Regerite, KFe6(PO4)4(OH)7(H2O)6 ⋅ 4H2O, the first new mineral species from the Kreuzberg pegmatite, Pleystein, Oberpfalz, Bavaria, Germany

Regerite, KFe6(PO4)(4)(OH)(7)(H2O)(6) & sdot; 4H(2)O, is the first new mineral species to be characterised from the Kreuzberg pegmatite, Pleystein, in the Oberpfalz, Bavaria. It was found in vugs on a specimen of drusy quartz, associated with rockbridgeite, strengite and phosphosiderite. Regerite occurs as clusters of yellowish-green prisms, typically 5 to 20 mu m wide and up to 0.1 mm long. The crystals are flattened on {100} and elongated along [001], and they display the forms {100}, {010} and {011}. The measured density is 2.69(2) g cm(-3). Optically, regerite crystals are biaxial (+), with alpha = 1.670(5), beta = 1.690(5) and gamma = 1.730(5) (measured in white light), and 2V (meas) is 76(2)(degrees). The empirical formula from electron microprobe analyses and crystal structure refinement is K-0.95(Fe5.663+ Ti0.45)(Sigma 6.11)(PO4)(3.95)(OH)(7)[(H2O)(5.33)(OH)(0.88)](Sigma 6.21) & sdot; 4H(2)O. Regerite has monoclinic symmetry, with space group P2(1)/c and unit-cell parameters a=15.408(11) & Aring;, b = 17.311(11) & Aring;, c = 9.870(11) & Aring;, beta = 95.42(2)(degrees), V = 2621(3) & Aring;(3) and Z = 4. The crystal structure was refined using synchrotron single-crystal data to wR(obs)=0.065 for 6088 reflections with I > 3 sigma(I). The structure type has not been previously reported. It is made of heteropolyhedral layers parallel to {100} that consist of Fe-centred octahedra and PO4 tetrahedra. The layers are interconnected via edge-shared octahedral dimers to form slit-like channels along [001] that are occupied by K+ ions and water molecules.