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Formula: BaTiSi3O9
Cyclosilicate (ring silicate), benitoite group,
barium and
titanium bearing mineral
Crystal System: Hexagonal
Specific gravity: 3.65 measured, 3.68 calculated
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)
Common impurities: Na
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)
(Spirifer #3).
At Ohmi, Japan, benitoite occurs in a
magnesio-riebeckite-quartz-phlogopite-albite
dike cutting
serpentinite, associated with
ohmilite, bario-orthojoaquinite
and leucosphenite
(HOM).
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, Spirifer #3, 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.
(Spirifer #3)
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).
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