Clinohumite

Clinohumite
General
Category	Nesosilicate
Formula (repeating unit)	(Mg,Fe)9(SiO4)4(F,OH)2
IMA symbol	Chu[1]
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 13.71 Å, b = 4.75 Å, c = 10.29 Å; β = 100.83°; Z = 2
Identification
Color	Brownish to orange, yellow, red
Crystal habit	Granular, prismatic, twinned
Twinning	Simple, lamellar common on {100}
Cleavage	Poor on {100}
Fracture	Subconchoidal to uneven
Mohs scale hardness	6
Luster	Vitreous to resinous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.17–3.35
Optical properties	biaxial (+)
Refractive index	nα = 1.623 – 1.702 nβ = 1.636 – 1.709 nγ = 1.651 – 1.728
Birefringence	+0.028
Pleochroism	X = golden yellow, yellow-brown, deep reddish yellow; Y = pale yellow, yellow-orange, light yellow; Z = pale yellow, yellow-orange, colorless
2V angle	Measured: 52° to 90°
References	[2][3][4][5]
Major varieties
Titanclinohumite	Titanoan; (Mg,Fe2+,Ti)9 [(F,OH,O)2|(SiO4)4] [6][7]

Clinohumite is an uncommon member of the humite group, a magnesium silicate according to the chemical formula (Mg, Fe)₉(SiO₄)₄(F,OH)₂. The formula can be thought of as four olivine (Mg₂SiO₄), plus one brucite (Mg(OH)₂). Indeed, the mineral is essentially a hydrated olivine and occurs in altered ultramafic rocks and carbonatites. Most commonly found as tiny indistinct grains, large euhedral clinohumite crystals are sought by collectors and occasionally fashioned into bright, yellow-orange gemstones. Only two sources of gem-quality material are known: the Pamir Mountains of Tajikistan, and the Taymyr region of northern Siberia. It is one of two humite group minerals that have been cut into gems, the other being the much more common chondrodite.