Alleghanyite

nesosilicate

chondrodite

Formula: Mn2+5(SiO4)2(OH)2
Nesosilicate (insular SiO4 groups), humite group
Alleghanyite forms a discontinuous solid solution with chondrodite.
Epitaxy: leucophanite has been found as an epitaxial growth on alleghanyite.
Specific gravity: 3.93 to 4.02
Hardness: 5 to 5½
Streak: Very pale pink
Colour: Pinkish to reddish brown, deep pink, greyish pink
Solubility: Soluble in hydrochloric acid leaving a silica gel (Dana)
Environments:

Metamorphic environments
Hydrothermal environments

Alleghanyite requires water-rich conditions to form in silica-undersaturated rocks during regional metamorphism (AM 68.951). It occurs in manganese-rich skarn with other manganese silicates and carbonates, and it is also hydrothermally deposited in lenses in manganese-bearing veins (Mindat) with tephroite and spessartine (Dana).

At the San Jose mine, California, USA, alleghanyite has been found in a boulder with tephroite, hausmannite, pyrochroite, ganophyllite, rhodochrosite, baryte, psilomelane (AM17.1-15) and alabandite (AM20.25-35).

At Franklin/Sterling Hill, New Jersey, USA, alleghanyite occurs as isolated crystals in the Franklin marble, and also in veins cross-cutting franklinite ore near pegmatites in a metamorphosed stratiform Zn-Mn deposit, formed in apparent equilibrium with rare species such as kolicite, holdenite, magnussonite, adelite, kraisslite and chlorophoenicite. Aside from these uncommon arsenates, other species associated with alleghanyite are franklinite, willemite, baryte and carbonates, all of secondary origin (AM70.379). At this locality alleghanyite always has some zinc content, but never more than 0.2 Zn per 2 Si. Some alleghanyite co-exists with zincite, but mostly willemite is the only associated zinc phase. Material from Sterling Hill is very rich in magnesium.

At the type locality, Bald Knob, North Carolina, USA, the environment is probably metamorphosed sediment with an estimated temperature of formation 575 +/- 40oC, pressure 5 +/- 1 kbar (AM66.278). Alleghanyite occurs here in lenses in a manganese-bearing calcite vein, mostly as grains embedded in the calcite and commonly intergrown with galaxite. It is chemically incompatible with quartz, and does not occur here associated either with quartz or with tephroite, although tephroite also occurs at this locality (AM17.1-15).

Alteration

manganhumite and H2O to alleghanyite and quartz
5Mn7(OH)2(SiO4)3 + 2H2O = 7Mn5(OH)2(SiO4)2 + SiO2 (AM68.975)

manganhumite and rhodochrosite and H2O to alleghanyite and CO2
2Mn7(OH)2(SiO4)3 + MnCO3 = 3Mn5(OH)2(SiO4)2 + CO2 (AM68.976)

Common impurities: Ti,Al,Fe,Mg,Ca,F

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