Dissolution at the surface of jarosite: an in situ AFM study

The development of the morphology of the rhombohedral {012} faces of jarosite [(K,H3O)Fe3(SO4)2(OH)6] has been observed in situ at the molecular scale in aqueous dissolution experiments using atomic force microscopy (AFM). Synthetically grown rhombohedral surfaces show growth spirals with monolayer steps about 0.5 nm high, corresponding to the d values of the (012) plane. Monolayer-deep etch pits formed on surface terraces during dissolution. The formation and expansion of these etch pits seem to contribute more to the overall dissolution than the retreat of steps that bound terraces. This observation indicates that the dissolution mechanism of jarosite is controlled by surface structure and differs from the step retreat typical for most minerals for which dissolution mechanisms at a molecular level have been investigated. In aqueous solution at pH 5.5, formation of thin, soft overlayers on the jarosite is observed. By adding Na2-EDTA, these overlayers are removed and dissolution takes place mainly by the formation and growth of etch pits. To investigate the influence of H3O+ substitution on the dissolution kinetics, K-dominant jarosite was compared with hydronium jarosite [(H3O)Fe3(SO4)2(OH)6]. Dissolution rates calculated in situ by measuring the vertical position of the surface vs. time on hydronium- and K-dominant jarosite {012} faces in deionized water are 4.41±0.7×10−7 and 1.45±0.98×10−7 mol m−2 s−1, respectively, confirming that H3O+ content on the A site in the structure accelerates jarosite dissolution.