1) Manganese mineral 2) Orthorhombic mineral 
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 occur...
• (n.) A silicate of manganese of an ash-gray color.
olivine mineral found only in iron-manganese ore deposits and skarns and in metamorphosed manganese-rich sediments, such as those of Cornwall, Eng., ... [2 related articles]
Teph'ro·ite noun [ See Tephrosia .] (Min.) A silicate of manganese of an ash-gray color. 
Tephroite is an ash-grey coloured silicate of manganese.
Manganese(II) orthosilicate