Pleysteinite, [.sub.10 â 4H.sub.2O, the Al analogue of benyacarite, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany

• Published in: European journal of mineralogy (Stuttgart) 2023-03, Vol.35 (2), p.189
• Main Authors: Grey, Ian E, Hochleitner, Rupert, Rewitzer, Christian, Kampf, Anthony R, MacRae, Colin M, Gable, Robert W, Mumme, William G, Keck, Erich, Davidson, Cameron
• Format: Article
• Language: English
• Subjects: Composition; Crystals; Mineralogical research; Pegmatites; Structure; Testing
• ISSN: 0935-1221

Pleysteinite, [(H.sub.2 O).sub.0.5 K.sub.0.5 ].sub.2 Mn.sub.2 Al.sub.3 (PO.sub.4).sub.4 F.sub.2 (H.sub.2 O).sub.10 â« 4H.sub.2 O, is the aluminium analogue of benyacarite, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany. It was found in specimens of altered zwieselite, in association with nordgauite, fluellite, rockbridgeite, pyrite and columbite. Pleysteinite occurs as isolated and small aggregates of colourless, stubby prisms that are typically 10 to 30 µm wide and up to 100 µm long. The crystals are flattened on {010} and bounded by {111}, {100} and {001} planes. The calculated density is 2.34 g cm.sup.-3 . Optically, pleysteinite crystals are biaxial (+), with α=1.566(2), β=1.580(2), γ=1.600(2) (measured in white light) and 2V(meas.) = 80(1).sup." . The empirical formula from electron microprobe analyses and structure refinement is [(H.sub.2 O).sub.0.50 K.sub.0.50 ].sub.2 (Mn.sub.1.20 Mg.sub.0.49 Fe0.272+Zn.sub.0.05).sub.â2.01 (Al.sub.1.63 Fe0.203+Ti0.194+).sub.â2.02 (Al.sub.0.56 Ti0.444+) (PO.sub.4).sub.4.02 [F.sub.0.58 O.sub.0.31 (OH).sub.0.11 ].sub.2 (H.sub.2 O).sub.10 â« 3.92H.sub.2 O. Pleysteinite has orthorhombic symmetry, with space group Pbca and unit-cell parameters a = 10.4133(8) Ã, b=20.5242(17) Ã, c=12.2651(13) Ã, V=2621.4(4) Ã.sup.3 and Z=4. The crystal structure was refined using single-crystal data to wR.sub.obs =0.054 for 1692 reflections with I3Ï(I). The crystal structure contains corner-connected linear trimers of Al-centred octahedra that share corners with PO.sub.4 tetrahedra to form 10-member rings parallel to (010). K.sup.+ cations and water molecules are located in the rings. Additional corner-sharing of the PO.sub.4 tetrahedra with Mn(H.sub.2 O).sub.4 O.sub.2 octahedra occurs along [010] to complete the 3D framework structure.