Petrography of roofing slate for quality assessment

The commercial term roofing slates refers to slate and stone for discontinuous roofing and external cladding, according to the European Norm EN-12326. This group comprises stones such as slate s.s., shale, phyllite, and carbonate slate, but even schist, cinerite, sandstone, meta-arkose, schistose quartzite, quartzitic flagstone, gneiss and mica schist. However, the main lithologies used for roofing are the first four. All these rocks have a well-developed and layered structure in common, allowing them to be split into large, thin and regular tiles. There are several places in the world where roofing stone could be quarried, but nowadays the main exploitations are located in Spain, China and Brazil. The difference in nature (petrology and characteristics) of the roofing stones from different parts of the world plays a major role in the final use, although builders often think that all stones are equally suited for the same purpose. Generally speaking, almost all of the World's roofing slates are mainly composed of quartz, mica and chlorite, together with some accessory minerals such as iron sulfides, iron oxides and carbonates. The weathering of these two minerals creates characteristic pathologies, oxidation and gypsification, respectively. Some iron sulfides alteration causes oxidation, which can be seen as brown to red stain marks over the surface of the tile. Oxidation is far the main cause of complaints and rejections, even when is just a aesthetical issue. On the other hand, gypsification can lead to the tile destruction, but it has less impact on the complaints. Since weathering is related to the mineralogical composition and so indirectly to the origin of the roofing slate, identifying and quantifying that mineralogy becomes critical to secure a proper use of the slate. In this sense, optical examination with Polarized Optical Light (POL) using both transmitted and reflected light and X-ray Diffraction are powerful methods, especially when combined with other techniques, such as Micro Computed X-ray Tomography (MCT). POL and XRD give information about the type of the rock, its internal structure and mineral components, while MCT gives information about the occurrence, size and distribution of the metallic minerals, specially the iron sulfides. This works revises the main types of roofing slates found nowadays in the market, together with their main characteristics found by POL, XRD and MCT.