Tephroite

(repeating unit)Mn₂SiO₄IMA symbolTep^[1]Crystal systemOrthorhombicCrystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)Space groupPnma (no. 62)Unit cella = 4.88(2) Å, b = 10.61(2) Å
c = 6.24(2) Å; Z = 4IdentificationColorOlive-green, bluish green, gray, °esh-red, reddish brown; pale green in thin section, may be colorlessCrystal habitCrystals typically short, prismatic, to 4 cm, or anhedral, equidimensional. Commonly in disseminated grains, compact, or massive.TwinningUncommon on {011}Cleavage{010}, distinct; {001}, imperfectFractureUneven to conchoidalTenacityBrittleMohs scale hardness6LusterVitreous to greasyStreakPale grayDiaphaneityTransparent to translucentSpecific gravity3.87 – 4.12Optical propertiesBiaxial (−)Refractive indexn_α = 1.759 n_β = 1.797 n_γ = 1.860Birefringenceδ = 0.101PleochroismWeak; X = brownish red; Y = reddish; Z = greenish blue.2V angleMeasured: 60° to 70°, Calculated: 78°References^[2][3][4]

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.^[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.

References

• Mineral galleries