Crystal structure of dicalcium (cobalt, magnesium) diarsenate dihydrate, Ca2(Coo.532 Mgo.468 )[AsO4]2 · 2H2O, hydrogen bonding in talmessite

As2Ca2Coo.532H4Mgo. 4680io, triclinic, P\ (no. 2), a = 5.884(1) A , b = 6.981(1) A , c = 5.564(1) A , a = 97.33(2)°, fl = 108.93(2)°,y = 108.18(2)°, V= 198.6 A 3 , Z = 1, Rgl(F) = 0.017, wR^F) = 0.047, T= 293 K. Source of material The crystals were taken from a mine near Bou-Azzer, Morocco. In contrast to the crystals investigated by Dunn, Sturman and Nelen, which had much more Mg than Co [1], our samples contain according to electron microprobe analyses in the average more Co than Mg (0.579:0.473). Experimental details The average Ha of the measured X-ray intensities is 76.6. The hydrogen atoms could be refined isotropically, nevertheless the measurements were carried out at room temperature. The largest peak and hole in the difference density map were 0.778 and -0.854 e/A, respectively. Despite die average If II equals 0.808, the adequate space group isPl . The refinement in PI with racemic twinning resulted in the same structure with slightly higher ^-values {RgiF) = 0.020, wR^F 2 ) = 0.055). Discussion According to Catti et al. [2] and Hawthorne & Ferguson [3] the triclinic compound of the chemical formula Ca2(Mg,Co)(AsC>4)2 2H2O is called talmessite, the monoclinic one roselite [3]; the name/7-roselite [4] will not be used by us. The top figure shows the krOhnkite-type chains. According to M. Fleck the structure should be assigned to the structural type A [5], roselite to structural type D. The high significance of the hydrogen positions allows an unambiguous identification of the water molecules and the hydrogen bonding [2]. 04 is the acceptor of two hydrogen bonds originating from the two hydrogens of the water molecule. The hydrogen bond 0W1-H2 0 4 (2.616 A ) connects twochains within the layers parallel (101), whereas OW1-H1— 0 4 (2.579A) connects the layers. There are, however, no hydrogen bonds within the chains [5]. The bottom figure shows a least squares fit between krohnkite-type chains of talmessite (bold lines) and roselite (dashed lines) [3]. The chains are very similar indeed. There seems to exist a solid solution seria between Mg and Co [ 1 ], whose relative content has apparently no influence on the crystal structure. There is an analysis of the miscibility in minerals, but not in crystals grown in the laboratory [1]. The average e.s. d. for metal-metal distances is 0.001 A , for metal-oxygen and oxygenoxygen distances 0.002 A and for the oxygen-hydrogen distances 0.04 A . The average e.s. d. of angles between metals with no oxygens in is 0.02°, with one oxygen 0.04° and with 2 oxygens 0.1°, in the water molecule it is 3°. Table 1. Data collection and handling. Crystal: Wavelength: PDiffractometer, scan mode: /V(MJ>roeasured, WltyJuniqi*: Criterion for labs, N(hkl) N(param)rcfmcd: Programs: pink plate, size 0.05 x 0.175 x 0.25 mm Mo K„ radiation (0.71073 A) 107.92 cm" Nonius CAD4,0-20-scan 59.92° 2318,1159 / „ b s > 2 a ( W , 1141 80 SHELXS-97 [6], SHELXL-97 [9], SHELXTL-PLUS [8] * Correspondence author (e-mail: e.paulus@kristall.uni-frankfurt.de) 342 Ca2(Coo.532Mgo.468)[As04]2 • 2H20 Table 2. Atomic coordinates and displacement parameters (in À).