Clay minerals identification in rock varnish by XRD: A one-step reduction approach

Abstract

Rock varnishes are known to be fine, dark, glossy submicron films found in deserts bare rock surfaces. The oxides and hydroxides of manganese and iron bind together the clay minerals present in the varnish layer. The processes of oxide-hydroxide accumulation at varnish sites are due to iron and manganese oxidizing bacteria which may require clay minerals for additional nutrition. Quantification and identification of clay minerals in this biofilm is needed to understand its formation. Past attempts to analyze the mineralogical composition of rock varnish have led to inconclusive results as varnish is a submicron thin layer composed of a complex mineral matrix. The elimination of non-crystalline cementing groups comprising of free iron oxides is a key step in the identification of many types of clay minerals, particularly in soil/sediment mineral studies.•The Fe-Mn oxide-hydroxide coatings, acting as cementing materials, can be easily removed using a one-step reduction method employing Na2S2O4 at 70 °C, leading to separation of clay minerals.•We have taken the lead from earlier reported Jackson (1958) method, wherein a combination of reagents was used such as sodium acetate, sodium citrate, hydrogen peroxide, sodium bicarbonate, and sodium dithionite for removing carbonate, organic carbon and Fe-Mn oxy-hydroxide coatings respectively from sediment grains to segregate individual grains from each other.•Our modification helps in the unveiling of clay minerals from a solid substrate and reports the X-ray diffraction peaks, which are elsewise hard to detect and therefore earlier studies are inconclusive.