Secondary Zn-bearing phosphate minerals associated with alteration of phosphophyllite at Hagendorf-Süd, Bavaria

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Marco E. Ciriotti
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Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Secondary Zn-bearing phosphate minerals associated with alteration of phosphophyllite at Hagendorf-Süd, Bavaria

Messaggio da Marco E. Ciriotti » sab 12 mag, 2018 10:25

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Referenza:
▪ Grey, I.E., Keck, E., Macrae, C.M., Glenn, A.M., Mumme, W.G., Kampf, A.R., Cashion, J.R. (2018): Secondary Zn-bearing phosphate minerals associated with alteration of phosphophyllite at Hagendorf-Süd, Bavaria. European Journal of Mineralogy, 30, (in press).

Abstract:
At the Hagendorf-Süd pegmatite, Bavaria, phosphophyllite plays an important paragenetic role in the formation of numerous Zn-bearing secondary phosphate minerals.We report the results of studies conducted on the alteration of phosphophyllite in a heavily corroded triphylite nodule from the 67m level of the pegmatite mine. The most abundant secondary phosphate minerals associated with phosphophyllite are oxidized schoonerite-group minerals, including the new mineral schmidite, [5]Zn[6]((Fe3+0.5Mn2+0.5)2ZnFe3+)(PO4)3(OH)3(H2O)6·2H2O, whose formation is contemporaneous with phosphophyllite. The phosphophyllite crystals in the nodule are characteristically covered with a composite rind of epitaxial Zn-rich jahnsite crystals on the phosphophyllite surface overlain by an orange-yellow amorphous Zn-bearing ferric phosphate and/or green mitridatite. The jahnsite crystals are compositionally zoned into two phase domains based on their Ca/Zn contents. The low-Zn domains are jahnsite-(CaMnMn) whereas the high-Zn domains correspond to a potentially new species, “jahnsite-(CaMnZn)”. The amorphous phase coating the jahnsite crystals has Zn levels similar to those in jahnsite, and is probably derived from its alteration. Its properties are consistent with it being a Zn-bearing santabarbaraite. Younger idiomorphic crystals of laueite, earlshannonite and zincostrunzite occur on the phosphophyllite rind. The laueite crystals have a surface layer, ∼5μm thick, in which Mn is replaced completely by a 1:1 atomic mixture of Zn+Mg, giving a composition (Zn0.5Mg0.5)Fe3+2(PO4)2(OH)2·8H2O. The composition of the surface phase is explained by configurational-entropy stabilisation. The earlshannonite crystals commonly grow epitactically on the laueite, yet despite the close association, they are Zn-bearing only, with very low Mg contents. Single-crystal structure refinements have been done on both (Zn+Mg)-bearing laueite and Zn-bearing earlshannonite and all H atoms were located in both cases. Structural models are presented for the epitactic growth of jahnsite on phosphophyllite and earlshannonite on laueite.
Marco E. Ciriotti

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