CRYSTAL CHEMISTRY FROM POWDER DATA. Ian E. Grey

CSIRO Division of Minerals, PO Box 124, Port Melbourne, Australia

A strength of structure analyses using powder data is the relatively short time required to obtain high quality data sets. This makes powder diffraction analysis particularly suitable for systematic structural studies on series of related compounds such as solid solutions in minerals and their synthetic equivalents. The high precision obtained when collecting and processing powder data sets under identical conditions allows the detailed study of subtle crystal chemistry correlations, for example variations in cation site occupancies, anion vacancies and bonding interactions.

The use of variable counting time (VCT) data collections (Madsen and Hill, 1994) enhances the capability to analyse subtle structure variations using X-ray data collected using a conventional laboratory configuration. In particular it yields more consistent thermal parameters, more accurate site occupancies and more stable refinement of light atoms.

Examples will be presented of the application of Rietveld analysis of VCT powder X-ray data to investigate chemical and structure variations in synthetic mineral systems. These include studies on cation substitutions in synthetic loveringite, (Ca, Ti³⁺, Ti⁴⁺, Mn)₂₂O₃₈ prepared under reducing conditions, studies on anion vacancy and Fe³⁺/Fe⁴⁺ variations in the solid solution of hexagonal BaTiO₃ with Ba₂Fe₂O₅, and crystal chemistry systematics of sulphate and oxysulphate phases formed during sulphuric acid digestion of ilmenite.

ref. I.C. Madsen and R.J.Hill, J.Appl. Cryst. (1994). 27, 385-392