INCONGRUENT AND CONGRUENT DISSOLUTION OF PLAGIOCLASE FELDSPAR - EFFECT OF FELDSPAR COMPOSITION AND LIGAND COMPLEXATION

Secondary ion mass spectrometry (SIMS) was used to characterize the surfaces of plagioclase feldspar (two labradorite compositions and one anorthite) leached in water (pH 5.8), and in 10(-4)M HCI, HF, and oxalic acid, all at the same pH (4.0-4.1). Labradorite (An54 and An60) leached at pH 4 for 72 days dissolved incongruently in HCl, oxalic acid, and HF, yielding altered layers strongly depleted in Al, Ca, and Sr and residually enriched in Si. The leached layers formed on the more calcic composition (An60) were approximately 3 times thicker than those formed on the more sodic (An54) composition. The thinnest leached layers were produced by HCl (up to 1000 angstrom on An54 and up to 3000 angstrom on An60), and the thickest leached layers by HF (up to 1700 angstrom on An54 and up to 5000 angstrom on An60). Because the H+ concentration was the same in each solution, it was possible to clearly separate the relative importance of proton-promoted dissolution (in HCl) and ligand-promoted dissolution (in oxalic acid and in HF) for a given feldspar composition. To do this, the SIMS depth profiles obtained from An60 labradorite leached in oxalic acid and in HF were normalized to those produced in response to HCl. This comparison clearly shows that the ligands oxalate and fluoride significantly increased the formation of leached layers, compared to the effect of H+ alone. In contrast, anorthite (An100) leached in the same solutions essentially dissolved congruently. In response to HCl only very thin leached layers formed in which Al and Ca were only weakly depleted relative to Si. Chemical analyses of the output solutions showed that the molar ratio of Si to Al in solution (1.1 to 1) is equal to the molar ratio of Si to Al in the fresh solid, confirming that the anorthite dissolved congruently. While the leached layers found on anorthite leached in oxalic acid and HF were thinner and less intensively depleted compared to the leached layers formed in response to HCl, the concentrations of Si and Al in the output solutions were significantly greater than those in the HCl solutions. While oxalate and fluoride again promoted the dissolution of the feldspar, Si and Al were still released to solution in the same molar proportion as they are found in the solid (congruent dissolution). The early stages of feldspar dissolution, therefore, may be either incongruent or congruent, depending upon the chemical composition of the feldspar.