Polytypic behaviour of zirconolite

Abstract

A polytypic family is described whose simplest member is zirconolite, (CaZrTi 2 O 7 ). The parent structure consists of calcium and zirconium atoms inserted between planes of titanium atoms, the latter being arranged in an hexagonal tungsten bronze (h. t. b.) type motif. Polytypes are generated by modifying the relative orientation of successive h. t. b. layers. Hettotypes are uniquely described by stacking formulae that are similar to those encountered in mica polytypes. Each stacking formula is com­posed of an interlayer stacking vector and a rotation. Zirconolite as a com­ponent of synroc (a proposed high level waste disposal matrix) was examined by selected area electron diffraction (s. a. d.) and high resolution electron microscopy (h. r. e. m.). Coherent polytypic intergrowths were observed which gave rise to laminar zirconolite domains. In crystallites that contain a large number of narrow domains, diffraction patterns contained sharp superlattice reflections of extremely small (100–200 Å) reciprocal lattice spacing; however, no superstructures with perfect translational periodicity were observed. (1 Å = 10 -10 m.) It is proposed that the intergrowth of zirconolite polytypes on a unit cell scale provides a means of incorporating cationic impurities in the parent structure.