Benitoite

Benitoite
Benitoite on natrolite
General
Category	Cyclosilicate
Formula (repeating unit)	BaTiSi3O9
IMA symbol	Bni[1]
Strunz classification	9.CA.05
Crystal system	Hexagonal
Crystal class	Ditrigonal dipyramidal (6m2) H-M symbol: (6 m2)
Space group	P6c2
Unit cell	a = 6.641, c = 9.7597(10) [Å]; Z = 2
Identification
Color	Blue, colorless
Crystal habit	Tabular dipyramidal crystals, granular
Twinning	On {0001} by rotation
Cleavage	[1011] poor
Fracture	Conchoidal
Mohs scale hardness	6–6.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.65
Optical properties	Uniaxial (+)
Refractive index	nω = 1.756 – 1.757 nε = 1.802 – 1.804
Birefringence	δ = 0.046
Pleochroism	O = colorless; E = purple, indigo, greenish blue
Dispersion	0.036–0.046[2]
Solubility	Insoluble: HCl, H2SO4 Soluble: HF
Other characteristics	Blue fluorescence under SW UV; intense blue cathodoluminescence
References	[3][4][5]

Benitoite (/bəˈniːtoʊaɪt/) is a rare blue barium titanium cyclosilicate mineral, found in hydrothermally altered serpentinite. It forms in low temperature, high pressure environments typical of subduction zones at convergent plate boundaries. Benitoite fluoresces under short wave ultraviolet light, appearing bright blue to bluish white in color. The more rarely seen clear to white benitoite crystals fluoresce red under long-wave UV light.

It was discovered in 1907 by prospector James M. Couch in the San Benito Mountains roughly halfway between San Francisco and Los Angeles. Due to its similar color, Couch originally believed it to be sapphire, a variety of corundum. In 1909, a sample was sent to the University of California, Berkeley, where mineralogist Dr. George D. Louderback realized it was a previously unknown mineral. Corundum (sapphire) has a defined Mohs hardness of 9, while benitoite is much softer. He named it benitoite for its occurrence near the headwaters of the San Benito River in San Benito County, California.^[6][7]

Benitoite occurs in a number of isolated locations globally, but gemstone quality material has only been found in California at the Benito Gem Mine where it was first discovered. It has been correctly identified in Montana, Arkansas, Japan, and Australia although they formed under slightly different conditions and only grow large enough to be considered an accessory mineral. In 1985 benitoite was named as the official state gem of California.^[8][9]

Benitoite typically crystallizes hexagonally. Non-gem crystals of benitoite can have a very rare, six-pointed twinned form.^[10]