Abstract

Loparite-(Ce) is a ubiquitous accessory mineral in modally diverse albite-rich metasomatic rocks of the Burpala alkaline complex, Siberia. Compositionally, the mineral approaches the ideal formula NaREETi 2 O 6 (REE = Ce>La>Nd>Pr>Sm), and contains minor CaTiO 3 (<4.8 mol.%), SrTiO 3 (<4.7 mol.%) and NaNbO 3 (<6.4 mol.%). The mineral is pseudocubic [a p = 3.8815(3) Aa], and produces an XRD pattern similar to that of synthetic NaCeTi 2 O 6 [Pnma, a = 5.4517(4), b = 7.7058(9), c = 5.4333(6) Aa]. The atomic coordinates and isotropic thermal parameters of synthetic NaCeTi 2 O 6 refined from an XRD powder pattern using the Rietveld method, are given. At Burpala, loparite precipitated from an alkaline REE-rich fluid during the metasomatic alteration of earlier-formed intrusive rocks. In some parageneses, loparite was replaced by "metaloparite" during the final stages of metasomatism. "Metaloparite" has the empirical formula REETi 2 O (sub 6-x) (OH,F)x.nH 2 O, and shows minor enrichment in Ca and depletion in Sr, compared to co-existing loparite. The formation of "metaloparite" involved cation leaching, hydration and ion-exchange between loparite and a fluid. "Metaloparite" is metamict at room temperature, but some samples regain the perovskite-type structure upon heating.

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