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Not to be confused with tephrite.
Tephroite Manganese silicate Kaao Mine Tochigi-ken Honshu Japan 1816.jpg
Tephroite from Japan
Category Silicate mineral
(repeating unit)
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Unit cell a = 4.88(2) Å, b = 10.61(2) Å
c = 6.24(2) Å; Z = 4
Color Olive-green, bluish green, gray, °esh-red, reddish brown; pale green in thin section, may be colorless
Crystal habit Crystals typically short, prismatic, to 4 cm, or anhedral, equidimensional. Commonly in disseminated grains, compact, or massive.
Twinning Uncommon on {011}
Cleavage {010}, distinct; {001}, imperfect
Fracture Uneven to conchoidal
Tenacity Brittle
Mohs scale hardness 6
Luster Vitreous to greasy
Streak Pale gray
Diaphaneity Transparent to translucent
Specific gravity 3.87 – 4.12
Optical properties Biaxial (-)
Refractive index nα = 1.759 nβ = 1.797 nγ = 1.860
Birefringence δ = 0.101
Pleochroism Weak; X = brownish red; Y = reddish; Z = greenish blue.
2V angle Measured: 60° to 70°, Calculated: 78°
References [1][2][3]

Tephroite is the manganese endmember of the olivine group of nesosilicate minerals with the formula Mn2SiO4. A solid solution series exists between tephroite and its analogues, the group endmembers fayalite and forsterite. Divalent iron or magnesium may readily replace manganese in the olivine crystal structure.

It was first described for an occurrence at the Sterling Hill Mine and Franklin, New Jersey, United States.[2] It occurs in iron-manganese ore deposits and their related skarns. It also occurs in metamorphosed manganese-rich sediments. It occurs in association with: zincite, willemite, franklinite, rhodonite, jacobsite, diopside, gageite, bustamite, manganocalcite, glaucochroite, calcite, banalsite and alleghanyite.[1] It can also be found in England and Sweden.

Tephroite has a hardness of 6 and a specific gravity of approximately 4.1, which is heavy for non-metallic minerals. Its name comes from the Greek tephros, "ash gray", for its color.[3] It can also be found olive-green, greenish-blue, pink, or brown. Other names for tephroite include mangan olivine and mangan peridot.