Alunite characterization in the upper Eocene clay deposits of Central Tunisia: an implication to its genesis

Abstract

Alunite mineral was recently identified at the upper Eocene clay deposits associated with laminated primary gypsum and iron oxide in Central Tunisia, particularly at El Gnater site. The alunite characterization has been performed by several analytical techniques [atomic absorption spectroscopy (AAS), X- ray diffraction (XRD), scanning electron microscope (SEM), thermogravimetric analysis (TGA) and differential thermal analysis (DTA)]. The mean features of their chemical composition are its deficiency of Al, water excess content, small replacement of K by Na and absence of impurities such as Fe. The X-ray diffraction pattern reveals the purity of the alunite samples. The SEM micromorphological study shows a compact facies which is made up by homogeneous pseudocubic microcrystals having approximately the same diameters of 1 μm. Thermal behaviour of El Gnater alunite closely matches published literature which shows two major endothermic peaks at 528 and 744 °C with total weight loss 17.27 and 29.09 %, respectively. The former corresponds to the dehydroxylation processes and the latter corresponds to sulphate decomposition reaction. The association of studied alunite with laminated primary gypsum, hematite and clay minerals implies its late diagenetic origin in reduced conditions and in evaporitic environment. However, jarosite was produced by oxidation of pyrite which was very abundant in the claystone host.