SECONDARY-MINERAL FORMATION DURING NATURAL WEATHERING OF PYROXENE - REVIEW AND THERMODYNAMIC APPROACH

Physical and chemical factors controlling natural weathering processes of pyroxenes through the formation of secondary minerals are reviewed. It has been shown that besides climate, mineralogy, and petrology of parent rocks, mineralogy and structural arrangement of secondary phases rule over the composition of the pyroxene-derived products during increased weathering. Drainage conditions, however, are the most critical factors in constraining weathering paths from the earlier stages of cation-depleted layer-silicate formation to the last stages of goethite/kaolinite formation. A succession of transitional phyllosilicates develops from Mg-endmembers to Fe- or Al-endmembers. The chemistry of natural-percolating waters remains poorly known, and local pH and Eh are generally unknown and difficult to measure in situ. Nevertheless these agents should play an important role at the microscale in the formation of weathering products. Therefore, thermodynamic simulation is required for a better understanding of the natural secondary products formed at the expense of pyroxene. In this, paper, computerized calculations were used to monitor the theoretical congruent dissolution, without kinetic effects, of a known mixture of minerals with a solution defined by initial pH, O2, and CO2 fugacities, temperature, and opening degree of the system. Results of this thermodynamic modeling generally agree with natural weathering paths, as, for example, successive replacements of pyroxene by Fe- and Al-bearing products such as goethite, kaolinite, and smectites. Changes of pO2 and Al-content strongly control the composition of the clay solid-solution. Consideration of kinetic effects, composition of weathering solutions in the field, and in situ oxygen fugacity measurement should rank high among future research priorities.