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

Abstract

Regerite, KFe6(PO4)4(OH)7(H2O)6 ⋅ 4H2O, 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 µm 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 α=1.670(5), β=1.690(5) and γ=1.730(5) (measured in white light), and 2V (meas) is 76(2)∘. The empirical formula from electron microprobe analyses and crystal structure refinement is K0.95(Fe5.663+Ti0.45)Σ6.11(PO4)3.95(OH)7[(H2O)5.33(OH)0.88]Σ6.21 ⋅ 4H2O. Regerite has monoclinic symmetry, with space group P21/c and unit-cell parameters a=15.408(11) Å, b=17.311(11) Å, c=9.870(11) Å, β=95.42(2)∘, V=2621(3) Å3 and Z=4. The crystal structure was refined using synchrotron single-crystal data to wRobs=0.065 for 6088 reflections with I>3σ(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.