In situ atomic force microscopy study of hectorite and nontronite dissolution: Implications for phyllosilicate edge surface structures and dissolution mechanisms

The dissolution behavior of two smectite minerals, hectorite (trioctahedral ) and nontronite (dioctahedral), was observed in situ, in acid solutions, u sing atomic force microscopy. As expected, the crystallites dissolved inwar d from the edges, and the basal surfaces appeared to be unreactive during t he timescale of the experiments. The hectorite (010) faces appeared to diss olve about 6x more slowly than the lath ends, usually broken edges. The edg es visibly dissolved on all sides, and appeared to roughen somewhat. On the other hand, the (010), (110), and (1 (1) over bar0) faces on nontronite cr ystals were exceptionally stable, so that ally dissolution fronts originati ng at broken edges or defects would quickly become pinned along these faces , after which no more dissolution was observable. These observations can be explained by using periodic bond chain theory to predict the topology of t he surface functional groups on the edge faces of these minerals. If a cert ain amount of predicted surface relaxation is allowed on the (110) and (1 ( 1) over tilde0) faces of nontronite, an important difference between the ex ceptionally stable faces and the others becomes apparent. That is, the oxyg en sites connecting the octahedral and tetrahedral sheets are all fully bon ded on the nontronite (010), (110), and (1 (1) over bar0) edge faces, where as all hectorite edge faces and nontronite broken edges would have coordina tively unsaturated connecting O atoms. This explanation for the differentia l reactivity of these crystal faces implies that the rate limiting step of the dissolution process is the breaking of bonds to connecting O atoms.