How do chlorite coatings form on quartz surface?

Abstract

Chlorite-coats on quartz surfaces are ubiquitous in various sedimentary environments. Chlorite-coats shield the surface of quartz from quartz cement overgrowths, thus preserving anomalously high porosity in deeply buried sandstone reservoirs. The inhibition of the quartz cement implies that the chlorite-coats on the surface of quartz grains can significantly influence the physicochemical behavior of the quartz grains. Therefore, failure to notice the initial thin microscale coatings forming during deposition can have serious consequences for modeling several geochemical reactions occurring at liquid-solid interfaces. Despite this huge implication, the fundamental mechanisms involved in chlorite-coat formation is not well understood. Here we present an experimental study to determine the parameters that control chlorite-coat formation on the surface of quartz grains. The batch experiments were conducted in a concoction of quartz and chlorite under different conditions of ionic strength, pH, and presence of humic acid (HA), iron- (Fe) and aluminum (Al) oxides). HA, Fe- and Al oxides are suggested to aid the emplacement of chlorite-coat precursors. At pH 7, the quartz–chlorite and quartz–chlorite–Fe/Al-oxides mixing experiments performed in saline and non-saline solution result in equal chlorite-coat coverages, suggesting neither salinity nor Fe and Al-oxides explain the mechanisms of chlorite-coat formation. At pH 5 and 9, however the chlorite-coat coverage was superior only in saline solution, indicating differences in coat coverage may be caused by variable electrokinetic charge distribution due to the distribution and transport of dissolved salt. The chlorite-coat barely formed in experiments that contain HA in quartz–chlorite mixtures regardless of ionic strength and pH. Against a long-standing notion, the presence of organic matter cannot necessarily be prerequisites for binding chlorite on the surface of quartz grains. The dynamic interactions between solution chemistry and surface chemistry of solid phases (quartz, chlorite, HA, Fe and Al oxides) can result in changing the electrokinetic properties in a region near the solid phases and at mineral-solution interfaces. We therefore propose that the electrokinetic response that arises in heterogeneous systems may explain the mechanisms of chlorite-coat formation.