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

Published online by Cambridge University Press:  05 July 2018

A. Guastoni ,
F. Nestola ,
G. Mazzoleni  and
P. Vignola
A. Guastoni
Affiliation:
Dipartimento di Geoscienze, Università degli Studi di Padova, Via Giotto 1, I-35137 Padova, Italy Centro di Ateneo per i Musei, Università di Padova, Via Orto Botanico 15, I-35122 Padova, Italy
F. Nestola*
Affiliation:
Dipartimento di Geoscienze, Università degli Studi di Padova, Via Giotto 1, I-35137 Padova, Italy di Geoscienze e Georisorse, CNR Sezione di Padova, Via Giotto 1, I-35137 Padova, Italy
G. Mazzoleni
Affiliation:
Stazione Valchiavenna per lo Studio dell’Ambiente Alpino, Via dei Cappuccini, I-23022 Chiavenna, Italy
P. Vignola
Affiliation:
CNR-Istituto per la dinamica dei processi ambientali, Via Mario Bianco 9, I-20131 Milano, Italy
*
E-mail: [email protected]
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Abstract

Mn-rich graftonite, (Ca,Mn2+)(Fe2+,Mn2+)2(PO4)2, ferrisicklerite, Li1–x(Fe3+,Mn2+)PO4, manganoan apatite, (Ca,Mn2+,Fe2+Mg)(PO4)3Cl, staně kite, 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 beendocumen ted in pegmatites. Inthe 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 AlpsItalySoè ValleypegmatiteX-ray diffractionmicroprobephosphates
Type
Research Article
Information
Mineralogical Magazine , Volume 71 , Issue 5 , October 2007 , pp. 579 - 585
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2007

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