Benitoite

(mineralogy) BaTi(SiO₃)₃ A blue to violet barium-titanium silicate mineral; at one time it was cut and sold as sapphire.

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
Identification
Color	Blue; Colorless
Crystal habit	Tabular dipyramidal crystals, granular
Crystal system	Hexagonal
Cleavage	[1011] Poor
Fracture	Conchoidal
Mohs scale hardness	6 - 6.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.6
Refractive index	1.757-1.759; 1.802-1.804
Pleochroism	Dichroic (blue to white)
Solubility	Insoluble: HCl, H2SO4 Soluble: HF
References	[1][2]

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 light blue in color.

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.^[3][4]

Uses of benitoite

Benitoite's main uses are as collector's specimens. Benitoite's hardness also makes it suitable for use as a gemstone, although the general lack of usable material has limited this use.

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

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

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 it occurs in a magnesio-riebeckite-quartz-phlogopite-albite dike cutting a serpentinite body.^[5] Benitoite is typically found with some combination of natrolite, joaquinite, and neptunite on a greenish-grey serpentinite base.

Benitoite, as of 1985, is the official state gem of California. The mineral is known to occur in gemstone quality, only in California. ^[6][7]

References

Wikimedia Commons has media related to: Benitoite

1. ^ http://webmineral.com/data/Benitoite.shtml WebMineral Listing
2. ^ http://www.mindat.org/min-624.html MinDat Listing
3. ^ 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
4. ^ http://www.mineralsocal.org/scfm/newsletters/2002%20march.htm Friends of Mineralogy review of benitoite
5. ^ http://rruff.geo.arizona.edu/doclib/hom/benitoite.pdf Handbook of Mineralogy
6. ^ Story of benitoite discovery. Accessed Nov. 10, 2009
7. ^ http://www.consrv.ca.gov/cgs/geologic_resources/mineral_resource_mapping/ Mineral Resources California Geologic Survey. Accessed December 31, 2005

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