Clinohumite

Clinohumite
Side View of a cushion cut clinohumite gemstone, 21.34 carats (4.268 g), 17.8 x 16 mm
General
Category	Nesosilicate
Formula (repeating unit)	(Mg,Fe)9(SiO4)4(F,OH)2
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
Major varieties
Titanclinohumite	Titanoan; (Mg,Fe2+,Ti)9 [(F,OH,O)2|(SiO4)4]

Titanoan; (Mg,Fe²⁺,Ti)₉
[(F,OH,O)₂|(SiO₄)₄]

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.

A monoclinic mineral, clinohumite is typically a dark to light brownish or orangy yellow, somewhat resembling the hessonite variety of grossular. Clinohumite's crystal habit is usually granular, but may also be prismatic; crystals are almost always small. Simple and multiple crystal twinning (on {001}) is common, resulting in a highly variable habit. Clinohumite is brittle with a hardness of 6 and a poor basal cleavage. Its specific gravity is 3.2–3.4, and its fracture is conchoidal to uneven; its streak is white.