Benitoite

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Environment

At San Benito Co., California (practically the only occurrence), crystals have grown on the sides of natrolite veins cutting a schistose serpentinite, and associated with black neptunite.

Good tabular triangular crystals, to 2Ɖ in. (5 cm) across, usually unevenly colored blue and white. The triangular base is likely to be duller in luster and whiter than the pyramids and prisms.

Blue to white (rarely pink). Luster glassy; Hardness 6-6Ɖ specific gravity 3.6; fracture conchoidal; cleavage poor pyramidal. Transparent to translucent; fluorescent blue in shortwave ultraviolet light.

Barium titanium silicate (36.3% BaO, 20.2% TiO ₂ , 43.5% SiO ₂ ).

All specimens are crystals with a shape so distinctive that tests are unnecessary.

Since there is but one significant occurrence, with constant associates and appearance, it has never been necessary to test specimens; it resembles no other mineral.

Mainly known from a limited deposit of compact granular natrolite veins cutting a gray-green fibrous schist interlayered with serpentine. Collectors might eventually turn up other occurrences of this attractive mineral. Small pinkish "roses" of benitoite have been found near the California occurrence, and it has been reported in six-sided blue crystals at Omi Machi Nishi-kubiki Gun, Niigata Prefecture, Japan.

Until this mineral was found in 1907, the mineral world had no naturally occurring representative of this crystal class. Even now it is the only mineral example. The official California gemstone, it is expensive when transparent, free of flaws, and of good color. One carat is fine; anything over five carats can be regarded as very special.

Benitoite
Benitoite on natrolite
General
Category	Silicate mineral
Chemical formula	BaTiSi3O9
Strunz classification	9.CA.05
Crystal symmetry	Hexagonal 6 m2 ditrigonal dipyramidal
Unit cell	a = 6.641 Å, c = 9.7597(10) Å; Z = 2
Identification
Color	Blue, colorless
Crystal habit	Tabular dipyramidal crystals, granular
Crystal system	Hexagonal
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
Solubility	Insoluble: HCl, H2SO4 Soluble: HF
Other characteristics	Blue fluorescence under SW UV; intense blue cathodoluminescence
References	[1][2][3]

Benitoite (ben-EE-toe-ite) 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.^[4][5]

Benitoite occurs in a number of sites, but gemstone quality material has only been found in California.^{[citation needed]} It is California's official state gem.

Associated minerals and locations

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.

Benitoite is a rare mineral found in very few locations including San Benito County, California, Japan and Arkansas. In the San Benito occurrence, it is found in natrolite veins within glaucophane schist within a serpentinite body. In Japan, the mineral occurs in a magnesio-riebeckite-quartz-phlogopite-albite dike cutting a serpentinite body.^[3]

In 1985 benitoite was named as the official state gem of California.^[6]

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  Blue Benitoite Crystals on white natrolite, Dallas Gem Mine, San Benito Co., California, USA

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  Benitoite crystals under UV light

References

1. ^ WebMineral Listing
2. ^ MinDat Listing
3. ^ ^{a b} Handbook of Mineralogy
4. ^ Louderback, George Davis. Bentiote, A New California Gen Mineral. Bulletin of The Department of Geology, Vol. 5, No. 9. University of California Publications. July, 1907
5. ^ http://www.mineralsocal.org/scfm/newsletters/2002%20march.htm Friends of Mineralogy review of benitoite
6. ^ http://www.consrv.ca.gov/cgs/geologic_resources/mineral_resource_mapping/ Mineral Resources California Geologic Survey. Accessed December 31, 2005

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