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Quantitative mineralogical analysis of Spanish roofing slates using the Rietveld method and X-ray powder diffraction data

¹ School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia
² Departamento de Ingeniera Minera, Universidad de León, C/Jesús Rubio 2, E-24004 León, Spain

^* Corresponding author: e-mail: C.Ward{at}unsw.edu.au

The relative proportions of the minerals in a series of slates from the Iberian Massif in northwestern Spain, quarried and used for roofing purposes, have been evaluated using X-ray powder diffraction and the Rietveld-based SIROQUANT data processing system. Information on the chemical composition of the chlorite, mica and feldspar in the slates was obtained separately from electron microprobe studies, and was used in conjunction with the mineral proportions to calculate an inferred chemical composition for each sample from the quantitative XRD data. The inferred compositions were then compared to the actual chemical compositions of the respective slate samples, obtained by X-ray fluorescence spectrometry, to evaluate the consistency between the XRD results and independent chemical analysis data.

A high level of consistency was observed between the element oxide percentages inferred from the XRD study and the equivalent oxide proportions determined by direct chemical analysis. The comparison suggests that Rietveld-based interpretation of X-ray powder diffraction data provides results that are consistent with other compositional parameters. Although no particular relationships were observed among the individual mineral percentages in the slates of the sample suite, a well-defined inverse relationship was found between the percentages of quartz plus feldspar on the one hand and the percentages of mica plus chlorite on the other. This may reflect variations in the relative proportions of coarse- and fine-grained sediment in the original parent materials that formed the slate deposits.

Resumen Se han calculado las proporciones relativas de los diferentes minerales de las pizarras de techar explotadas en el Macizo Ibérico del noroeste de España. Para ello, datos obtenidos por difracción de rayos X sobre muestras de polvo de pizarra han sido procesados utilizando el sistema SIROQUANT, basado en la técnica de Rietveld. A partir de estudios de microsonda electrónica se determinó la composición qumica de cloritas, micas y feldespatos de las pizarras, lo que junto con las proporciones minerales obtenidas a partir de los datos de DRX, permitió inferir una composición qumica para cada muestra de pizarra. Estas composiciones inferidas fueron entonces comparadas con composiciones qumicas de las respectivas muestras de pizarra, obtenidas mediante fluorescencia de rayos X, evaluándose la coherencia entre los resultados de las dos técnicas empleadas.

Un alto nivel de coherencia fue observado entre los porcentajes de óxidos de cada elemento inferidos a partir del estudio de DRX y las proporciones de óxido equivalentes determinadas por FRX. La comparación sugiere que la interpretación de la DRX basada en la técnica de Rietveld aporta resultados que son coherentes con otros parámetros composicionales. Aunque no se observaron relaciones entre porcentajes de minerales individuales en las pizarras muestreadas, se encontró una relación inversa bien definida entre los porcentajes de cuarzo más feldespato y los porcentajes de mica más clorita, lo que se interpreta como una herencia de las proporciones relativas de limo y arcilla en los sedimentos a partir de los que se formaron las pizarras.

Key-words: slate, Iberian Massif, Spain, X-ray powder diffraction, mineralogy, Rietveld method.

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