How elemental composition influences the color of igneous and sedimentary rocks: Case of the High Atlas rocks of Morocco.

In spite of color being one of the physicochemical parameters most commonly used to characterize a rock, very limited studies have studied the correlation between the nature, chemical composition, and color of a rock. This study presents a new approach for quantitatively assessing the relationship between these three parameters for specific rocks (example of igneous and sedimentary rocks) collected from the High Atlas of Morocco. A spectrophotometer was used to measure the color of samples, and the measurements were expressed in CIE L*a*b* color system units then converted to Hex color codes. Whereas, the chemical composition of samples was carried out by X-ray fluorescence.
The most abundant oxides in magmatic rock samples are SiO2, Al2O3, Fe2O3, MgO, and CaO, while K2O, Na2O, TiO2, and P2O5 are generally found in trace concentrations. Two categories of clays were studied, non-calcareous raw materials without carbonate contents (<4% CaO) and calcareous clays or marls (CaO > 10%). Phosphate samples are rich in phosphorus (4.4%-17.5%) and CaO (11.2%-42.7%) with relatively low contents of SiO2 (28.5%-52.2%), Al2O3 (3.1%-17.5%), and Fe2O3 (1.1%-6.6%). Results show that the change in the content of these elements from one rock type to another may be indicative of rocks with particular characteristics that do have an impact on color. The main coloration agent of clays was iron, Fe2+, and Fe3+ ions can color clay minerals either red or green or in various shades of orange and brown. However, in marls and phosphates, the high concentration of carbonates inhibits this iron effect by affecting a* (red) and b* (yellow) color parameters, which leads to grayish materials. The same applies to magmatic rocks rich in Fe2O3 and CaO.