ARE DISSOLUTION MECHANISMS THE SAME FOR CRYSTALLINE

Amorphous materials are characterized by the lack of long-range structural order, yet possess quantifiable short- to medium-range order. In many cases, the bonding arrangement between cations and ligands (oxygen) are similar and bond lengths are identical. For example, d²⁹Si MAS-NMR measurements indicate no significant difference between Si—O bonds in conjugate crystalline and glassy materials. In addition, there are reported cases in which measured dissolution rates of “mineral glass” and crystalline materials (e.g., feldspar glass and crystals) are indistinguishable from each other. Despite these similarities the mechanisms that govern dissolution are different for amorphous and crystalline species, especially as equilibrium is approached. Equilibrium with respect to the rate-limiting phase can be approached experimentally by adding either dissolved Al or Si (or both) to solution. Based on our work on alkali-rich (Na>Al+^ivB) boroaluminosilicate glass and feldspar crystals, we will show that, on the one hand, crystalline dissolution is governed by the presence or absence of etch pits. In contrast, glass dissolution is controlled by surface reaction control at far from equilibrium settings, and by ion-exchange kinetics near equilibrium. For situations in which the borosilicate glass is phase separated, dissolution of the sodium metaborate (Na₂B₄O₇) domains is the principal mechanism by which glass components are released to solution. In all of the above cited cases, as the system approaches equilibrium with respect to the rate-limiting secondary phase, dissolution rates typically show a non-linear response to the saturation state of the solution. These data indicate that linear Transition-State Theory (TST) models are likely inappropriate for most silicate materials (crystalline or amorphous). As well, the data demonstrates that rate models used to describe the dissolution of crystalline substances cannot be applied to models of glass dissolution.