Benitoite

Benitoite
Benitoite on natrolite
General
Category	Cyclosilicate
Formula (repeating unit)	BaTiSi3O9
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	3.6
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
Solubility	Insoluble: HCl, H2SO4 Soluble: HF
Other characteristics	Blue fluorescence under SW UV; intense blue cathodoluminescence
References

Benitoite (pronunciation: /bɛn.ˈi.toʊ.aɪt/) is a rare blue barium titanium silicate mineral, found in hydrothermally altered serpentinite. 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 first described in 1907 by George D. Louderback, who named it benitoite for its occurrence near the headwaters of the San Benito River in San Benito County, California.

Benitoite occurs in a number of sites, but gemstone quality material has only been found in California. In 1985 benitoite was named as the official state gem of California.

Benitiote has a rare 5 pointed crystal form, and an even rarer 6 pointed form, "star of David", with about 24 samples known.

Benitoite typically occurs with an unusual set of minerals, along with minerals that make up its host rock. Frequently associated minerals include: natrolite, neptunite, joaquinite, serpentine and albite.