Soils regulate the movement of potentially toxic trace elements from natural or anthropogenic sources into food and water supplies. Because soil matrices are complex assemblages of minerals and organic matter, our most advanced analytical techniques are limited in their ability to elucidate specific geochemical reaction mechanisms that control trace-element mobility. This research aims to determine how the natural complexity of soil matrices affects arsenic reactivity. We characterized mineral-organic coatings on a quartz soil sand grain, and we measured progressive oxidation and accumulation of As following treatments with As(III). Synchrotron x-ray computed microtomography (CMT) indicated grain-coating thicknesses of tens of microns, micro x-ray fluorescence microprobe (μ-XRF) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) showed heterogeneous element distributions across the region mapped, and micro X-ray absorption near edge structure (μ-XANES) analysis revealed that a significant proportion of reacted As(III) was oxidized to As(V). Spatial statistical analyses showed a strong correlation between accumulated As and Fe except at high relative concentrations of Cr, suggesting an influence of Fe mineralogical differences. Direct analyses of chemical reactivity at a microscale in soil materials can potentially infer reaction mechanisms that control As mobility in the environment.