The geology of roofing slate

Abstract

This paper reviews the geological factors linked to the quarrying and quality of roofing slate deposits, and gives recent research results on the loss of fissility by dewatering, and the oxidation of iron sulphides. Mineralogy, resulting (from the original protolith composition and the metamorphic grade, controls the colour, brightness and durability of slate tiles. The microtexture of the rock mainly controls the split fitness of slate (fissility). The structure: (folding, slaty cleavage development, So-S1 angle, lack of crenulation cleavage and kink-bands, joints and faults) controls the exploitability. Additional factors, such as bed thickness and changes in facies are also important. Both very cold environments, giving frost, and arid conditions, drying the rock enough to remove water, produce a loss of fissility in slate. Ostwald-ripening by dewatering and some collapse of the crystallographic structure of phyllosilicates cause the loss. A mineralogical study of the iron sulphides in roofing Spanish slate (pyrite, pyrrhotite, marcasite), encompassing crystal habit and size, and their biochemical alteration processes (stain spots), allow the oxidation behaviour of different slates to be predicted.