CONTROL OF DISSOLUTION RATES OF ORTHOSILICATE MINERALS BY DIVALENT METAL OXYGEN BONDS

COMMON rock- and soil-forming minerals are complicated structures of varying composition. Despite some encouraging progress 1,2 there is as vet no comprehensive rationale for predicting the dissolution rates of these minerals. Here we test the hypothesis 3 that dissolution rates of compositionally distinct orthosilicate minerals scale in a fashion similar to rates of water exchange around the corresponding dissolved, divalent cation. Although dissolution rates span several orders of magnitude, the hypothesis is sustained. Minerals containing alkaline-earth cations dissolve at rates that correlate with ionic size, whereas minerals containing first-row transition metals dissolve at rates that vary with the number of cation d-electrons. Both types of behaviour are consistent with the control of dissolution rate by the character of the bonds between the divalent cation and neighbouring oxygen atoms. This result supports the proposed link 3-6 between the mechanisms of mineral dissolution and the mechanisms by which a dissolved metal exchanges ligands. With this link it may be possible to predict dissolution rates for other nearly isostructural minerals that vary in composition.