Nano-scale analysis of polymineralic surface coatings on aeolian quartz grains with palaeoenvironmental implications

Abstract

The combined application of SEM/FIB cross-section lamella preparation and S/TEM characterization of individual aeolian quartz silt grains revealed two types of polymineralic coating (composed of Fe oxides/hydroxides, Al-silicates, together with relatively coarse incorporated crystals such as anhedral potassium feldspar and idiomorphic hematite) on the quartz grain, which differ in their internal structure (Type A – an openwork and relatively thick outer layer; Type B – a thinner and finer layer adherent to the grain surface). The highly porous Type B polymineralic coating may by disordered or arranged as laminae, and may lie directly on the quartz grain surface or may be separated by a thin layer of opal (with lepispheres) or cryptocrystalline quartz, most likely of diagenetic origin. These quartz grains, even in the silt fraction are not monomineralic monocrystals: monomineralic polycrystalline and even polymineralic polycrystalline internal structures with visible primary and secondary defects were found. All these characteristics of a coated quartz grain (including the opal/cryptocrystalline rims), in addition to the direct palaeoenvironmental interpretations, most likely can affect the accuracy and precision of age estimation using the optically stimulated luminescence (OSL) method; the rate of HF etching (hydrofluoric acid) is probably not uniform and isotropic as is assumed in OSL protocols.