Wollastonite: Preferred Orientation Study


by M Vickers, Dept. of Chemisry, UCL

Powder X-ray Diffraction of wollastonite comparing flat-plate reflection geometry data with capillary transmission data using Rietveld refinement. Quartz is present at about 5 wt% and was included in the refinement.

Back-packed flat-Plate "reflection" data from a Siemens D500 with preferred orientation function included. This was very difficult to refine and gives an R-factor of 26% with a March function PO value of 1.6. Several hkl combinations were tried with the 010 having the greatest effect (with no PO included the R-wp was 75% but the refinement was very unstable):

Capillary transmission data from a Stoe StadiP. This had no refinement of any possible preferred orientation effects but gave a stable refinement with 11% R-wp:

Capillary transmission data from a Stoe StadiP with 010 March preferred orientation function included. This returned a value of 0.9 and reduced the R-wp to 6%:

Conclusion:

Transmission geometry minimizes preferred-orientation effects. Despite lower background and higher resolution, the reflection geometry gives unsatisfactory results and almost unrefinable parameters even when back-packed.

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