Clay minerals from rock salt of Salt Range Formation (Late Neoproterozoic–Early Cambrian, Pakistan)

Abstract

The clay minerals of Late Neoproterozoic–Early Cambrian rock salt of Salt Range Formation of Pakistan have been studied by means of X-ray diffraction, scanning electron microscopy and dispersive X-ray spectrometry, complex thermal and chemical analyses. The clay minerals association of pelitic fraction of water-insoluble residue of these deposits consists of corrensite, chlorite and illite with the admixture of unordered mixed-layered chlorite–corrensite and chlorite–smectite; in some samples, the admixture of smectite occurs. The expandable layers in corrensite are determined as smectite. In studied samples the chlorite, corrensite and mixed-layered species are presented by trioctahedral Mg-rich type and illite is dioctahedral and enriched by Fe; this association of clay minerals is typical for evaporite deposits. Transformation of clay minerals proceeded under the impact of several factors of different direction and intensity. In evaporite basin, the elevated salinity of brines reinforces the processes of clay minerals structure ordering causing disappearance of mixed-layered minerals and thus decreasing the number of clay mineral species; in the brines originated from SO4-rich seawater, the clay mineral associations are richer comparing to Ca-rich brines. Local factors—volcanic ash input, elevated content of organic matter slow down the transformation processes thus also increasing the number of clay mineral species. We explain the unexpectedly rich clay mineral association (as for the halite stage of evaporation) in studied rocks by the strong effect of local factors.