Peretaite, CaSbnOo(OH)2(SO4)2 '2HrOz its atomic arrangement and twinning

  • Su-vro MeNcnrrrr aNo, CnseRn SenBLt
  • Published 2007


Peretaite, CaSboOn(OHL(SOn), '2H2O, crystallizes in the monoclinic space group C2/c, with a: 24.665, b: 5.6006, c: 10.185A, F:95.98'. The structure was determined by Patterson and direct methods and refned by least-squares tecbnique to a final R index of 0.037. The dominant structural feature is the arrangement of the two independent Sb polyhedra into sheets parallel to (100). The Ca ion is coordinated by six O and two HrO in a square antiprism. These polyhedra are edge414 e6ms1-linked to SO4 groups into chains which run parallel to the Sb-O sheets. The hydroxyl groups form hydrogen bonds between Sb-O sheets and Ca-S chains; these bonds provide for the three-dimensional cohesion of the structure. An interpretation of twinning with (100) as twin plane is given. Introduction Peretaite is a new sulfate of antimony and calcium found at Pereta, Tuscany, Italy. Its chemical formula, CaSboOo(OH)r(SOo),' 2I{2O, and other mineralogical data have been reported in the preceding article (Cipriani et al.,1980). It occurs ai aggregates oftabular crystals, associated with quartz, pyrite, calcite, sulfur, gypsum, and other antimony minerals, stibnite, valentinite, kermesite, and klebelsbergite. Klebelsbergite and peretaite are the only tws Sb sulfate minerals so far known. This study was undertaken to solve the crystal stnicture of peretaite and to investigate its possible crystal-chemical relationships with the structures of klebelsbergite (Menchetti and Sabelli, 1980) and of other synthetic Sb sulfates such as the anhydrous SbrO. '2SO. (Merciet et al . , 1975), SbrO. '3SO, (Mercier et al.,1976), SbuO,(SO4), (Bovin, 1976), and the hydrate Sb,(OH),(SO')2.zH,O (Dolglade et al., 1978). Experimental For the X-ray structure analysis a small tabular crystal (approx. 0.1 X 0.4 x 0.2 mm) was taken from an original sample of peretaite. Monoclinic symmetry and possible space groups C2/c and Cc were determined by Weissenberg photographs. The centrosymmetric space group C2/c was chosen for the structure determination and was subsequently found to be correct. The unit-cell dimensions, determined 0oo3-004x/80/09 l -0940$02.00 from 25 high-angle reflections measured on a fourcircle automatic diffractometer, ate a : 24.665(4), b : 5 .6006(9) , c : 10.185( l )A, B : 95.98( l ) ' . T[re sample used for intensity data collection and all other crystals tested for X-ray work were found to be twinned. Figure I reports the reflection population due to the whole twinned crystal and referred to reciprocal lattice levels with k : 2n (levels with k : 2n*l can be represented by a similar scheme with ft-odd lattice points occupied instead of ft-even ones). One can readily see that overlapping of points from the two individuals takes place only in the lattice rows with I : 4n. From the intensities of 363 pairs of well-separated reflections (with I : 4n + 2) the relative volumes of the two members of the twin were easily computed. The knowledge of the volume ratio so determined (A/B : 3.55i0.02) allowed the composite intensities of the overlapped reflections to be subdivided into the intensities of A and B individuals, by solving the following equations: I^r,: ltu, * lP,o,, lo'*'r: lf 'o'r+I"ro, where I is the observed intensity of the twinned crystal and [" and IB are the intensities of the two superimposed reflections; h,k,l are the Miller indices of reflections belonging to the A individual and h' : -(h + t), k' : k, Il : / those of overlapped reflections of the B individual of the twin. In conclusion, according to Friedel's (1926) notation, the crystals of peretaite 940 MENCHETTI AND SABELLI: PERETAITE ATOMIC ARRANGEMENT

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@inproceedings{aNo2007PeretaiteC, title={Peretaite, CaSbnOo(OH)2(SO4)2 '2HrOz its atomic arrangement and twinning}, author={Su-vro MeNcnrrrr aNo and CnseRn SenBLt}, year={2007} }