The crystal structure of a second antigorite polysome (m=16), by single-crystal synchrotron diffraction

A model for the modulated crystal structure of all antigorite polysome with m = 16 (where in is related to the number of tetrahedra spanning a wavelength along a) was refined by single-crystal synchrotron diffraction data in C2/m, using crystals coexisting with the in = 17 polysome from Val Malenco, Italy, which was previously determined structurally. Lattice parameters [a = 81.664(10), b = 9.255(5), c = 7.261(5) angstrom, beta = 91.409(5)degrees] were determined using a single-crystal diffractometer equipped with an area detector at the Desy synchrotron (Hamburg). The structure was solved by direct methods, and the model refined using 19222 symmetry-related reflections. The final R-4 sigma factor was 0.0951, Calculated for 7246 reflections. The Structure of the m = 16 antigorite polysome strongly resembles that of the in = 17 polysome. A Continuous. wavy octahedral sheet is linked to a tetrahedral sheet, reversing its polarity through sixfold tetrahedral and eightfold tetrahedral rings. The half-wave has a Curvature radius of 80.1 angstrom. Polyhedral geometry, ditrigonalization angles, and interlayer O-O distances are similar in the two polysomes. The only differences concern the number of tetrahedra for the in = 16 polysome (an even number which leads to symmetric half-waves) and the periodic b/2 shift involving the eightfold rings (to produce file doubling of the a parameter and a C-centered cell).