Loparite and‘metaloparite’ from the Burpala alkaline complex, Baikal Alkaline Province (Russia) A. R. CHAKHMOURADIAN 1 , R. H. MITCHELL 1 , A. V. PANKOV 2 AND N. V. CHUKANOV 3 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B 5E1 2 Department of Mineralogy, St. Petersburg State University, 7/9 University Emb., St. Petersburg, Russia 199034 3 Institute of Chemical Physics, Chernogolovka, Moscow Region, Russia 101026 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) A ˚ ], 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) A ˚ ]. 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 6Àx (OH,F) x ÁnH 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. KEYWORDS: loparite-(Ce), ‘metaloparite’, structure, albite metasomatites, Burpala complex, Baikal Alkaline Province. Introduction COMPLEX titanates of alkaline and alkaline-earth elements have recently been studied extensively because of their potential use in ceramic nuclear- waste forms, and significance as repositories for the high field-strength and large-ion lithophile elements in the lower crust and upper mantle. Studies of naturally-occurring and synthetic titanates (Kesson et al., 1983, Giere ´ and Williams, 1992; Harley, 1994; Lumpkin and Ewing, 1996; Mitchell and Chakhmouradian, 1996, 1998 a,b; and Chakhmouradian and Mitchell, 1998a) show that their structures are remarkably tolerant towards cationic substitutions involving the rare-earth (REE) and actinide (ACT) elements, typical components of high-level radio- active waste (Ringwood et al. , 1979). Significantly less information is available on the stability of the Ti-based REE-ACT hosts in various geological environments. There is a very limited number of experimental studies on the behaviour of these phases under hydrothermal conditions (Nesbitt et al., 1981; Myhra et al., 1984; Kastrissios et al., 1987), and mineralogical publications describing primary or secondary alteration of naturally-occurring titanates (Banfield and Veblen, 1992; Lumpkin and Ewing, 1996). One of the most important REE-ACT hosts in alkaline rocks is loparite, a member of the perovskite mineral group (Mitchell, 1996; Mitchell and Chakhmouradian, 1996, 1998b; Chakhmouradian and Mitchell, 1997, 1998a). Naturally-occurring loparite exhibits a broad variation in major and minor components, including Na, REE, Ca, Sr, Th, Ti and Nb. Most compositions of this mineral can be adequately described in terms of four perovskite-type end- members (Mitchell, 1996): loparite Mineralogical Magazine, August 1999, Vol. 63(4), pp. 519–534 # 1999 The Mineralogical Society