Palygorskite in Miocene rocks of northern Iraq: environmental and geochemical indicators

Abstract

The mineralogical characteristics of palygorskite from the Lower Miocene Euphrates Formation and the Middle Miocene Fat’ha (Lower Fars) succession in north and northwestern Iraq were studied by X-ray diffraction (XRD), Infrared spectroscopy (IR) and scanning electron microscopy (SEM) with EDAX analysis. Palygorskite is the common clay mineral in the rocks studied, together with small amounts of illite, chlorite, and kaolinite, in addition to various amounts of quartz, feldspar, dolomite and calcite. It is believed that most of the clay minerals are detrital in origin, except palygorskite, which is formed by authigenesis in evaporitic environments. Marl beds of the Euphrates Formation were deposited in a lagoonal, relatively saline environment, suitable for the neoformation of palygorskite. the Fat’ha Formation is composed mainly of an evaporitic sequence. It consists of numerous shallowing-upward cycles of alternating mudrock, limestone, gypsum and/or anhydrite and halite in the basin centre. this depositional environment favours the authigenic formation of palygorskite and partly its diagenetic formation by transformation of precursor clays, mainly smectite and mixed-layer illite-smectite, as revealed by scanning electron microphotographs.