Rb-Cs-rich rasvumite and sector-zoned "loparite-(Ce)" from Mont Saint-Hilaire (Quebec, Canada) and their petrologic significance

Abstract

Rasvumite and “loparite-(Ce)” from the Mont Saint-Hilaire alkaline complex in Quebec were re-examined using a variety of analytical techniques. Rasvumite crystals from a marble xenolith and tawite (“sodalite xenolith”) entrained in nepheline syenite contain significant amounts of Rb and Cs (up to 7.2 and 2.6 wt.%, respectively). Our data indicate that these elements are more compatible with respect to rasvumite than sodalite, tainiolite, or perovskite-type phases. Cubo-octahedral crystals and penetration twins of “loparite-(Ce)” from the tawite comprise {100} growth sectors composed of loparite-(Ce) and {111} sectors composed of lueshite; the proportion of Na 0.5 Ce 0.5 TiO 3 and NaNbO 3 components varies by ≥15 mol.% between the sectors. In addition to the light rare-earth elements and Ti, the {100} sectors are enriched in K, Sr, Ba, Y, Th, U, Fe, Si and Zr with respect to the {111} sectors, which show higher levels of Na, Ca, Nb and Ta. Some elements (Ba, Th and U) exhibit a two-fold or greater difference in D between the sectors. Crystal-chemical analysis of the sector zoning indicates that higher-charged cations partition into surface protosites with fewer bonds satisfied (in agreement with Dowty’s model). Among isovalent A -site cations, the larger partition into the {100} sectors. This observation is at variance with Dowty’s predictions, but can be readily explained in terms of the relative differences in bond strength between large and small cations (estimated from their bond-valence parameters). The distribution of B -site cations is highly charge-dependent (but size-independent) and constrained mostly by heterovalent substitutions in the A site within a given sector. Comparison with the published data shows that the inter-sectorial distribution of cations in the perovskite structure is controlled not only by their charge, radius and involvement in coupled substitutions, but also by the chemistry of crystallization environment ( e.g. , availability of Nb). The implications of these data for the study of element partitioning in perovskites are discussed. The loparite-lueshite intergrowths and Rb-Cs-rich rasvumite in the tawite are interpreted to have crystallized in equilibrium with sodalite, aegirine and tainiolite from halogen-rich peralkaline magma. The tawite and its host nepheline syenite may have formed from cognate immiscible magmas, as proposed earlier by Piilonen, McDonald and Lalonde.