Rewitzerite, K(H2O)Mn2(Al2Ti)(PO4)4[O(OH)](H2O)10⋅4H2O, a new monoclinic paulkerrite-group mineral, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany

Abstract Rewitzerite, K(H 2 O)Mn 2 (Al 2 Ti)(PO 4 ) 4 [O(OH)](H 2 O) 10 ⋅4H 2 O, is a new monoclinic member of the paulkerrite group, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany. It was found in specimens of altered zwieselite, in association with rockbridgeite. Rewitzerite forms clusters of colourless elongated hexagonal-shaped prisms, up to 0.1 mm long. The crystals are flattened on {010} and elongated along [100], with forms {010}, {001}, {111} and { $\bar{1}$ 11}. The calculated density is 2.33 g⋅cm –3 . Optically, rewitzerite crystals are biaxial (+), with α = 1.585(2), β = 1.586(2), γ = 1.615(2) (measured in white light) and 2V(meas) = 25(2)°. The empirical formula from electron microprobe analyses and structure refinement is A 1 [K 0.77 (H 2 O) 0.23 ] A2 [H 2 O] M 1 (Mn 2+ 0.82 Mg 0.64 Fe 3+ 0.43 □ 0.11 ) Σ2.00 M 2+ M 3 (Al 1.51 Ti 4+ 1.06 Fe 3+ 0.43 ) Σ3.00 (PO 4 ) 4 X [(OH) 0.54 F 0.42 O 1.04 ] Σ2.00 (H 2 O) 10 ⋅4H 2 O, where □ = vacancy. Rewitzerite has monoclinic symmetry with space group P 2 1 / c and unit-cell parameters a = 10.444(2) Å, b = 20.445(2) Å, c = 12.2690(10)Å, β = 90.17(3)°, V = 2619.8(6) Å 3 and Z = 4. The crystal structure was refined using synchrotron single-crystal data to wR obs = 0.068 for 5894 reflections with I > 3σ( I ). The crystal structure has the same topology as that for orthorhombic paulkerrite-group minerals but differs primarily in having an ordering of K + and H 2 O molecules in different A sites, whereas they are disordered at a single A site in the orthorhombic members of the group.