A nanoscale study of the formation of Fe-(hydr)oxides in a volcanic regolith: Implications for the understanding of soil forming processes on Earth and Mars

Abstract Iron(hydr)oxides are one of the most important constituents of regoliths and soils derived from volcanic rocks both on Earth and Mars, often giving them their characteristic red color. This study deciphers for the first time an underlying mechanism for the formation of Fe-(hydr)oxides in a regolith which can occur during the weathering of basaltic glass; Fe-(hydr)oxides are prominent alteration products of volcanic regolths. An excellent example of these conditions is the early stage of basaltic glass weathering in the Martian regolith simulant JSC MARS-1A. In this study, a combination of focused ion beam technology and analytic transmission electron microscopy is employed in order to characterize basaltic glass weathering down to the nanometer level. Our results show that the formation of Fe-(hydr)oxide phases such as ferrihydrite, magnetite/maghemite and hematite during alteration of basaltic glass is based on complex and formerly unknown sequences of dissolution-precipitation reactions and pressure induced coalescence, segregation, aggregation, densification and growth processes. The weathering of the glass starts with its dissolution and subsequent precipitation of hydrous amorphous silica-bearing pockets rimmed by nano-size domains of ferrihydrite. An increase in molar volume during this process leads to an overall volume expansion, which promotes (a) growth of the hydrous silica through coalescence of individual pockets, (b) agglomeration of ferrihydrite domains to larger and denser aggregates in between layers or along the surfaces of plagioclase, hydrous amorphous silica and amorphous Al-(hydr)oxides, (c) formation of hematite within dense aggregates of ferrihydrite or as larger nanoparticles within an hydrous amorphous Si-Al-rich phase and (d) the break-up of plagioclase crystals and the replacement of these fragments by an hydrous amorphous Fe-Al-Si-bearing phase. At a later weathering stage, ferrihydrite nano-domains can also transform into magnetite/maghemite nanoparticles, which occur as layers around and on the surface of larger plagioclase crystals. This study also indicates the presence of past nano-environments in close proximity to each other, as for example layers of imogolite and ferrihydrite/hydrous amorphous silica occur only nanometers apart from each other on the opposite sides of unaltered glass. In accord with previous mineralogical studies of JSC MARS-1, the observed bulk and nano-mineralogical composition indicate that early alteration processes of basaltic glass under dry and cold conditions are mainly controlled by the formation of Fe-(hydr)oxides and minor imogolite and kaolinite. Recent mineralogical studies indicate that alteration processes at these conditions may have been the dominant weathering processes over long time periods on the Martian surface.