Methods of evaluating slate and their application to the Scottish slate quarries
Methodology The basis of the research was to identify those physical, chemical and geological attributes of slate which make it a good roofing material. These were often related to specific Scottish quarries in order to select those which showed the best potential for resumed production. However, as no fresh samples of Scottish slate were available, the work was based initially on slate of known and reliable quality from producing quarries elsewhere. The parameters examined were mineral composition, metamorphic grade, fabric and weathering properties. When the parameters most closely related to the quality of finished slates were established then the Scottish slate was examined for these same parameters. Mineral Composition Recent progress in the determination of the chemical formulae of the phyllosilicates of fine-grained material has enabled the mineral composition of slate to be calculated from its total oxide composition. Calculations based on representative samples from each group of quarries gave good results when taken from sites in close proximity, but application to samples from more remote sites gave a weaker correlation. Metamorphic Grade Metamorphic grade affects the crystallinity and the grain size of a material. Traditionally for sedimentary rocks both properties were evaluated by the sharpness of peaks in XRD analysis, using Full Width at Half Magnitude (FWHM) as the criterion. However, this method is not effective when applied to slate, and an alternative method of determining its crystallinity was developed, based on the intensities of peaks of the main minerals.