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Formula: Mn2+2(SiO4)
Nesosilicate (insular SiO4 groups),
olivine group, forms a series with fayalite,
manganese-bearing mineral.
Crystal System: Orthorhombic
Specific gravity: 3.87 to 4.12 measured, 4.15 calculated
Hardness: 6
Streak: pale grey
Colour: grey, olive green, flesh red or reddish brown, dark brown
Environments:
Tephroite is a manganese-bearing 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.
Localities
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Tephroite is a primary silicate that occurs as greyish
areas in black manganese oxide aggregates at the contact between granite
and magnetite-rich skarn at the
Two Mile deposit
(AJM 22.1.36).
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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