Microtwinning hypothesis for a more ordered vaterite model

An orthorhombic fully ordered structural model is proposed for vaterite [space group Ama2, a =8.4721(5) angstrom. b=7.1575(7) angstrom. c=4.1265(4) angstrom. Z=4. and V=250.23(4) angstrom(3)]. It is based on a microtwinning hypothesis, with three domains rotated by 120 degrees along the orthorhombic a axis, regenerating a pseudohexagonal habit. The solution came from direct space oh initio calculations applied to the powder diffraction data. However, five weak superstructure reflections seen in single-crystal and powder diffraction experiments, leading to a six times larger unit cell, are still unexplained. (C) 2011 International Centre for Diffraction Data. [DOI: 10.1154/1.3552994]