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

Abstract

A model for the modulated crystal structure of an antigorite polysome with m = 16 (where m is related to the number of tetrahedra spanning a wavelength along a) was reÞ ned by single-crystal synchrotron diffraction data in C2/m, using crystals coexisting with the m = 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) A, β = 91.409(5)°] 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 reÞ ned using 19 222 symmetry-related res ections. The Þ nal R4σ factor was 0.0951, calculated for 7246 res ections. The structure of the m = 16 antigorite polysome strongly resembles that of the m = 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 A. 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 m = 16 polysome (an even number which leads to symmetric half-waves) and the periodic b/2 shift involving the eightfold rings (to produce the doubling of the a parameter and a C-centered cell).