The role of carboxylic acids in feldspar and quartz dissolution and secondary porosity development : an experimental study under diagenetic conditions

Abstract

Amelia albite and Brazilian hydrothermal quartz were reacted with 4200 ppm acetate, 4200 ppm acetate-500 ppm oxalate solutions, distilled water, and a 5% CO2 solution at selected pH values. Pore-fluid chemistry was monitored through time to obtain equilibrium and steady-state concentrations as well as dissolution rate constants. The experiments utilized flow-through hydrothermal systems at 100°C and 347 bars. These systems simulate subsurface flow rates and low water/rock mass ratios. Acetate and oxalate both significantly increase albite solubility at temperatures, pressures and pH values typical of diagenetic environments. A steady-state concentration of 15 ppm Al was obtained in the acetate solution, whereas 50 ppm Al was obtained in the acetate-oxalate solution. Al concentrations in distilled water and the CO2 -water solution were below detection limits of about 1 ppm. Al-acetate and Al-oxalate complexes are most effective at maintaining Al in solution at intermediate pH Acetate and oxalate did not significantly increase quartz solubility or dissolution rate. Acetate and oxalate both increase albite dissolution rates. Rate laws obtained for albite dissolution in acetate-oxalate solutions indicate that dissolution rate is weakly dependent on pH relative to oxalate concentration. This suggests that oxalate adsorption and Al-oxalate complex formation are dominant processes controlling dissolution. Log dissolution rate constants for acetate and acetate-oxalate solutions are as much as an order of magnitude higher than those for distilled water and the 5% CO2-water solution. Scanning electron microscope analyses of reacted albite grains show extensive dissolution features concentrated along structural dislocations such as twin lamellae, cleavage planes, and fluid inclusions. This suggests surface-controlled reaction kinetics. No authigenic aluminosilicate minerals were observed. The lack of authigenic clays indicates the efficiency of oxalate and acetate in mobilizing and transporting Al...