Abstract
The mineralogy and microstructure, which affect the slaty cleavage, are linked with the strong preferred orientation of phyllosilicates and this enables the rock to be split into large, thin, flat sheets. Roofing slate samples with different commercial fissilities have been analyzed by radioluminescence (RL), thermoluminescence (3DTL), by X-ray diffraction (XRD), by scanning electron microscopy (SEM) using the back-scattered mode (BSEI) and by electron microprobe (EMP). They are made up of white micas, chlorite, quartz, detrital feldspars, ilmenite, pyrite, rutile apatite and tourmaline. Texturally, all consist of silt-sized clasts of detrital quartz, feldspars, chlorite-mica stacks, muscovite and ilmenite in a recrystalline, lepidoblastic matrix of white micas and chlorite with quartz lenses, all showing a very strong preferred orientation. The luminescence emission centers are a low broad blue band around the 400 nm spectra positions linked with alkali losses and formation of [AlO 4 ]o defects; a peak at 473 nm interpreted as a the first thermal step (150-300oC) of a non-isothermal dehydroxylation of the slate phyllosilicates; and a 568 nm peak which agrees with Mn 2+ point defects in aluminosilicate lattices. The studies on the slaty cleavage could be significant because Spain is the largest producer of roofing slate tiles in the world (87% of world production).
Highlights
Slaty cleavage is due to the strong preferred orientation of phyllosilicates and this enables the rock to be split into large, thin, flat sheets
La medidas termoópticas (TL3D) y la radioluminiscencia, evidencian procesos de autodifiísión iónica y deshidroxilaciones no isotermales en los filosilicatos de la pizarra y defectos cristalinos puntuales debidos a iones metálicos que siguen los modelos de la teoría de conducción de valencia
All consist of silt-sized clasts of detrital quartz, feldspars, chloritemica stacks, muscovite and ilmenite in a recrystalline lepidoblastic matrix of white micas, chlorite and quartz lenses, all showing a very strong preferred orientation Tourmaline, rutile, pyrite and apatite are dispersed through the matrix and frequently show
Summary
The mineralogy and microstructure, which affect the slaty cleavage, are linked with the strong preferred orientation of phyllosilicates and this enables the rock to be split into large, thin, flat sheets. In the 90s, the slate industry has acquired new techniques using diamond wire and diamond saw cutting methods, the development of block sawing and the use of automatic devices to split blocks along the slaty cleavage into roofing slates. Roofing slates are very fine-grained rocks having a penetrative (or slaty) cleavage They develop mostly from muddy sediments during very low grade metamorphism. Slaty cleavage is due to the strong preferred orientation of phyllosilicates (white micas and chlorites) and this enables the rock to be split into large, thin, flat sheets. Some slates develop a penetrative lineation, parallel to the X-direction of the strain ellipsoid, which is seen on the cleavage surfaces This is termed "hilo " in Spain, "grain " by British quanymen, "longrain " in the Ardennes and "down-dip-lineation " ^v Hobbs et al [13]. The great majority of the geological formations of slate are not exploitable massifs of rock because a narrow set of limitations as mineralogy, metamorphic grade, texture, layer thickness and discontinuities of the rock massif are necessary
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