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Formula:
Formerly the formula was given as
(Na,Ca)(Mn,Mg,Fe2+)(Fe3+,Mn2+)2(PO4)3,
but it has been redefined as
☐NaMnFe3+2(PO4)3
(CM 59.243-263(2021)).
Anhydrous phosphate, alluaudite group, forms a series with
ferroalluaudite,
manganese-bearing mineral
Crystal System: Monoclinic
Specific gravity: 3.45
Hardness: 5½ to 6
Streak: Dirty yellow
Colour: Dirty yellow to brownish yellow, greyish green, black
Solubility: Soluble in acids
Melting point: Approximately 1000oC (AM 50.1647)
Environments
Alluaudite is a very common secondary mineral in
granite
pegmatites, formed by sodium metasomatism of triphylite -
lithiophilite, heterosite
– purpurite or
ferrisicklerite
(Webmin, Dana, HOM). It is an alteration product of varulite and
arrojadite-(KFe) (Mindat), and it alters to
heterosite and purpurite with
leaching of sodium Na and oxidation of iron Fe and manganese Mn (Dana). It also occurs in phosphatic nodules
in shale (HOM).
Associated minerals include triphylite,
arrojadite, satterlyite,
wicksite, wolfeite and
pyrite (HOM).
Localities
At the Kibingo pegmatite, Gitarama – Kibuye Road, Southern Province, Rwanda, a sample from the
granitic
pegmatite shows a striking assemblage of an
alluaudite-group mineral with heterosite. The core of the
alluaudite-group mineral is strongly pleochroic in blue green to pinkish grey hues, and is rimmed by an
irregular border, weakly pleochroic in yellow brown to greenish yellow tints. This mechanism represents the
progressive transition of hagendorfite to true
alluaudite, and results from oxidation of Fe2+ to Fe3+
(CM 42.697-704).
The type locality is Varuträsk, Skellefte mining district, Skellefteå, Västerbotten County, Sweden.
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