Crystal chemistry of the variscite and metavariscite groups: Crystal structures of synthetic CrAsO4 center dot 2H(2)O, TlPO4 center dot 2H(2)O, MnSeO4 center dot 2H(2)O, CdSeO4 center dot 2H(2)O and natural bonacinaite, ScAsO4 center dot 2H(2)O

摘要

We report the crystal structures of four synthetic members of the variscite group (space group type Pbca) and of bonacinaite, the first naturally occurring scandium arsenate member of the metavariscite group. All structures were determined using single-crystal X-ray intensity data. The following members were all synthesised under either mild hydrothermal conditions or by wet-chemical methods (<90 degrees C). CrAsO4 center dot 2H(2)O (deep green): a = 8.894(2), b= 9.946(2), c= 10.206(2) angstrom and V= 902.8(3) angstrom(3); R-1 = 2.14%. Tl3+PO4 center dot 2H(2)O (colourless): a = 10.2848(7), b = 8.8578(6), c= 10.3637(7) angstrom and V= 944.14(11) angstrom(3) (data at -173 degrees C); R-1 = 2.56%. MnSeO4 center dot 2H(2)O (pale pink): a = 10.441(2), b = 9.2410(18), c = 10.552(2) angstrom and V= 1018.1(3) angstrom(3); R-1 = 2.19%. A different method of preparation of MnSeO4 center dot 2H(2)O yielded crystals with very similar unit-cell parameters, a = 10.4353(5), b= 9.2420(5) and c= 10.5349(6) angstrom; R-1 = 2.25%. CdSeO4 center dot 2H(2)O (colourless) has a = 10.481(1), b = 9.416(1), c = 10.755(1) angstrom and V= 1061.4(2) angstrom(3); R-1 =1.53%. The thermal behaviour of the two selenate members was studied by a combination of DSC and TG, supplemented by PXRD. Bonacinaite (IMA2018-056), metavariscite-type natural (Sc,Al)(As,P)O-4 center dot 2H(2)O (ideally ScAsO4 center dot 2H(2)O), crystallises in the space group P2(1)/n, with a = 5.533(1), b = 10.409(2), c = 9.036(2) angstrom, beta= 91.94(3)degrees and V= 520.11(18) angstrom(3); R-1 = 3.66%. The structural formula, supported by chemical analysis, is (Sc0.807(1)Al0.193)(As0.767(7)P0.233)O-4 center dot 2H(2)O. All structures are based on frameworks built by corner-sharing of TO4 tetrahedra (T = P5+, As5+ or Se6+) with MO4(H2O)(2) (M = Mn2+, Cd2+, Cr3+, Sc3+ or Tl3+) octahedra. The flexible frameworks are reinforced by partly bifurcated, strong to weak hydrogen bonds.