Although aqueous silica (H4SiO4) is one of the most abundant components of surficial and hydrothermal fluids, the current knowledge on metal-silica aqueous complexes and their role in metal mobility is very limited. In this study the stability of aluminum-silica complexes has been determined by combining potentiomentric, solubility and Raman spectroscopic methods in a wide range of pH (1-9) and temperatures (25-300°C).
The equilibrium constant of the reaction
Al3++ H4SiO4°(aq) = AlH3SiO42++ H+
(K) has been determined using a new pH-selective glass electrode (Pokrovski et al., 1993) at temperatures between 25 and 150°C from the pH of aqueous solutions containing H4SiO4(aq), HCl and KCl with and without added AlCl3. Log K values obtained in this study at 25, 90 and 150°C are -2.38±0.10, -0.35±0.20, and 1.07±0.30, respectively. These values are linearly related to the reciprocal temperature and yield an association enthalpy of 66.6 kJ/mol. The log K value derived at 25°C is in close agreement with that determined by Farmer and Lumsdon (1994) but is ~1.3 log units lower than that proposed by Browne and Driscoll (1992). Raman spectroscopic measurements of H4SiO4 and H4SiO4-AlCl3 solutions performed between 25 and 150°C and at pH ranging from 1 to 2.5 show a significant decrease of the H4SiO4 symmetric band (783 cm-1) intensity in H4SiO4 - AlCl3 solutions with increasing temperature, while no changes were observed in aluminum-free solutions. This decrease is consistent with the formation of aluminum-silica complexes having no symmetric vibrations. Although AlH3SiO4+2 stability increases rapidly with increasing temperature, Al+3 hydrolysis also increases with temperature thus moving the field of predominance of Al+3 to lower pH. As a result, AlH3SiO4+2 complexes are significant only in acid solutions (pH<~3). For example, calculations with K values derived from our data indicate that at 200°C and pH=2 AlH3SiO4+2 complex accounts for >50% of the total Al in equilibrium with boehmite in the presence of only 0.002m aqueous silica.
In most crustal fluids whose pH's are neutral, hydrolysed aluminum species (Al(OH)n3-n) are much more significant than Al+3 (Castet et al., 1993). To investigate their possible complexation with aqueous silica, boehmite solubility measurements were performed at 300°C and pH ranging from 3 to 9. Preliminary results show that aluminum concentration increases by about 2 times in the presence of only 0.01m aqueous silica. This increase in Al concentration demonstrates the formation of complexes between silicic acid and the various Al hydrolysed species including Al(OH)4-. These results strongly suggest that Al-silica complexes can greatly increase Al mobility in crustal fluids. Explicit account must be made for Al-silica complexing to model chemical equilibria in the presence of quartz and aluminosilicate minerals at both hydrothermal and metamorphic conditions.
Analysis of available literature data on other metal-silica complexes indicates that there is a positive correlation between the first hydrolysis constant of a metal and the stability constant of the complexes that it forms with silica. This correlation implies that silica can form very stable complexes with strongly hydrolysed metals such as Fe, Zr, U, Au, and thus can control the transport of these metals in crustal fluids.
Browne, B.A. & Driscoll, C.T., Science 256, 1667-1670 (1992).
Castet, S., Dandurand, J-L., Schott, J. & Gout, R., Geochim. Cosmochim. Acta 57, 4869-4884 (1993).
Farmer, V.C. & Lumsdon, D.G., Geochim. Cosmochim. Acta 58, 3331-3334 (1994).
Pokrovski, G., Zotov, A., Sergeyev, A., Gout, R. & Schott, J., In Proceedings of the 4th International Symposium on Hydrothermal Reactions, Nancy, 189-192 (1993).