Kyanite

Kyanite
General
Category	Nesosilicate
Formula (repeating unit)	Al2SiO5
IMA symbol	Ky[1]
Strunz classification	9.AF.15
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 7.1262(12) Å b = 7.852(10) Å c = 5.5724(10) Å α = 89.99(2)°, β = 101.11(2)° γ = 106.03(1)°; Z = 4
Identification
Color	Blue, white, rarely green, light gray to gray, rarely yellow, pink, orange, and black, can be zoned
Crystal habit	Columnar; fibrous; bladed
Twinning	Lamellar on {100}
Cleavage	[100] perfect [010] imperfect with 79° angle between
Fracture	Splintery
Tenacity	Brittle
Mohs scale hardness	4.5–5 parallel to one axis 6.5–7 perpendicular to that axis
Luster	Vitreous to white
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.53–3.65 measured; 3.67 calculated
Optical properties	Biaxial (-); high relief
Refractive index	nα = 1.712 – 1.718 nβ = 1.720 – 1.725 nγ = 1.727 – 1.734
Birefringence	δ = 0.012 – 0.016
Pleochroism	Trichroic, colorless to pale blue to blue
2V angle	78°–83°
References	[2][3][4]

Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generally indicates metamorphism deep in the Earth's crust. Kyanite is also known as disthene or cyanite.^[5]

Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction. In kyanite, this anisotropism can be considered an identifying characteristic, along with its characteristic blue color. Its name comes from the same origin as that of the color cyan, being derived from the Ancient Greek word κύανος. This is typically rendered into English as kyanos or kuanos and means "dark blue."

Kyanite is used as a raw material in the manufacture of ceramics and abrasives, and it is an important index mineral used by geologists to trace metamorphic zones.