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

Authors: Guastoni, A.; Nestola, F.; Mazzoleni, G.; Vignola, P.

Source: Mineralogical Magazine, Volume 71, Number 5, October 2007 , pp. 579-585(7)

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Abstract:

Mn-rich graftonite, (Ca,Mn²⁺)(Fe²⁺,Mn²⁺)₂(PO₄)₂, ferrisicklerite, Li_1-x(Fe³⁺,Mn²⁺)PO₄, manganoan apatite, (Ca,Mn²⁺,Fe²⁺Mg)(PO₄)₃Cl, staněkite, Fe³⁺Mn²⁺O(PO₄) and Mn-rich vivianite, (Fe²⁺)₃(PO₄)₂˙8H₂O, occurring in a granitic pegmatite at Soè 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 + staněkite has rarely been documented in pegmatites. In the Soè Valley pegmatite, ferrisicklerite forms exsolution lamellae with Mn-rich graftonite associated with manganoan apatite and staněkite. 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 Mn²⁺ content. Staněkite shows a distinctly smaller unit-cell volume with respect to previously reported staněkites, probably due to reduced Mn²⁺. Vivianite with significant Mn²⁺ 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 staněkite forms by alterationof ferrisicklerite and Mn-rich vivianite due to circulation of late-stage hydrothermal fluids.

Keywords: CENTRAL ALPS; ITALY; SOÈ VALLEY; PEGMATITE; X-RAY DIFFRACTION; MICROPROBE; PHOSPHATES

Document Type: Research article

DOI: http://dx.doi.org/10.1180/minmag.2007.071.5.579

Publication date: 2007-10-01

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