Caracterización de pizarras comerciales del NO de España a partir de DRX, microscopía electrónica (EMPA , SEM, TEM) y FRX

Abstract

[EN] The aim of this paper is to characterize thoroughly the mineralogy and petrography of a set of ten different types of commercial roofing slates from the NW of Spain. Representative samples of each type have been studied by transmitted and reflected-light optical microscopy, scanning and transmission electron microscopy, electron microprobe, X-ray fluorescence and X-ray diffraction techniques. Quantitative analyses of the main mineral percentages in each slate sample were made from XRD data using the Siroquant™ interpretation software system, based on Rietveld principles. The microscopic textures of the slates vary depending on the samples, and lepidoblastic to porphyroblastic textures have been described. Some slates with bands of sand layers display grano-lepidoblastic textures. White mica, quartz, chlorite and feldspar (albite) are the main minerals. The white micas and the chlorites have similar compositions in all the analyzed samples. Electron microprobe analyses indicate that the micas are K-rich dioctahedral varieties, having a composition close to muscovite with a phengitic component. They show the following average formula: (Na0.16 K1.39 Ca0.01) (Al3.83 Ti0.03 Fe0.15 Mg0.14) (Si6.22 Al1.78) O20 (OH)4. The chlorites are magnesian chamosites with an average formula of (Na0.02 K0.04 Ca0.01Al3.24 Ti0.01 Fe5.13 Mn0.03 Mg3.28) (Si5.23 Al2.77) O20 (OH)16. Accessory minerals are ilmenite, rutile, sulphides, carbonates, graphite, monazite, apatite, zircon and tourmaline. Two different associations of sulphide minerals are distinguished: 1) Type I with pyrite, pyrrhotite, pentlandite, cobaltite, chalcopyrite, sphalerite, galena, ullmannite and gersdorffite occurring as pre-kinematic porphydic aggregates; b) Type II mainly with post-kinematic pyrite and sporadic chalcopyrite and galena. The main carbonate mineral is ankerite ((Mg0.26, Ca0.49, Mn0.05, Fe0.19) CO3) with minor calcite and siderite also present.