THE FUTURE OF QUANTITATIVE X-RAY POWDER DIFFRACTION (QXRD) METHODS IN GEOSCIENCE

More than 75 years ago, it was recognized that the weight fraction of a mineral in a rock was proportional to the intensity of its x-ray diffraction (XRD) peaks, and quantitative XRD techniques have become increasingly sophisticated in geoscience since then. A greater use of QXRD with increased accuracy is predicted.

Present QXRD methods fall into two categories. Quantitative representations (QR) produce a numerical estimate of relative changes in mineral abundances that are assumed to vary directly with the mineral wt %. They are best expressed as peak-height-percentages (PHP) and differ from truly quantitative phase analysis (QPA) results. QPA results are an accurate measurement of the actual quantity of the mineral present. QR values are more dependent on individual laboratory operating conditions and are more difficult to correlate with those produced by other scientists.

QPA is based on the simple relationship between the intensity of diffraction and the amount of a component in a mixture:

I_min=K_minW_min/m_avg

I, K, and W represent intensity, experimental constants and weight fraction of a specific mineral, respectively. m_avg is the average mass attenuation coefficient of the mixture. This coefficient and the various experimental parameters incorporated in K are difficult to evaluate and are generally responsible for the shortcomings of quantitative methods.

In recent years, QPA determinations have been improved by a “Rietveld” approach employing least-squares minimization techniques to adjust mineral structure variables and instrumental parameters until a simulated XRD pattern matches an experimental one. The crystal data may be based on known mineral structures or hypothetical ones. Results obtained by winners in The Clay Minerals Society Reynolds Cup competition illustrate a high degree of accuracy for abundant minerals (>20wt%) and moderate agreement at the <5wt% level for prepared mixtures containing 10-15 minerals common in sediments. Studies in progress are exploring ways to use the Rietveld method to model the effects of stacking irregularities and mixed layering exhibited by clay minerals. An estimate of accuracy is provided by comparing modal and chemical results from North Sea shales.

The future looks bright for QXRD methods in geoscience and materials science.