THE ALBITE-WATER SYSTEM: PART II. THE TIME-EVOLUTION OF THE STOICHIOMETRY OF DISSOLUTION AS A FUNCTION OF PH AT 100, 200, AND 300°C

Abstract

Albite feldspar was hydrolyzed over a wide-range of pH conditions at 100, 200 and 300°C. The release rates of Na, Al, and Si were measured as a function of time from the initial, pre-steady-state phase to the attainment of steady-state, congruent dissolution conditions. Leached layers were developed during the initial stages of dissolution due to the preferential release of Na with respect to Al and Si at nearly all pH and temperature conditions. The preferential release of Na is due to the higher rate of ion exchange reactions vs. hydrolysis reactions associated with the release of Si and Al. The depths of Na preferential leaching show a pH dependence. Maximum depths on the order of 1500 and 1200 a were recorded at acid and basic pH conditions, respectively, whereas minimum depths were observed in the neutral pH range. Leached layers deficient in Al (with respect to Si) were recorded at acid and neutral pH conditions. At mildly basic pH conditions, Al and Si were congruently released, or alternatively, either Al or Si was preferentially released. At more extreme basic pH conditions, only Al was preferentially released. The depths of Al preferential leaching were not determinable under all conditions due to the precipitation of an Al surface phase; a maximum recorded depth of 250 a was determined at basic pH conditions. The preferential release behavior of Al and Si is ascribed to the pH dependency of the speciation of Al-OH and Si-OH groups at the surface and within the leached layers. However, at very basic pH conditions, where Al-O- and Si-O- groups are postulated to predominate, the preferential release of Al is probably due to the intrinsically greater reactivity of Al-bridging oxygen bonds.Calculations of diffusion coefficients for Na diffusion within the leached layers suggest that leached layers are structurally more open and porous than unaltered, crystalline albite. In addition, the calculated diffusion coefficients show a strong pH dependence. This result implies that leached layers formed at acid and basic pH conditions undergo a greater degree of structural modification since they are more open than those formed at neutral pH. The pH-dependent nature of the structural and transport properties of leached layers can be considered to be a contributing factor to the overall pH dependence of feldspar dissolution rates.The formation of leached layers is an important factor in providing hydrolyzing molecules (H+, H2O, OH-) access to Na+ exchange sites and Si-O-Si and Al-O-Si hydrolysis sites deep within the structure. Evidence for this is shown by the positive relationship between the overall rates of dissolution and the depths of Na and Al leaching. The data from this study suggest that reactions at the surface and within leached layers control the overall dissolution behavior of feldspars and other similar, multi-oxide silicates. It is therefore proposed that a ''leached layer-surface reaction'' model more accurately describes the dissolution process at elevated temperatures than the traditionally accepted ''surface reaction'' model.