Incommensurately modulated structure of isotropic lazurite as a product of twinning of two-dimensionally modulated domains

The incommensurately modulated structure of isotropic lazurite (the Baikal region) was established for the first time. The structure was solved within a pseudocubic unit cell with the parameter a = 9.077(l) Angstrom and the (3 + 2)-dimensional superspace group Pnn2(gammagamma0, gamma-gamma0) based on a twin model consisting of three two-dimensionally modulated orthorhombic components related by a threefold axis along the [I I I] direction. The structure was refined with isotropic thermal parameters to wR(0) = 1.98% and wR(1) = 7.5% using 257 main reflections and 2392 first-order satellite reflections, respectively. (C) 2003 MAIK "Nauka/Interperiodica".