Analysis of the methodology of the petrographic examination test (European Standard EN 12326-2) and the relation between petrography and modulus of rupture for Spanish roofing slates

This work aims to re-assess the methodology of the petrographic examination test established in European Standard EN 12326-2, and also to evaluate the results of recent work by Cárdenes et al. (2010) based on application of this t'est to establish relationships between petrography of roofing slates and modulus of rupture (MoR). A comparative study of 21 commercial varieties of Spanish slate using light microscopy and scanning electron microscopy has shown that the optical microscope does not adequately resolve the fine-grained textures that dominate Spanish roofing slates, and hence that the methodology set out in EN 12326-2 does not seem to provide the most appropriate mechanism to establish the structural type of the mica fabric and the mica stacking index (MSI). Samples of ten different slate varieties were also subjected to bending strength tests (EN 12326-2), which define the MoR. The MoR longitudinal is positively correlated with the chlorite content and the angle between the stratification and cleavage, but is negatively correlated with the presence of porphyric elements. However, the MoR transverse is independent of all these factors, suggesting that the parallel orientation of the loading bar with respect to the intersection lineation stratification/cleavage (L1) is a determining factor in the test result. Many unmeasured factors, such as porosity, calcite or quartz veins, and fractures, may also influence the test results, indicating a need to avoid simplifications associated with assessing likely MoR values based only on microscopic observations.

► This work evaluates the petrographic methodology of the European Standard EN 12326-2.
► The optical microscopy does not resolve the dominant microtextures of roofing slates.
► We also evaluate the relationships between slate petrography and modulus of rupture.
► Bending strength depends on multiple structural and microtextural factors.