Monazite-(Ce) and Xenotime-(Y) From An Lct, NYF Tertiary Pegmatite Field: Evidence From A Regional Study In the Central Alps (Italy and Switzerland)

Abstract

Monazite-(Ce) and xenotime-(Y) from tertiary pegmatites of the central Alps (Italy and Switzerland) were studied by EMPA and SCXRD analytical methods. The REE-phosphate minerals analyzed are from LCT and NYF pegmatite dikes of the Central Alps, which includes three main areas: Vigezzo (LCT and NYF from the Vigezzo valley), Bodengo (NYF from the Leggia valley and LCT from the Cama valley), and Codera (LCT from Codera and Chiavenna valleys and Alpe Rossaccio). SEM-BSE images of monazite-(Ce) show primary growth zoning, sector zoning, and patchy zoning. SEM-BSE images of xenotime-(Y) show faint patchy-type zoning and spongiform texture. Both REE-phosphates evidence secondary mineral replacements that include thorianite, thorite, and uraninite. EMPA compositions show that the monazite-(Ce) is characterized by variable amounts of huttonite (ThSiO 4 ) and cheralite [Ca(Th,U)(PO 4 )] molecules. SCXRD analyses shows that the unit-cell volume of monazite-(Ce) correlates with its chemical composition; the b cell parameter shows an evident relationship with the huttonite molecule. The unit-cell volume of xenotime-(Y) is well correlated to its chemical composition. Xenotime-(Y) related to the NYF family shows a significant increase in yttrium and a lower U þ HREE content.The Tertiary pegmatite field of the central Alps is related to the Masino-Bregaglia intrusion, the San Fedelino granite, and younger magmatic pulses generated by the migration of the metamorphic thermal Lepontine Dome towards the west. The presence of poikiloblastic thorite, thorianite, and uraninite in monazite and xenotime can be induced to form from alkali-bearing fluids via coupled dissolution-reprecipitation, under moderate to high-grade and post-peak Alpine metamorphic conditions.

ON THE CRYSTAL-CHEMISTRY OF A NEAR-ENDMEMBER TRIPLITE, Mn2+2(PO4)F, FROM THE CODERA VALLEY (SONDRIO PROVINCE, CENTRAL ALPS, ITALY)

“…The dike is asymmetric, with zones characterized by different mineral associations. Triplite masses, up to 5 mm in diameter, occur in a zone strongly enriched in dark green to brownish yellow elbaite and colorless to pale pink beryl (Guastoni 2012). In thin section, triplite occurs as deeply fractured rounded masses; fractures are sharp and filled with Mn oxides.…”

Crystal chemistry and miscibility of chernovite-(Y), xenotime-(Y), gasparite-(Ce) and monazite-(Ce) from Mt. Cervandone, Western Alps, Italy

“…However, the presence of REE at the Asite, according to Speer (1982), may enlarge the stability field of huttonite to lower temperatures. Harlov et al (2007) found out that the crystallization of metastable huttonite at the expenses of monazite-(Ce) crystals can take place at 400° C (and 500 MPa), in the thorite field, as also reported by Guastoni et al (2016) for pegmatitic monazites of the Central Alps. In this light, given the difficulty to tell apart thorite and huttonite by means of EPMA analyses Harlov and Föster, 2002) and the lack of information on the (P,T) conditions of the hydrothermal fluids of Mt.…”

REE-bearing minerals in Drava river sediments, Slovenia, and their potential origin

“…5d) shows very scattered results, exhibiting relatively consistent concentrations of Y, while concentrations of HREE vary extensively. Lack of grouping and scattered results point towards the origin of xenotime from variable geological backgrounds (Guastoni et al, 2016).…”