Phlogopite

Phlogopite
General
Category	Mica, phyllosilicates
Formula (repeating unit)	KMg3(AlSi3O10)(F,OH)2
IMA symbol	Phl[1]
Strunz classification	9.EC.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Color	Brown, brownish red, dark brown, yellow, yellowish brown, green, white and gray
Crystal habit	Tabular, scaly masses, rarely perfect phenocryst tablets
Twinning	Composition twinning
Cleavage	Perfect basal (001)
Fracture	None
Tenacity	Tough, flexible thin laminae
Mohs scale hardness	2–2.5
Luster	Pearly, sometimes slightly metallic on cleavage surfaces
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.78–2.85
Optical properties	Biaxial (−), 2V=12
Refractive index	nα = 1.530–1.573 nβ = 1.557–1.617 nγ = 1.558–1.618
Birefringence	δ =0.0280–0.0450
2V angle	16–20°
Other characteristics	Fluorescent
References	[2][3][4][5]

Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.

Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg₃AlSi₃O₁₀(F,OH)₂. Iron substitutes for magnesium in variable amounts leading to the more common biotite with higher iron content. For physical and optical identification, it has most of the characteristic properties of biotite.