Galeaclolusite, [Al6(AsO4)3(OH)9(H2O)4]•8H2O, a new bulachite-related mineral from Cap Garonne, France

Galeaclolusite, [Al-6(AsO4)(3)(OH)(9)(H2O)(4)]center dot 8H(2)O, is a new secondary hydrated aluminium arsenate mineral from Cap Garonne, Var, France. It forms crusts and spheroids of white fibres up to 50 mu m long by 0.4 mu m wide and only 0.1 mu m thick. The fibres are elongated along [001] and flattened on (100). The calculated density is 2.27 g.cm(-3). Optically, galeaclolusite is biaxial with alpha = 1.550(5), beta not determined, gamma = 1.570(5) (white light) and partial orientation: Z = c (fibre axis). Electron microprobe analyses coupled with crystal structure refinement results gives an empirical formula based on 33 O atoms of Al5.72Si0.08As2.88O33H34.12. Galeaclolusite is orthorhombic, Pnma, with a = 19.855(4), b = 17.6933(11), c = 7.7799(5) angstrom, V = 2733.0(7) angstrom(3) and Z = 4. The crystal structure of galeaclolusite was established from its close relationship to bulachite and refined using synchrotron powder X-ray diffraction data. It is based on heteropolyhedral layers, parallel to (100), of composition Al-6(AsO4)(3)(OH)(9)(H2O)(4) and with H-bonded H2O between the layers. The layers contain [001] spiral chains of edge-shared octahedra, decorated with corner-connected AsO4 tetrahedra, that are the same as in the mineral liskeardite.