Tephroite

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
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.