Influence of Chemical-Mineralogical Composition on the Color and Brightness of Iberian Roofing Slates

Abstract

This study examines the relationship between the aesthetic parameters of color and brightness and conversely, chemical-mineralogical composition in roofing slates from different Spanish and Portuguese quarries. Chemical and mineralogical characterization of 50 commercial types of roofing slates was carried out using X-ray diffraction and fluorescence, along with petrographical microscope examination. Reflectance color measurements were made using a portable spectrophotometer and analyzed by considering the Commission Internationale de l'Eclairage Lab (CIELAB) color parameters. Brightness was measured at an angle of incidence of 60° in accordance with ASTM D 523-95. The results showed that the slates were composed of quartz, mica, chlorites, and feldspar, along with some other minerals in lesser amounts. Organic matter (graphite) was present in concentrations between 0 and 4% and was closely related to the slate’s age. A strong correlation was found between an abundance of organic matter, and CIELAB coordinates