Loparite and ‘metaloparite’ from the Burpala alkaline complex, Baikal Alkaline Province (Russia)

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 NaREETi206 (REE -Ce > La > Nd > Pr > Sin), and contains minor CaTiO3 (<4.8 omOl.%), SrYiO3 (<4.7 tool.%) and NaNbO3 (<6.4 tool.%), The mineral is pseudocubic lap = 3.8815(3) A], and produces an XRD pattern similar to that of synthetic NaCeTi206 [Pnma, a = 5.4517(4), b = 7.7058(9), c -5.4333(6) A]. The atomic coordinates and isotropic thermal parameters of synthetic NaCeTi206 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;06 x(OH,F)x.nH20, 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 strncture upon heating.

A structural study of the perovskite series Na0.75Ln0.25Ti0.5Nb0.5O3

“…As the A-site in these minerals is dominated by Na, La and Ce, it is suggested that intermediate (50 mol%) members of the solid solution series NaNbO 3 (lueshite)-NaLnTi 2 O 6 (loparite) should adopt the space group Cmcm. As loparite-rich compositions, and the compound NaCeTi 2 O 6 , are orthorhombic Pbnm [29,30], at least three compositionally-driven space group changes can be expected to occur within this series i.e., Pbma (#57) through Cmcm (#63) to Pbnm (#62). We are currently investigating the crystal structure of this solid solution series.…”

Minerals and Natural Analogues

“…Lumpkin et al 55,56 and Chakhmouradian et al 86 described the hydrothermal alteration of Na-bearing perovskite (ideally Na 0.5 Ln 0.5 TiO 3 ) in alkaline igneous rocks. The primary result of this alteration is removal of Na from the original perovskite, producing lucasite, LnTi 2 O 6Àx (OH,F) x ÁH 2 O. Lucasite is isostructural with kassite, CaTi 2 O 4 (OH) 2 ÁH 2 O, another alteration product of perovskite (CaTiO 3 ).…”

Investigation of the hydrothermal crystallisation of the perovskite solid solution NaCe1−La Ti2O6 and its defect chemistry

“…Although the end member NaCeTi 2 O 6 has been reported in the literature with the space group Pnma [34,39] there were fewer peaks observed in the neutron diffraction pattern than were expected for this model. A tetragonal model (I4/mcm) was also tested but it gave a slightly poorer fit than the R c model.…”

“…NaCeTi 2 O 6 has been reported to have an orthorhombic Pnma structure [34,39] while several different structures have been reported for NaLaTi 2 O 6 at room temperature including cubic [32], rhombohedral [40,41], tetragonal [42] and orthorhombic [33,43,44] unit cells. Ranjan et al [40] attributed the contradicting reports for NaLaTi 2 O 6 in the literature to the fact that the distortion of the perovskite structure is weak and that other authors had used only X-ray diffraction data for structure analysis.…”