A GIXAFS study of the Fe oxide precipitates on quartz surfaces

This study investigates the interaction between quartz surfaces and iron-bearing solutions through the precipitation of Fe oxide phases under controlled physicochemical conditions. Three differently oriented quartz crystal surfaces were exposed to buffered and unbuffered Fe(II) and Fe(III) solutions, and the resulting surface precipitates were analysed by GIXAFS spectroscopy. Due to the extremely low amount of deposited material, GIXAFS represented the only suitable technique capable of simultaneously probing iron oxidation state and local structural arrangement. The results demonstrate that Fe precipitates as nanoscale clusters rather than continuous surface coatings, independently of the crystallographic orientation of the quartz substrate. XANES, pre-edge, and EXAFS analyses reveal that the final Fe speciation frequently differs from the nominal redox state imposed by the starting solutions, with widespread occurrence of mixed Fe(II)–Fe(III) species and hematite-like local structures. Thermodynamic modelling performed with PHREEQC and Pourbaix diagrams successfully explains part of the observed behaviour, particularly the tendency toward Fe(III) oxide formation during evaporation and maturation of the precipitates. However, some experimental observations, including unexpected redox transformations, cannot be fully accounted for by bulk thermodynamics alone, suggesting an additional contribution from quartz surface effects or surface defects. These findings provide new insights into Fe oxide nucleation and redox evolution on silica surfaces, with potential implications for geochemical processes, environmental remediation, and the reactivity of crystalline silica in biological environments. In particular, the observed mixed-valence Fe precipitates may represent an additional contribution to the complex mechanistic framework linking crystalline silica exposure, persistent oxidative stress, and silica-related lung diseases.