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

Abstract

The com bined ap pli ca tion of SEM/FIB cross-sec tion lamella prep a ra tion and S/TEM char ac ter iza tion of in di vid ual ae olian quartz silt grains re vealed two types of polymineralic coat ing (com posed of Fe ox ides/hy drox ides, Al-sil i cates, to gether with rel a tively coarse in cor po rated crys tals such as anhedral po tas sium feld spar and idiomorphic he ma tite) on the quartz grain, which dif fer in their in ter nal struc ture (Type A -an openwork and rel a tively thick outer layer; Type B -a thin ner and finer layer ad her ent to the grain sur face). The highly po rous Type B polymineralic coat ing may by dis or dered or ar ranged as laminae, and may lie di rectly on the quartz grain sur face or may be sep a rated by a thin layer of opal (with lepispheres) or cryptocrystalline quartz, most likely of diagenetic or i gin. These quartz grains, even in the silt frac tion are not monomineralic monocrystals: monomineralic polycrystalline and even polymineralic polycrystalline in ter nal struc tures with vis i ble pri mary and sec ond ary de fects were found. All these char ac ter is tics of a coated quartz grain (in clud ing the opal/cryptocrystalline rims), in ad di tion to the di rect palaeoenvironmental in ter pre ta tions, most likely can af fect the ac cu racy and pre ci sion of age es ti ma tion us ing the op ti cally stim u lated lu mi nes cence (OSL) method; the rate of HF etch ing (hy dro flu oric acid) is prob a bly not uni form and iso tro pic as is as sumed in OSL pro to cols.