MASS-SPECTROMETRIC STUDY OF THE DISSOLUTION BEHAVIOR OF SANIDINE

Secondary ion mass spectrometry has been used to measure the silicon i sotope ratio as a function of depth in feldspars after their interacti on with dilute acid solutions. The reaction of feldspars with aqueous solutions implies the formation of relatively thick K, Na and Al deple ted surface layers, especially when the dissolution experiments are co nducted in the presence of silica powder. The latter is used to lower the silicon concentration gradient at the solid-solution interface. Th is generates an uncertainty in the interpretation of the results: the depleted surface layer may have been formed by leaching of the above-m entioned elements from the solid, or by precipitation of silica from t he solution. Measurements using isotopically enriched silica powder (( SiO2)-Si-30) were therefore performed in the hope that they would solv e the uncertainty. The measured silicon isotope ratio at the surface o f the mineral appears to be different from its normal terrestrial valu e and from the enriched powder value, having a value somewhere in betw een which decreases towards its normal value as a function of depth. I n addition the silicon isotope ratio of the solution, which was measur ed using thermal ionization mass spectrometry, shows an altered value compared with the enriched silica powder. The results of both analyses therefore indicate an exchange in silicon between solid and solution and suggest the combination of two or more mechanisms during dissoluti on.