Peraluminous rocks of Bou-Azzer region (Morocco): Geology and firing transformations

The general geology of the peraluminous rocks of the Bou-Azzer region (Central Anti-Atlas, Morocco) and their firing transformations were investigated by several techniques: optical microscopy, microprobe analysis, X-ray diffraction (XRD), X-ray fluorescence (XRF), thermal analysis, scanning electron microscope (SEM), energy dispersive spectrometry (EDS), and by measuring selected physical properties. The results of the geological study show that these rocks consist mainly of a quartz–pyrophyllite assemblage (70–74 wt.% SiO2 and 14–17 wt.% Al2O3), associated with minor amounts of muscovite and nacrite. They formed from a progressive deformation and hydrothermal alteration of felsic volcanic rocks. The principal mineralogical transformations recorded from rhyodacite to peraluminuous rocks are: K-feldspar → muscovite → pyrophyllite. Regarding the firing transformations, it is found that up to about 1100 °C the rock samples are subjected to expansion associated with the destruction of the pyrophyllite hydroxyl framework. The estimated amount of energy associated with this process is 50.226 kJ/mol. Beyond 1100 °C, a marked shrinkage was observed, due to the formation of a glassy phase, and the precipitation of mullite, cristobalite and K-feldspar. The reaction pathways for these phases are proposed and the evolved heat is determined (−160.928 kJ/mol).