Lueshite, pyrochlore and monazite-(Ce) from apatite-dolomite carbonatite, Lesnaya Varaka complex, Kola Peninsula, Russia

Citation
Ar. Chakhmouradian et Rh. Mitchell, Lueshite, pyrochlore and monazite-(Ce) from apatite-dolomite carbonatite, Lesnaya Varaka complex, Kola Peninsula, Russia, MINERAL MAG, 62(6), 1998, pp. 769-782
Citations number
49
Categorie Soggetti
Earth Sciences
Journal title
MINERALOGICAL MAGAZINE
ISSN journal
0026461X → ACNP
Volume
62
Issue
6
Year of publication
1998
Pages
769 - 782
Database
ISI
SICI code
0026-461X(199812)62:6<769:LPAMFA>2.0.ZU;2-G
Abstract
Apatite-dolomite carbonatite at Lesnaya Varaka, Kola Peninsula, Russia, hos ts intricate mineral intergrowths composed of lueshite in the core and pyro chlore-group minerals in the rim. Lueshite is a primary Nb-bearing phase in the carbonatite and ranges in composition from cerian lueshite to almost p ure NaNbO3. For comparison, the compositional variation of lueshite from th e Kovdor and Sallanlatvi carbonatites is described. At Lesnaya Varaka, lues hite is replaced by nearly stoichiometric Na-Ca pyrochlore due to late-stag e re-equilibration in the: carbonatite system. X-ray powder diffraction dat a for both minerals are presented. Barian strontiopyrochlore, occurring as replacement mantles on Na-Ca pyrochlore, contains up to 43% Sr and 8-18% Ba at the A-site, and shows a high degree of hydration and strong ionic defic iency at the A- and Y-sites. This mineral is metamict and, upon heating, re crystallises to an aeschynite-type structure. Monazite-(Ce) found as minute crystals in fractures, represents the solid solution between monazite-(Ce) CePO4, brabantite CaTh(PO4)(2) and SrTh(PO4)(2). Our data indicate the hig h capacity of the monazite structure for Th and accompanying divalent catio ns at low temperatures and pressures that has a direct relevance to solving the problem of long-term conservation of radioactive wastes. Monazite-(Ce) and barian strontiopyrochlore are products of low-temperature hydrothermal or secondary (hypergene) alteration of the primary mineral assemblage of t he carbonatite.