DISSOLUTION OF A LUNAR BASALT SIMULANT AS AFFECTED BY PH AND ORGANIC-ANIONS

The dissolution of a lunar simulant (MLS-I) basalt was examined at 298 K; pH 3, 5, and 7; and in the presence of citrate and oxalate anions. The basalt was mined from an abandoned quarry in Duluth, Minnesota. T he relative abundance of minerals in the basalt are plagioclase > pyro xene > olivine > ilmenite. The chemical composition and mineralogy of the basalt most closely resembles Apollo Il Mare soil, For most of the experiments the order of major ion release (Mg, Ca, Fe, Al, and Si) f rom the MLS-1 basalt was controlled by the solubility of its mineral c omponents. The amount of major ion release followed the order Fe appro ximate to Mg > Si > Al > Ca. Deviations in this release order were rel ated to the effect of the treatments on the quantity of MLS-1 basalt d issolved and the precipitation of secondary minerals, For the pH exper iments dissolution followed a two-stage process. The first stage was c haracterized by a rapid rate of release of the major ions into solutio n, followed by a slower, more linear rate during the second stage. Rat e coefficients were calculated from the linear portions of the rate cu rves and were inversely related to pH, The first stage was attributed to the dissolution of ultrafine particles created during the sample gr inding process. During the second stage, dissolution occurred on the l arger mineral surfaces, at higher energy sites (i,e. dislocations, twi nning planes, fluid inclusions, etc), For the organic anion experiment s, dissolution followed a one-stage parabolic process. Dissolution was greater in the presence of the citrate anion compared to the oxalate anion and decreased with a decrease in concentration, It is proposed t hat the organic anions accelerate the dissolution of the MLS-1 basalt through chemisorption and subsequent disruption of metal-oxygen bonds. The rate-limiting step of the reaction involves the diffusion of the cation-organic complex formed at thr mineral surface.