Tephroite

manganese

fayalite

Formula: Mn2+2(SiO4)
Nesosilicate (insular SiO4 groups), olivine group, forms a series with fayalite. Manganese mineral.
Specific gravity: 3.87 to 4.12
Hardness: 6
Streak: pale grey
Colour: grey, olive green, flesh red or reddish brown, dark brown
Environments:

Metamorphic environments

Tephroite is a manganese mineral that is a common component of metamorphic iron-manganese deposits and skarn, formed by low grade metamorphism. It is associated with minerals such as rhodonite, bustamite, manganoan calcite, hausmannite and spessartine.

In the Tamworth area, New South Wales, Australia, tephroite occurs with hausmannite, rhodonite and quartz (Lauf p32).

At Buritirama, Para, Brazil, tephroite is associated with rhodochrosite, pyroxmangite, rhodonite, manganese-rich pyroxene, amphibole, spessartine and phlogopite (Lauf p32).

At Bonneval-sur-Arc, Haute-Maurienne, French Alps, tephroite occurs with rhodochrosite, rhodonite, pyroxmangite, manganese-rich amphibole, spessartine, sonolite, alleghanyite and freidelite as a product of high-pressure, low-temperature metamorphism of manganese deposits (Lauf p32).

At the core of the Suretta nappe, Italian Alps, tephroite occurs with kutnohorite in calc-silicate rocks (Lauf p32).

At the Kaso mine, Honshu, Japan, tephroite is a product of metamorphism associated with the intrusion of rhodonite veins into rhodochrosite ore (Lauf p32).

At the Noda-Tamagawa mine, Iwate prefecture, Japan, tephroite occurs with rhodonite, galaxite, manganosite, baryte and alabandite (Lauf p32).

In the Oberhalbstein-Malenco area in the Swiss Alps tephroite occurs with rhodonite and quartz in greenschist facies metamorphic rocks (Lauf p32).

At Sterling Hill, Ogdensberg, New Jersey, USA, tephroite is associated with zincite and franklinite (Lauf p31).

Alteration

braunite to tephroite, hausmannite and O2
3Mn2+Mn3+6O8(SiO4) ⇌ 3Mn2+2(SiO4) + 5Mn2+Mn3+2O4 + 2O2
(AM80.572)

bustamite, tephroite and calcite to glaucochroite and CO2
CaMn2+Si2O6 + Mn2+2(SiO4) + 2CaCO3 ⇌ 3CaMn2+(SiO4) + 2CO2
(DHZ 1A p348)

hausmannite and rhodonite to tephroite and O2
2Mn3O4 + 6Mn2+SiO3 ⇌ 6Mn2+2SiO4 + O2
Increasing temperature favours the forward reaction (AM80.565).

manganosite and quartz to tephroite
2MnO + SiO2 ⇌ Mn2+2SiO4
(AM80.571)

manganosite and rhodonite to tephroite
MnO + Mn2+SiO3 ⇌ Mn2+2SiO4
(AM80.571)

rhodonite and braunite to tephroite and O2
10Mn2+SiO3 + 2Mn2+Mn3+6O8(SiO4) ⇌ 12Mn2+2(SiO4) + 3O2
(AM80.571).

rhodonite and rhodochrosite to tephroite and CO2
Mn2+SiO3 + Mn(CO3) → Mn2+2(SiO4) + CO2
Increasing temperature favours the forward reaction (DHZ 1A p344, 348, AM80.571).

tephroite and quartz to rhodonite
Mn2+2(SiO4) + SiO2 → 2Mn2+SiO3
(AM80.571)

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