Hydrothermal syntheses of REE-rich silicates: A kinetic study of allanites

Hydrothermal experiments were performed on natural epidote and REE-bearing fluids as starting material to understand the formation of allanite from epidote. The experiments were performed at 30 to 100 MPa and at 250, 550 and 800 degrees C with a fluid containing 1000 mu g/g of each of the following elements: Y, La, Ce, Nd, Eu, Yb; and of 10 mu g/g of Th and U. At 550 degrees C, the only new mineral phase containing trivalent elements is located on grain boundaries and cracks of the starting epidote. Its composition corresponds to allanite. Except for the rim of the starting epidote no reaction is observed. At 800 degrees C the same reaction is observed at the rim of the former epidote. However, epidote is completely transformed to zoisite, clinopyroxene, spinel and garnet. The allanite seems to be formed by a solution-precipitation mechanism directly out of the trace element enriched fluid.