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 L*, a*, hab and brightness. Organic matter is the most influencing component of aspect (color and brightness) in the Iberian roofing slates color and, in a lesser extent, chlorite abundance. Chromatic effect of the chlorite is only perceptible in absence of organic matter, being this effect is more intense for the magnesic chlorite, clinochlore. Also, other correlations were found between chlorite and the slate’s main elements by means of CIELAB parameters. Thus, color quantification in roofing slates can be used as a first step to learning about their mineralogical composition.
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