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 67 m level of the pegmatite mine. The most abundant secondary phosphate minerals associated with phosphophyllite are oxidized schoonerite-group minerals, including the new mineral schmidite, Zn, whose formation is contemporaneous with phosphophyllite. The phosphophyllite crystals in the nodule are characteristically covered with a composite rind of epitactic 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, ∼5 µm thick, in which Mn is replaced completely by a 1:1 atomic mixture of Zn + Mg, giving a composition . 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.
Secondary Zn-bearing phosphate minerals associated with alteration of phosphophyllite at Hagendorf-Süd, Bavaria
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Ian E. Grey, Erich Keck, Colin M. MacRae, A. Matt Glenn, W. Gus Mumme, Anthony R. Kampf, John D. Cashion; Secondary Zn-bearing phosphate minerals associated with alteration of phosphophyllite at Hagendorf-Süd, Bavaria. European Journal of Mineralogy doi: https://doi.org/10.1127/ejm/2018/0030-2763
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