THE MINERAL TEPHROITE

Chemistry: Mn2SiO4, Manganese Silicate.
Class: Silicates
Subclass: Nesosilicates
Group: Olivine
Uses: A very minor ore of manganese and as mineral specimens.
Specimens

Tephroite is one of the unusual minerals found at the famous mines of Sterling Hill and Franklin, New Jersey, USA. It is named for the typical color which is ash gray ("tephros" is Greek for ash colored). Tephroite forms a series with the olivine mineral fayalite. Tephroite is the manganese rich member of the series and fayalite is the iron rich member with a formula of Fe2SiO4. Unlike fayalite which is more commonly found in volcanic rocks, tephroite is a contact metamorphic and hydrothermal replacement mineral.

PHYSICAL CHARACTERISTICS:

Color is commonly ash gray but also is found olive-green, greenish-blue, pink or brown.
Luster is vitreous to greasy.
Transparency: Crystals are transparent to translucent.
Crystal System is orthorhombic; 2/m 2/m 2/m
Crystal Habits include short prismatic to blocky crystals Also massive, granular and compact.
Cleavage is good in two directions at 90 degrees.
Fracture is uneven to conchoidal.
Hardness is 6
Specific Gravity is approximately 4.1 (heavy for non-metallic minerals)
Streak is off white to gray.
Associated Minerals are calcite, quartz, spessartine, rhodonite, willemite, franklinite and pyroxmangite.
Notable Occurrences include the Sterling Hill and Franklin, New Jersey and California, USA locations; Cornwall, England and Langban, Sweden.
Best Field Indicators are crystal habit, cleavage, environment, color, hardness and density.