Rietveld refinement for minerals : a tirilite synchrotron powder diffraction study

Rietveld refinement and synchrotron powder diffraction are both rather unknown for the common geologist, although very powerful methods in academic and industry applications. The aim of the study is to resolve phase composition of a complex geological sample and investigate the capabilities of the methods. Complex granitic rock, tirilite was selected for quantitative phase analysis to be carried out by Rietveld refinement utilizing Synchrotron Radiation X-Ray powder diffraction at the Swiss Light Source (SLS) synchrotron radiation facility maintained by Paul Scherrer Institute (PSI). Samples were measured using Vibrating Sample Holder (VSH), a novel sample handling approach developed originally for Martian exploration by NASA. This invention was further modified and developed with PSI and Stenman Minerals Ab for synchrotron beamline setup to answer increased demand of industrial diffraction analyses. The new approach might serve industrial as well as academic interest. A large bulk sample of tirilite were processed for the study. The sample was divided to different weight fractions using heavy liquid separation to reduce the number of mineral phases present in a powder diffraction sample. The motif for this was to decrease the number of diffraction peaks within a diffractogram for achieving better refinement for individual mineral phases. Mineral phases were recognized from the separate weight fractions. Each mineral in a rock was first refined from the weight fraction in which it was most represented. Refinements were carried out against database structure references. After refining the structure- and profile parameters of the present phases from the weight fractions, they were used as structural references for whole rock refinement. Quality of the refinements were compared to chemical analyses carried out by WD-XRF and ICP-MS. Correlation between analysed and refined chemical composition and lattice parameter references and refinement quality parameters suggests that the method is not limited for determination of accurate modal compositions. Also, a set of crystallographic parameters can be both accurately and precisely derived from the refinements: lattice parameters, compositions, and site occupancies. Determination of inter-site cation distribution can be used for geothermobarometry and samples can be measured faster than before. The method seems a promising new application of synchrotron powder diffraction for academic and industry applications.