Canadian Mineralogist Yol, ?.4,pp. 469474 (1986) A COMPARATIVE STUDY OF SOME PEGMATITIC AND FISSURE MONAZITE FROM THE ALPS GREGORIO MANNUCCI Dipartimento di Scienze della Terra, Universitd degliStudi di Milano, via Botticelli 23, I-20133 Milan, kaly. VALERIA DIELLA C.AL.R., Centro di Studi per la Stratigralia e la Petrografia delle Alpi Centrali, via Botticelli23, I-20133 Milan, Italy, CARLO MARIA GRAMACCTOLI Dipartinento di Scienze della Terra, Universitd desli Studidi Milano, via Botticelli 23, I-20133 Milan, holy TULLIO PILATI C..|V.R., Centro per le Relazioni tra Reattivitd e StrutturaChimica, via Golgi 19, I-20133 Milan, Italy. ABSTRACT Elevenspecimens of monazitefrom different occurence$ in fissuresand pegmatitesin the Alps have been analyzed by electron microprobe. For some samplesfrom pegma- tites, 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 enrichedin uranium and 1fusrirrm. In all ttre samples, tlte rare-earthdistribution shows a low Nd and high Sm content, reliativeto the averages given in litera- ture. Appreciable variations of such distribution do occur, especiallywhere large quantities of nonessentialelements. axe present. Suchdeviations, howwer, are irregular. In view of the constancy of the unit<ell volume, evenfor large sub- stitution of the &EE by U and Th, the monazite crystals behave as ideal solutions, and the REE distribution is prac- tically 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 evolu- tion of the melt or solution before monazite crystals were formed. The Y content seems larger than usual, especially for pegmatitesamples. The Y-REEfractionation doesnot depend exclusively on differences in ionic radii. Keywords: monazite, rare earths, yttrium, thorium, ura- nium, pegmatite, fissure, Alps, electron-microprobe analysis. SoMMAIRE On a analys€,i la microsonde 6lectronique, onze sp6ci- mens de monazite provenant de divers gites dans des fis- sureset des pegmatitesdesAlpes. Pour certains €chantil- lons de pegmatites, on a d6termind les donndes de la maille, La monazite desfissuresest plus pauwe en €ldments non- essentiels (Si, Ca, Y, Th, I-I) que celle qui provient despeg- matites; cettederni0repeut, par endroits, €tre extr6mernent enrichieen uranium et thorium. Dans tous les 6chantillons, la dislribution desterres rares indique une teneur basse en Nd et haute en Sm, relativemenl aux moyennesdonnees dansla litterature. Des variations appr6ciables de pareilles distributions sont observes, surtout li orl les 6l6ments non- essentiels sont pr6ents en €rande quantitd. De tellesvaria- tions sont cependant irr€guliires. Vu la constance du volume de la maille, 6p6s dans le cas de substitution importante desterres rares (IR) par U et Th, les cristaux de mo[azite se comportenl en solutions id6ales, et la distribution des terres rares est pratiquement ind€pendantede la temp6ra- ture ou de la pression, ou encore de la teneur des autres composants i l'€tat solide. Par cons6quenl, si quelques cor- r6lations dtaient ddcouvertes, ellesne pourraient d6pendre que de processus de fractionnement qui sepr€sentbrent au cours de 1'6volution du bain fondu ou de la solution avant que ne se forment les cristaux de monazite. La teneur en Y paralt plus grandequ'd I'ordinaire, particulerement pour les €chantillons de pegmatites. Le fractionnement Y-TR ne d6pend pas uniquement de diff6rences entre rayons ioniques. Mots-clds: monazite, terres rares, yttrium, thorium',ura- nium, pegmatite, fissure,Alpes, analyse A la microsonde dlectronique. INTRODUCflON Monazite was di$covered long ago in the Alps (Ldvy 1823) but it is considered to be rate, at least in macroscopic specimens. The crystals that have been analyzed in recent times are still comparative- ly few, and many of the older analyses are not very reliable. However, some modern results are particularly in- teresring; for instance, in the Alpine region some oc- currences correspond to the present t'world record" in uranium content: 15.64 wl.tlo UO2 in the Piona monazite (Gramaccioli & Segalstad 1978); 9.5 wt.9o UO2 in the Bergell monazite (Gulson & Krogh 1973). Moreover, at least two different groups of monatzite occurrences, i.e., the "classic" fissure 469