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 Na 2 S 2 O 4 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.

Birnessite-clay mineral couple in the rock varnish: a nature's electrocatalyst

“…This increase in the electrocatalytic property of rock varnish might be due to the presence of clay minerals in addition to oxides of Mn and Fe, as reported in a recent study. 98 As clay minerals have thin sheet silicate structures with extremely fine particle sizes and charged surfaces, the adsorption and desorption processes in clay minerals empower the adsorption of electrolytes with electrode material and thereby enhance the rate of OER. To explain the mechanism, the reaction order of OER was determined by recording the anodic polarization curve with both Ni and Pt coated electrodes (Fig.…”

Rock Varnish: Nature’s Shield

“…800 Dremel) with a diamond-coated sterilized coarse bit. This method was employed to prevent contamination while scraping off the varnish layer from the underlying host rock. , The extracted varnish samples were then stored in small Eppendorf tubes for isotopic and organic biomarker analysis. In the current study, we denote the varnish layers scraped from these samples as 1V, 2V, 3V, and 4V, while we designate the respective underlying host rocks as 1H, 2H, 3H, and 4H.…”

“…Rock varnish exists as a thin, dark, lustrous coating frequently seen on exposed rock surfaces worldwide in desert environments, including its accretion on top of ancient petroglyphs. , Rock varnish hosts range of microorganisms, , with a composition dominated by clay minerals cemented to rock surfaces by iron and manganese oxides and hydroxides. − Depending on different factors, varnish coatings range in thickness from ≤1 μm to several 100 μm. The rate of accretion varies tremendously, ranging from a few micron per millennia in harsh warm deserts to microns per century in wetter settings. − Scholars have proposed a host of alternative competing hypotheses to explain the tremendous concentration of manganese found in varnish.…”

Rock varnish revisited

“…Rock varnish consists of over half clay minerals, about a third Mn and Fe oxides, as well as minor and trace elements that deposits as a paper-thin accretion on an underlying rock. This definition remains basically unchanged from Potter and Rossman (1977) and has been adopted widely: (Andreae et al, 2023; Aulinas et al, 2015; Chaddha et al, 2021, 2022, 2024; Dietzel et al, 2004; Dorn and Oberlander, 1982; Dragovich, 1988; Drake et al, 1993; Elshabrawy and Ali, 2021; Esposito et al, 2019; Forster et al, 2021; Glazovskiy, 1985; Harrington, 1987; Hodge et al, 2005; Hungate et al, 1987; Jahn and Maneck, 1991; Jones, 1991; Krinsley, 1998; Krinsley et al, 2013; Krumbein and Jens, 1981; Kuhlman and Abrecht, 2008; Kuhlman et al, 2006; Lingappa et al, 2021a; Liu et al, 2021; Martínez-Pabello et al, 2021a, 2021b; McKeown and Post, 2001; Nagy et al, 1991; Northup et al, 2010; Otter et al, 2020; Pineda et al, 1988; Reneau et al, 1992; Spilde et al, 2007, 2013; Wang et al, 2011; Wayne et al, 2004; Whalley et al, 1990; Xu et al, 2019; Zerboni et al, 2022; Zhang et al, 2012; Zhou et al, 2000).…”