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From Wikipedia, the free encyclopedia
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.
Tephroite | |
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![]() Tephroite from Japan | |
General | |
Category | Silicate mineral |
Formula (repeating unit) | Mn2SiO4 |
IMA symbol | Tep[1] |
Crystal system | Orthorhombic |
Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
Space group | Pnma (no. 62) |
Unit cell | a = 4.88(2) Å, b = 10.61(2) Å c = 6.24(2) Å; Z = 4 |
Identification | |
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 | [2][3][4] |
It was first described for an occurrence at the Sterling Hill Mine and Franklin, New Jersey, United States.[3] 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.[2] 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.[4] It can also be found olive-green, greenish-blue, pink, or brown. Other names for tephroite include mangan olivine and mangan peridot.
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