Phlogopite

Formula: KMg₃Si₃AlO₁₀(F,OH)₂

Origin of name: from Greek for firelike

Crystal system: monoclinic

Crystal class: 2/m

Twinning: common on {001} about [310] axis

Unit cell: a = 5.3 Å, b = 9.2 Å, c = 10.22 Å, b = 99.8°

Colour: yellowish brown to brownish red, colourless, white

Diaphaneity: 

Luster: pearly, submetallic on cleavage

Habit: crystals prismated, tapered, plates, scales

Hardness: 2 to 2.5

Specific gravity: 2.76 to 2.90

Cleavage: 1; {001} perfect

Tenacity: thin lamellae flexible and elastic, tough

Streak: colourless

Synonyms/varieties: flogopite, chromephlogopite (Cr-rich variety)

Comments: series with biotite, the -1M, -3T and -2M₁ polytypes are known