Benitoite

cyclosilicate

neptunite

Formula: BaTiSi3O9
Cyclosilicate (ring silicate), benitoite group
Specific gravity: 3.65
Hardness: 6 to 6½
Streak: White
Colour: Sapphire blue, white to colourless, rarely pink
Solubility: Insoluble in hydrochloric acid, but easily attacked by HF (Dana)
Environments:

Metamorphic environments
Hydrothermal environments

Benitoite is a mineral of the blueschist facies, found in only a few localities worldwide. It fluoresces a very bright sky blue colour under shortwave ultraviolet radiation, and the fluorescence appears to be stronger in crystals with crossite (an amphibole intermediate between the riebeckite group and the glaucophane group).

At the type locality, the California State Gem Mine (previously the Benitoite Gem Mine), San Benito county, California, USA, benitoite occurs in natrolite veins cutting glaucophane schist interlayered with hydrothermally altered serpentine, associated with neptunite and joaquinite. Crossite (an amphibole intermediate between the riebeckite group and the glaucophane group) inclusions are sometimes present, especially in twinned crystals (Dana, MinIssue, Webmin, HOM, AM 57.85-102).
A large body of serpentinite was tectonically emplaced into surrounding sedimentary and metamorphic rocks in the Jurassic period. Subsequently the serpentinite experienced the low temperature and high pressure metamorphism of the blueschist facies. Later numerous hydrothermal calc-silicate veins developed, altering the blueschist rock and depositing the minerals found here. Finally natrolite deposited in the veins, encasing most, but not all, of the minerals deposited in the earlier phases. The upper zone of the mine has a large concentration of albite veins and is devoid of natrolite. Here benitoite is associated with albite. The lower level portions of the veins were typically filled with natrolite. (Minerals Issue #3, The Collector's Newspaper 2011)

At Magnet Cove, Hot Spring county, Arkansas, USA, benitoite occurs in pectolite in gas cavities in pseudoleucite (a mixture of nepheline and orthoclase, pseudomorph after leucite) syenite (Dana).

Common impurities: Na

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