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Formula: Mn2+Mn2+Fe2+2Fe3+2(PO4)4(OH)2.8H2O
Hydrated phosphate with hydroxyl, jahnsite subgroup,
jahnsite group,
manganese-bearing mineral
Crystal System: Monoclinic
Specific gravity: 2.654 calculated
Hardness: 4
Streak: Pale greenish brown
Colour: Dark brown
Luminescence: No fluorescence under UV
Environments
Localities
At the type locality, the Malpensata pegmatitic dyke, Piona Peninsula, Colico, Lecco Province, Lombardy, Italy, the
albite pegmatite shows
an interesting primary and
secondary phosphate association and is the type locality for
karenwebberite. The
primary phosphate association is dominated by minerals belonging
to the
graftonite group in close association with
sarcopside and triphylite.
Arrojadite-(KNa),
arrojadite-(BaNa), wyllieite,
ferrowyllieite and
karenwebberite are rare
primary phosphates occurring in the association. Alteration of
primary phosphates in the dike is quite pervasive and results in
the formation of about 15 secondary phosphates. Among these are
jahnsite-(CaMnFe) and
jahnsite-(CaMnMn), which occur as prismatic crystals growing on the
surfaces of hydrothermal cavities.
Jahnsite-(MnMnFe) was found in one cm diameter hydrothermal cavity, hosted in the
albite matrix. The dark orange-brown crystals are prismatic, up to 0.2 mm in
length, and grow on the surface of the cavity in close association with
rockbridgeite and
mitridatite
(CM 57.2.225-233).
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