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
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.