Mineralogy and modulus of rupture of roofing slate: Applications in the prospection and quarrying of slate deposits

Abstract

The European norm UNE 12326 for roofing slate defines the mechanical properties of a slate tile through the Bending Strength test, which defines the modulus of rupture (MoR) of the tile. This MoR is related to the durability of the slate, and is one of its main commercial quality properties. During the prospection task previous to open a new slate quarry, it is crucial to define the MoR. The Bending Strength test needs proper slate tiles and a flexion press. To supply these slate tiles it is frequently necessary to open experimental quarry fronts, which are very expensive, and do not guarantee the viability of the quarry. Therefore, a method which could reduce the costs and time for the mechanical characterization would be very useful. In this work we have tried to relate the MoR to the mineralogical and petrographic characteristics of slate, using the Petrographic Examination test, also part of UNE 12326. This test contemplates the finding of a numerical index called Mica Stacking Index (MSI) which quantifies the mica layers fabric. We collected fifteen samples of roofing slates from the Northwest of the Iberian Peninsula and we studied them by using the Bending Strength and Petrographic Examination tests, plus mineralogical quantification using X-ray diffraction and measure of the illite crystallinity (Kubler Index) to determine the metamorphic grade. We have found a strong positive correlation between MoR values and MSI, and a negative correlation between MoR and Mica percentages. These correlations may be used to deduce the MoR, reducing costs during the exploitation labours in roofing slate mining, and also can be used in the quality controls on a working slate quarry.