Abstract
Knowledge of clay minerals composition can be used to understand paleoenvironmental and/or paleoclimatic conditions, particularly for soils derived from volcanic parent materials that were subjected to diverse climatic conditions during pedogenesis. In this study, the clay minerals composition in soils developed from basaltic parent materials during the Cenozoic (mostly during the Early Miocene to Early Pleistocene) in the harrats region of the Arabian Shield in Saudi Arabia were investigated. Forty-seven clay samples were examined using selective dissolution analyses, X-ray diffraction (XRD), thermogravimetric analysis (TG), Fourier-transform infrared spectroscopy (ATR/FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to obtain a better understanding of the clay minerals developed in basaltic parent materials, particularly under a dry-hot environment. The XRD results revealed that the clay mineralogy was dominated by smectite, kaolin (kaolinite/halloysite), and interstratified vermiculite–chlorite (Vm–Ch), with mica, pseudo-chlorite, palygorskite, and Ca–zeolites as minor components. The TEM provided strong evidence for the presence of allophane and imogolite, which is in line with numerous previous studies on soils formed from basaltic parent materials. The existence of poorly-crystalline minerals in these soils was strongly supported by the selective dissolution analysis results. The combined influence of paleoclimates and parent material is evidenced in the clay minerals in the studied soils. The formation of smectite, Vm–Ch, and chlorite minerals in the harrats soils could be attributed to the simultaneous effect of temperate paleoclimates that dominated the Pliocene and the wet transition periods that occurred through the Pleistocene into the mid-Holocene, while the minor palygorskite and Ca–zeolites could be related to the dry conditions that started in the late-Holocene and persist to the present day. Additional studies regarding the clay mineral assemblages in similar geoenvironmental settings are highly recommend and should account for paleoclimatic conditions to provide a better understanding of the relationship between clay minerals and paleoclimates.