Editorial for special issue "clays, clay minerals, and geology"

Sedimentary rocks covering most of the Earth's crust are mainly composed of clays, making clay minerals widespread globally. Clay minerals are mainly derived from the interaction of the surface of the Earth's crust with the atmosphere and hydrosphere, without neglecting the contribution of organic matter and bio-agents mainly characterizing the soil. Since the beginning of the last century, with the development of the X-ray diffraction technique, a strong interest has arisen in the study of these sediments, particularly clayey minerals. Various specialized journals, many books, and many national and international conferences have been dedicated to these minerals, and national and international associations have been established (e.g., AIPEA-Association Internationale pour l'Étude des Argiles). The main topics covered in journals, books, and conferences concern the determination of crystallo-chemical and chemical-physical characterizations, as well as qualitative and quantitative analyses of the minerals present in clays, genesis, depositional environments, and post-depositional evolution [1]. The mineralogical composition of clayey sediment consists of a non-phyllosilicate component, such as quartz, feldspars, Fe-(hydr)oxides, and components of phyllosilicates, among which clay minerals are represented by smectites, illite, chlorite, and kaolin groups, wherein mixed layers of illite/smectite and chlorite/smectite are the most frequent minerals. Phyllosilicates have a small grain-size, while the others are concentrated in the silt and sand grain-size. Clay minerals are characterized by tetrahedral sheets occupied by Si and subordinately Al and Fe3+. These sheets join together along the surfaces with different combinations, giving rise to different layers, which in turn join to form different clay minerals. In the various clay minerals, the layers have different layer charges as a function of the substitution of higher charge cations with cations with lower charges (e.g., replacement of Si4+ with Al3+ in tetrahedral sheets). Cations such as K, Ca, and Na can be linked to the layers more or less strongly depending on the layer charge (e.g., smectite < illite < muscovite). In the smectites, the cations can be easily exchanged and this group of minerals also has the ability to exchange organic molecules. Minerals such as kaolinite, pyrophyllite, and chlorite have no interlayer cations. Due to their structure and small grain-size, clays and clay minerals are characterized by specific properties, such as cation-exchange capacity (CEC), sorption of water and organic substance, thixotropy, swelling, impermeability, and plasticity, making them very versatile and therefore useful in various technological fields and industrial productions. The above properties control the geotechnical parameters; therefore, mineralogical knowledge of clayey materials is important for landslide/mass movement and natural hazard assessment [2,3]. In addition, based on these properties, clay minerals respond differently to thermo-chemical treatments, which are useful for their identification through X-ray diffraction [4]. The characteristics of the clayey sediments depend on geological history, such as the nature of the parent rocks, from which they derive through alteration, tectonic activity and climate of the source area, transportation, depositional environments, and the subsequent diagenetic processes or very low metamorphic degree (Reference [5] and reference therein). Based on the above and as already mentioned, clay minerals are useful proxies for the reconstruction of the dynamics of previous geological processes affecting the Earth.