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 Québec 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.

Oscillatory- and sector-zoned pyrochlore from carbonatites of the Kerimasi volcano, Gregory rift, Tanzania

“…This mechanism requires different atomic arrangements of proto-sites on different growth faces, a feature that was observed for a series of compounds with sector zoning, e.g. staurolite (Hollister, 1970), clinopyroxene (Hollister and Gancarz, 1971), monazite (Cressey et al , 1999) and perovskite (Chakhmouradian et al , 2007) among others. Below we explore the latter hypothesis in more detail.…”

“…This mechanism is plausible, and it typically accompanies mechanism (3), though it cannot be verified owing to the lack of information on the growth rate dependence of the distribution coefficient. (3) An additional plausible explanation is the selective adsorption of components to the face due to the differences in atomic structure of crystallographically non-equivalent crystal surfaces (Hollister, 1970; Hollister and Gancarz, 1971; Dowty, 1976; Chakhmouradian et al , 2007). This mechanism requires different atomic arrangements of proto-sites on different growth faces, a feature that was observed for a series of compounds with sector zoning, e.g.…”

Sector Zoning In Uraninite

“…The causes of sector zoning in minerals are not universally agreed upon (e.g., review in Chakhmouradian et al 2007). One common model suggests that small differences in field strength (defined as ion charge versus ionic radius) are the main cause for the development of sector zoning in most crystalline materials (e.g., Dowty 1976, Paquette & Reeder 1995, Cressey et al 1999: simplistically, cations with higher charge and smaller size will be more strongly bound and would adsorb preferentially on sector surfaces with relatively higher density of negatively charged protosites (Dowty 1976).…”

“…Sector zoning has been described in several minerals, for example fluorite (Bosze & Rakovan 2002), baryte, brookite (Frondel et al 1942), staurolite (Hollister 1970), pyroxene (Hollister & Gancarz 1971, Shearer & Larsen 1994, fluorite (Dowty 1976), ankerite (Searl 1990), dolomite (Fouke & Reeder, 1992), titanite (Paterson & Stephens 1992), arsenopyrite (Vesselinov & Kerestedjian 1995), and loparite-(Ce) (Chakhmouradian et al 2007), among others. The general cause for sector zoning is the differential incorporation of some components on a specific set of symmetry-related growth surfaces (Watson & Liang 1995).…”

First occurrence of moskvinite-(Y) in the Ilímaussaq alkaline complex, South Greenland – implications for rare-earth element mobility

“…They typically occur in highly alkaline rocks (e.g. Sokolova et al, 1970;Czamanske et al, 1979;Dobrovolskaya et al, 1981;Lisitsin et al, 2002;Yakovenchuk et al, 2003;Chakhmouradian et al, 2007;Andersen et al, 2012Andersen et al, , 2014, carbonatites (e.g. Mitchell, 1997;Jago and Gittins, 1999;Mitchell, 2006), kimberlites (e.g.…”