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

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, ^[5]Zn^[6]$((Fe0.53+Mn0.52+)2ZnFe3+)(PO4)3(OH)3(H2O)6⋅2H2O$⁠, whose formation is contemporaneous with phosphophyllite. The phosphophyllite crystals in the nodule are characteristically covered with a composite rind of 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 $(Zn0.5Mg0.5)Fe23+(PO4)2(OH)2⋅8H2O$⁠. 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.