TEPHROITE

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Orthorhombic
    Chemistry : Mn2SiO4
    Rarity : Uncommon

Tephroite belongs to the olivine group. It forms two series, one with fayalite (ferriferous) and the other with forsterite (magnesian). It is a fairly rare olivine that appears in skarn-type iron and manganese deposits, or in metamorphic sediments rich in manganese. Its name comes from the Greek tephros (ash), illustrating the most common color of the mineral. Tephroite is variable in color, olive green to gray, sometimes purple-red, red-brown or yellow-brown, it generally occurs in the form of disseminated grains, or xenomorphic masses, rarely in prismatic or isometric crystals.

Main photo : Tephroite from In den Dellen quarries, Eifel © Volker Betz

Tephroite from Sterling Hill, New Jersey, USA © Christopher O'Neill
Tephroite from Wessels Mine, South Atlantic © Bruce Cairncross
Tephroite from Wessels Mine, South Atlantic © Bruce Cairncross
Tephroite from Laacher See, Germany © Stephan Wolfsried

Tephroite in the World

The most beautiful samples of tephroite are the 2 cm translucent purple-red crystals discovered in South Africa in the Hotazel manganese district. They rival the magnificent 4 cm crystals of Franklin and Sterling Hill (New Jersey), the original localities of this mineral. Tephroite is reported in many localities of the Japanese island of Honshû and Kazakhstan, almost everywhere in the United States, in the large mine of Broken Hill (Australia), etc...

Tephroite in France

In France, tephroite is known in masses of several kilograms of brown-yellow color in the micaschists of Locqueltas, on the island of Groix (Morbihan). It is also reported in St-Jean-de-Maurienne (Savoie) and in the Aure valley (Hautes-Pyrénées).

Twinning

Twinning is reported on {011}, but uncommon.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 6
Density : 3.87 to 4.12
Fracture : Irregular to conchoidal
Streak : Light gray

TP : Translucent to transparent
RI : 1.770 to 1.825
Birefringence : 0.047
Optical character : Biaxial -
Pleochroism : Low
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : ParamagneticRadioactivity : None