Factors influencing the development of microtextures on cold‐climate aeolian quartz grains revealed by experimental frost action

Abstract

Aeolian‐originated quartz grains of coarse‐sand size (0.5–1 mm) were subjected to experimental frost weathering. A total of 1,000 freeze–thaw cycles with temperature ranges from −5 to +10°C were simulated under full water availability conditions. Scanning electron microscope microtextural analysis of grain surfaces conducted after 0, 50, 100, 300, 700, and 1,000 freeze–thaw cycles resulted in different‐sized conchoidal fractures and breakage blocks as frost‐induced microtextures. The vast majority of these microtextures were encountered on the most convex parts of aeolian grains and their number increased with ongoing freeze–thaw cycles. However, the number of recorded frost‐originated microtextures remained relatively small up to 700 freeze–thaw cycles and increased after 1,000 freeze–thaw cycles. Transmission electron microscope microstructural analysis of grains after 0, 100, and 1,000 freeze–thaw cycles showed both primary (e.g., inclusions, grain boundaries) and secondary (e.g., cracks) defects in quartz crystals. The frequency of the latter remained unexpectedly low. The susceptibility of aeolian‐originated sand‐sized quartz grains to frost‐induced modifications is interpreted here to depend mainly on their internal characteristics. These include aeolian‐driven development of a subsurface impact zone that determines the depth to which frost‐originated microtextures develop. The outer impact zone consists of a thin layer of surficial crust and a series of more or less parallel ridges arranged into mechanically upturned plates. The inner impact zone consists of intact or cracked quartz crystals. The susceptibility of aeolian‐originated quartz grains to frost‐induced modifications depends therefore on a combination of internal (i.e., original crystallography of quartz grains) and external (i.e., aeolian and frost processes acting upon the grains) factors.