Mn-rich graftonite, ferrisicklerite, stanekite and Mn-rich vivianite in a granitic pegmatite at Soe Valley, central Alps, Italy

摘要

Mn-rich graftonite, (Ca,Mn~(2+))(Fe~(2+),Mn~(2+))2(P04)2, ferrisicklerite, Li,_x(Fe~(3+),Mn~(2+))PO_4, manganoan apatite, (Ca,Mn~(2+),Fe~(2+)Mg)(P04)3Cl, stanekite, Fe~(3+)Mn~(2+)O(PO_4) and Mn-rich vivianite, (Fe~(2+))3(PO_4)_2 centre dot 8H_O, occurring in a granitic pegmatite at Soe Valley (central Alps, Italy) were characterized by powder and single-crystal X-ray diffraction (XRD) and electron microprobe analyses. Geochemically, the Mn-rich graftonite phases are poorly evolved Fe/Mn-phosphates of rare-earth elements-lithium (REE-Li) granitic pegmatites. The assemblage Mn-rich graftonite + ferrisicklerite + stanekite has rarely been documented in pegmatites. In the Soe Valley pegmatite, ferrisicklerite forms exsolution lamellae with Mn-rich graftonite associated with manganoan apatite and stanekite. Graftonite is associated with Mn-rich vivianite. Powder and single-crystal XRD data indicate that the unit-cell volume of graftonite increases as a function of Mn~(2+) content. Stanekite shows a distinctly smaller unit-cell volume with respect to previously reported stanekites, probably due to reduced Mn~(2+). Vivianite with significant Mn~(2+) has a unit-cell volume similar to nearly Mn-free vivianite. The formation of Mn-rich graftonite and manganoan apatite is related to destabilization of Mn-rich almandine and biotite during pegmatite formation. Ferrisicklerite forms exsolution lamellae along the 010 cleavage planes of Mn-rich graftonite, whereas stanekite forms by alteration of ferrisicklerite and Mn-rich vivianite due to circulation of late-stage hydrothermal fluids.