A comparative study of some pegmatitic and fissure monazite from the Alps.

Eleven specimenso f monazite from different occurence$ in fissures and pegmatites in the Alps have been analyzed by electron microprobe. For some samples from pegmatites, unit-cell data have been obtained. Fissure monazite is poorer in nonessential elemenls (Si, Ca, Y, Th, [.I) than monazite coming from pegmatites; tlle latter can in places be extraordinarily enriched in uranium and thorium. In all the samples, tlte rare-earth distribution shows a low Nd and high Sm content, reliative to the averages given in literature. Appreciable variations of such distribution do occur, especially where large quantities of nonessential elements. axe present. Such deviations, howwer, are irregular. In view of the constancy of the unit-cell volume, even for large substitution of the REE by U and Th, the monazite crystals behave as ideal solutions, and the REE distribution is practically independent of temperature or pressure, or of the presence of other components in the solid state. Therefore, if some correlations are found, these only depend on processes of fractionation that occurred during the evolution of the melt or solution before monazite crystals were formed. The Y content seems larger than usual, especially for pegmatite samples. The Y-REE fractionation does not depend exclusively on differences in ionic radii.