Characterization, nomenclature and factors controlling the stability of quartz and opal-CT of Burdigalian and Ypresian siliceous rocks from Tunisia

Abstract The present work aimed to specify the nomenclature and factors controlling the stability of the Burdigalian siliceous rocks of the Babouchite series by comparison to the results of the Ypresian siliceous rocks of the Chouabine formation. The selected samples were investigated using optical microscopy, X-ray diffraction (XRD), chemical and infrared analyses, and Scanning Electron Microscopy (SEM). The data obtained shows that the Babouchite sample is mainly made up of microquartz (more than 70%). The detrital cryptocrystalline quartz represents a start for crystalline growth of microquartz as clusters in the dissolved edges of the relatively preserved radiolarian test pores enhancing chertification. The preservation of frustules is encouraged by the adsorption of clay minerals at their surfaces. However, the porcelanite rock of the Gafsa-Metlaoui basin is composed mainly of opal-CT associated with calcite, dolomite, smectite, sepiolite and palygorskite. Based on the percentage of SiO 2 , mineralogical composition (high microquartz content) and geochemical data (Al 2 O 3 rich and CaO, MgO poor), the chert is the appropriate nomenclature for Babouchite sample. But, porcelanite is the nomenclature for the bedded silica-rich rock of Chouabine formation. In addition, the high Al/(Al + Fe + Mn) and Al 2 O 3 /TiO 2 ratios and a low Fe 2 O 3 /TiO 2 and MnO/TiO 2 ratios and the presence of radiolarian tests indicate a non-hydrothermal environment of Babouchite cherts formation with relatively terrigenous detrital input.