New secondary phosphate mineral occurrences and their crystal chemistry, at the Hagendorf Süd pegmatite, Bavaria

Abstract

First occurrences from the Hagendorf Süd pegmatite of the secondary phosphate minerals kenngottite, Mn32+Fe43+(PO4)4(OH)6(H2O)2; allanpringite, Fe33+(PO4)2(OH)3 ⚫ 5H2O; iangreyite, Ca2Al7(PO4)2(PO3OH)2(OH,F)15 ⚫ 8H2O; and nizamoffite, MnZn2(PO4)2(H2O)4, are reported with characterisation of their crystal chemistry and phase associations. A synchrotron single-crystal structure refinement for kenngottite shows that it has the same level of disordering (75 %/25 %) of Fe3+ in adjacent octahedra along the 5 Å axis as for type-locality kenngottite from Krásno, Czech Republic. This is explained in terms of 1 : 1 fine-scale mixing of domains of ordered kenngottite-type and souzalite-type structures. Allanpringite occurs in an unusual epitaxial relationship to associated strunzite. The epitaxy is explained by the close metrical and structural match of common planes in the two minerals, (010) for allanpringite and (100) for strunzite. Iangreyite occurs in close association with perhamite in 30 µm spheroids. The characterisation results support a paragenesis of iangreyite from perhamite by selective leaching of silica from the layer structure of perhamite and rejoining of the layers by fusion of AlO4 tetrahedra from adjacent layers into AlO2(OH)3 trigonal bipyramids.