Crystal structure control of the dissolution of rare earth elements in water-mineral interactions

Leaching experiments with pyroxene and plagioclase minerals and whole-rock basalt and distilled water were performed to clarify rare earth element (REE) behavior during water-rock interactions. A system was developed to recover the leachate quickly under nearly neutral conditions (pH similar to 6) with little re-precipitation or adsorption. The release ratios (amount of an element in the leachate from the minerals or rock relative to the amount in the original minerals or rock) of REE were comparable to those of Mg and Ca. Rare earth elements were easily leached. More heavy REE (HREE) than light REE (LREE) were leached from the plagioclase, and more LREE than HREE from the pyroxene. Very little europium was leached from the plagioclase. These release patterns show an inverse relationship with the elemental partition coefficient between the minerals and magma. The trends in the REE release ratios thus depend mostly on the compatibility of each element with the crystal structure of the mineral, rather than on chemical characteristics such as the solubility of the element itself. During the basalt-water interaction, a negative Eu anomaly in the leachate increased with time because Eu was initially leached from interstitial minor phases and later was dominantly from the plagioclase which is the major host mineral of Eu.