, pyroxene, talc, and other mica group minerals.

Uses and significance

Industrial uses of phlogopite arise from its sheet-like structure, providing thermal and electrical insulation. Sheet mica from phlogopite has historically been used in electrical equipment and as a heat-resistant insulating material. Some transparent or translucent crystals are mined for ornamental or gemstone purposes, although this is less common than for other micas. Phlogopite is also a useful indicator mineral for geological exploration due to its association with specific metamorphic environments.

See also: muscovite, biotite, annite, lepidolite.