Benitoite - Encyclopedia

    Class : Silicates
    Subclass : Cyclosilicates
    Crystal system : Hexagonal
    Chemistry : BaTiSi3O9
    Rarity : Very rare


Benitoite is a very rare mineral found in small veins of natrolite crossing a glaucophane schist intercalated with a serpentine, where it associates with neptunite. It owes its name to its place of discovery : San Benito County in California. It occurs in tabular prismatic crystals, truncated by three triangular pyramidal faces, more rarely in trigonal bipyramids. Its appeal is due to its superb sapphire blue to light blue, sometimes dark blue color. Benitoite is sometimes cut in jewelry and used as a fluorescent standard in certain electronic microprobes. But these uses remain marginal due to the rarity of its crystals.

Benitoite in the World

Benitoite is only known as crystals in the Dallas Gem Mine (renamed since Benitoite Gem Mine), in the San Benito Valley in California, a unique deposit that has provided beautiful blue gemmy crystals up to size to 5.6 cm ! Two other deposits in the San Benito valley (Victor Mine and Numero Uno Mine) provided good specimens but of lower quality. Benitoite is also reported in Arkansas and Montana, the Czech Republic and Japan.

Benitoite in France

This mineral is not present in the French underground.

Twinning

The twins are present by rotation on [001] and can form 6-pointed stars (extremely rare) like these world-famous specimens shown on the right.


Fakes and scams

No fake recorded for this mineral species.



Hardness : 6 to 6.5
Density : 3.65
Fracture : Conchoidal
Trace : White



TP : Translucent to transparent
RI : 1.756 to 1.804
Birefringence : 0.046
Optical character : Uniaxial +
Pleochroism : Visible
Fluorescence : Blue under SW UV


Solubility : Hydrofluoric acid

Magnetism : None
Radioactivity : None