Microtwinning hypothesis for a more ordered vaterite model

An orthorhombic fully ordered structural model is proposed for vaterite [space group A m a 2, a =8.4721(5) Å, b =7.1575(7) Å, c =4.1265(4) Å, Z =4, and V =250.23(4) Å 3 ]. It is based on a microtwinning hypothesis, with three domains rotated by 120° along the orthorhombic a axis, regenerating a pseudohexagonal habit. The solution came from direct space ab 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.