Tephroite

Tephroite
Tephroite from Japan
General
Category	Silicate mineral
Formula (repeating unit)	Mn2SiO4
Crystal symmetry	Orthorhombic 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
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.
Crystal system	Orthorhombic – Dipyramidal
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 a non-metallic manganese silicate mineral with the formula, Mn₂SiO₄.

It was first described for an occurrence at the Sterling Hill Mine and Franklin, New Jersey, USA.^[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.

References

• Mineral galleries