The solubility of an hydroxyaluminosilicate

Hydroxyaluminosilicates (HAS) are critical secondary mineral phases in the biogeochemical cycle of aluminium. They are formed from the reaction of silicic acid (Si(OH)(4)) with an aluminium hydroxide template and act as a geochemical control of the biological availability of Al. There are two main forms of HAS which we have called HAS(A) and HAS(B) and which of these will predominate will depend upon the Si(OH)(4) to Al ratio in any one environment. In all but the most heavily weathered environments or those undergoing a progressive acidification Si(OH)(4) will be present in significant excess to Al and HASB will be the dominant secondary mineral phase. We have tried to determine the solubility of HAS(B(s)) so that its contribution to Al solubility control might be compared with other secondary minerals such as Al(OH)(3(gibbsite)). In preliminary experiments, the dissolution of HASB(s) was found to be non-congruent with almost no Al being released during 18 months ageing. We then demonstrated that HASB(s) was significantly less soluble than AI(OH)(3(s)) prepared under identical experimental conditions. We have used this information to describe a solubility expression for HAS(B(s)) at a predefined quasi-equibrium and to calculate a solubility constant. K*Al2Si2O5(OH)(4) = [Al2O4+][SiO2](2)[OH-](4) This unconventional solubility expression was derived to take account of the non-stoichiometric dissolution of HAS(B(S)) and included theoretical dissolution products which could then be substituted for the dissolution products which were measured experimentally. K*HAS(B) = [Al-r][Si(OH)(4)](2)[OH-](4) K*HAS(B) = 10(-40.6+/-0.15) (n = 17) (20 degreesC/I = 0.1 mol/L) The derivation of the solubility expression, though non-standard in approach, was validated by its application to Al(OH)(3(s)) and the calculation of a realistic solubility constant. K*Al2O(OH)(4) = [Al2O4+][OH-](4) K*Al(OH)(3(s)) = [Al-r][OH-](4) =10(-30.8+/-0.21) (n = 7) (20 degreesC/I = 0.1 mol/L) K*HASB(s) was found to be independent of [Si(OH)4] and predicted that HASB(s) could be the predominant secondary mineral phase controlling the solubility of Al in environments in which the pH 4.00 and [Si(OH)(4)] > 100 mumol/L. (C) 2004 Elsevier Ltd. All rights reserved.