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Formula: Mn2+5(SiO4)2(OH)2
Nesosilicate (insular SiO4 groups), humite group,
manganese-bearing mineral
Alleghanyite forms a discontinuous solid solution with chondrodite.
Epitaxy: leucophanite has been found as an epitaxial growth on
alleghanyite.
Crystal System: Monoclinic
Specific gravity: 3.93 to 4.02 measured, 3.96 calculated
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)
Common impurities: Ti,Al,Fe,Mg,Ca,F
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)
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