MINERALOGY, CHEMICAL-COMPOSITION, AND INTERACTION OF LUNAR SIMULANTS WITH HUMIC ACIDS

An investigation was conducted on the mineralogy, chemical composition, and interaction of two NASA lunar simulants, Minnesota basalt and Coming glass, with humic acid (HA). Chemical composition was determined by a fluoro-boric acid digestion process. Mineral composition was investigated by x-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and energy dispersive analysis by x-rays (EDAX). Interaction was studied by shaking samples of the lunar simulants with HA solutions at pH 2.45 and 7.0 for 1 to 40 days. Silica, Fe, and Al released were determined by atomic absorption spectroscopy. Changes in mineral structure and elemental composition of single crystals were examined by SEM and EDAX. Minnesota basalt was composed of approximately 60% plagioclase and 40% ferromagnesian minerals. The presence of ilmenite was detected by SEM and EDAX. Corning glass was amorphous to XRD. The rate and kinetics of dissolution of the two lunar simulants were similar to those reported for soil silicates and carbonate minerals. The release of Si, Al, and Fe was curvi-linear in regression, and the rate of dissolution was greater at low than at high pH. Interaction with HA resulted in crust formation covering the lunar mineral surfaces, which acted as a barrier, slowing down further dissolution with time. The concentrations of the dissolved metals were too small to create toxicity in plants, especially in the presence of HA with its high complexation capacity. If these lunar simulants are considered representative of the real lunar regolith, it is expected that the interaction between lunar minerals and soil organic matter will be similar as that between soil minerals and organic matter.