Alkali partitioning between beryl and hydrothermal fluids; an experimental study at 600 degree C and 1.5 kbar

A natural lithian-caesian beryl was re-equilibrated with hydrothermal chloride solutions of Li, Na or Cs, at 600 degrees C and 1.5 kbar. The alkali elements substitute for one another in the mineral, suggesting that they occupy the same structural position. Beryl composition depends on the absolute amount of alkalis present in the solution and not on the alkali concentration. Partition coefficients are defined; they are independent of chlorinity, but depend slightly on the alkali composition. Using these coefficients, and assuming equivalent crystallization conditions, the Li-, Na- and Cs-ratios of mineralizing fluids have been estimated on the basis of the compositions of several natural beryls; their relative constancy may reflect the buffering of the pegmatite system by alkali- aluminosilicates.