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Formula: Mn2+Mn2+Fe2+2Fe3+2(PO4)4(OH)2.8H2O
Hydrated phosphate with hydroxyl, jahnsite group, manganese-bearing mineral
Specific gravity: 2.654 calculated
Hardness: 4
Streak: Pale greenish brown
Colour: Dark brown
Luminescence: No fluorescence under UV



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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