Crystal Chemistry and Solid Solutions Amongst the Pseudowollastonite-Like Polytypes of the Calcium, Strontium, and Barium Silicates CaSiO3, SrSiO3, and BaSiO3

Abstract

SrSiO3 occurs in two distinct modifications, shown by selected-area electron diffraction to be polytypes. The thermodynamically more stable form, SrSiO3 (30 .ANG.), has a geometrically near-rhombohedral unit cell, with a 7.16, c 30.36 .ANG. on the pseudohexagonal axes, and is an analog of SrGeO3. The other polytype, SrSiO3 (20 .ANG.), is pseudohexagonal with a 7.16, c 20.24 .ANG., and is an analog of pseudowollastonite, a CaSiO3. SrSiO3 (20 .ANG.) occurs in twin lamellae of width 20 ´ n .ANG. (where n is usually between one and four), compn. plane (00.1). The true cells are monoclinic or triclinic for both types of SrSiO3. The system a-CaSiO3-SrSiO3 is characterized by a discontinuous change in symmetry: a two-polytype region exists in the compn. range 50 to 60 mole % CaSiO3. Both 20 and 30 .ANG. polytypes were obsd. in a BaSiO3 prepn.