Monoclinic Pleysteinite and Hochleitnerite from the Hagendorf-Süd Pegmatite: Synchrotron Microfocus Diffraction Studies on Twinned Members of Paulkerrite-Group Minerals

Abstract Single-crystal X-ray diffraction data sets for pleysteinite, (H2O)KMn2Al3(PO4)4F2(H2O)10·4H2O, and hochleitnerite, (H2O)KMn2(Ti2Fe)(PO4)4O2(H2O)10·4H2O, collected using a synchrotron microfocus beam, gave monoclinic unit cells, in contrast to previously reported orthorhombic cells obtained using laboratory-based sealed-tube diffractometer data. The differences are attributed to twinning in crystals of the two minerals. As the laboratory diffractometer beam diameter of the sealed-tube SXRD is larger than the crystals that were analyzed, the diffraction results were averaged over the different twin domains, whereas those obtained with the synchrotron microfocus beam gave diffraction data wholly or predominantly from a single domain. Data from the synchrotron datasets give a = 10.440(5) Å, b = 20.588(5) Å, c = 12.234(2) Å, β = 90.38(1)° for pleysteinite and a = 10.547(2) Å, b = 20.577(4) Å, c = 12.373(2) Å, β = 90.09(3)° for hochleitnerite. The crystal structures for both minerals were refined in P21/c, giving Robs = 0.059, 0.058 for 5951, 6718 reflections with I 3σ(I), respectively. The ideal formulae for the two minerals in this setting conform to the general formula A1A2M12M22M3(PO4)4X2(H2O)10·4H2O, with K and H2O ordered at separate A sites, whereas K and H2O were disordered over a single A site in the orthorhombic models (space group Pbca). Crystal-chemical aspects of the paulkerrite-group minerals, pleysteinite, hochleitnerite, paulkerrite, rewitzerite, and macraeite, are compared and discussed.