Regerite, KFe ₆(PO ₄) ₄(OH) ₇(H ₂O) ₆  ⋅  4H ₂O, the first new mineral species from the Kreuzberg pegmatite, Pleystein, Oberpfalz, Bavaria, Germany

Regerite, KFeinline-formula₆(POinline-formula₄)₄(OH)inline-formula₇(Hinline-formula₂O)inline-formula₆ inline-formula⋅ 4Hinline-formula₂O, 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 inline-formulaµm wide and up to 0.1 mm long. The crystals are flattened on inline-formula{100} and elongated along [001], and they display the forms inline-formula{100}, {010} and {011}. The measured density is 2.69(2) g cminline-formula⁻³. Optically, regerite crystals are biaxial (inline-formula+), with inline-formulaα=1.670(5), inline-formulaβ=1.690(5) and inline-formulaγ=1.730(5) (measured in white light), and 2inline-formulaV (meas) is 76(2)inline-formula^∘. The empirical formula from electron microprobe analyses and crystal structure refinement is Kinline-formula_0.95(Feinline-formula $M33inlinescrollmathml{}_{\mathrm{normal\; 5.66}}^{\mathrm{normal\; 3}+}$ 18pt17ptsvg-formulamathimgf08d9cf95de36c6c2e907d3507055e88 ejm-35-805-2023-ie00001.svg18pt17ptejm-35-805-2023-ie00001.png Tiinline-formula_0.45)_Σ6.11(POinline-formula₄)_3.95(OH)inline-formula₇[(Hinline-formula₂O)inline-formula_5.33(OH)inline-formula_0.88]inline-formula_Σ6.21 inline-formula⋅ 4Hinline-formula₂O. Regerite has monoclinic symmetry, with space group inline-formula $M43inlinescrollmathmlP{\mathrm{normal\; 2}}_{\mathrm{normal\; 1}}/c$ 34pt14ptsvg-formulamathimg387f17258900b89bb2b7f34c3d41ad1f ejm-35-805-2023-ie00002.svg34pt14ptejm-35-805-2023-ie00002.png and unit-cell parameters inline-formulaa=15.408(11) Å, inline-formulab=17.311(11) Å, inline-formulac=9.870(11) Å, inline-formulaβ=95.42(2)inline-formula^∘, inline-formulaV=2621(3) Åinline-formula³ and inline-formulaZ=4. The crystal structure was refined using synchrotron single-crystal data to inline-formulawR_obs=0.065 for 6088 reflections with inline-formulaI>3σ(I). The structure type has not been previously reported. It is made of heteropolyhedral layers parallel to inline-formula{100} that consist of Fe-centred octahedra and POinline-formula₄ tetrahedra. The layers are interconnected via edge-shared octahedral dimers to form slit-like channels along [001] that are occupied by Kinline-formula⁺ ions and water molecules.