A new scheme of nomenclature for the pyrochlore supergroup, approved by the CNMNC-IMA, is based on the ions at the A, B and Y sites. What has been referred to until now as the pyrochlore group should be referred to as the pyrochlore supergroup, and the subgroups should be changed to groups. Five groups are recommended, based on the atomic proportions of the B atoms Nb, Ta, Sb, Ti, and W. The recommended groups are pyrochlore, microlite, roméite, betafite, and elsmoreite, respectively. The new names are composed of two prefixes and one root name (identical to the name of the group). The first prefix refers to the dominant anion (or cation) of the dominant valence [or H 2 O or □] at the Y site. The second prefix refers to the dominant cation of the dominant valence [or H 2 O or □] at the A site. The prefix "keno-" represents "vacancy". Where the first and second prefixes are equal, then only one prefix is applied. Complete descriptions are missing for the majority of the pyrochlore-supergroup species. Only seven names refer to valid species on the grounds of their complete descriptions: oxycalciopyrochlore, hydropyrochlore, hydroxykenomicrolite, oxystannomicrolite, oxystibiomicrolite, hydroxycalcioroméite, and hydrokenoelsmoreite. Fluornatromicrolite is an IMA-approved mineral, but the complete description has not yet been published. The following 20 names refer to minerals that need to be completely described in order to be approved as valid species: hydroxycalciopyrochlore, fluornatropyrochlore, fluorcalciopyrochlore, fluorstrontiopyrochlore, fluorkenopyrochlore, oxynatropyrochlore, oxyplumbopyrochlore, oxyyttropyrochlore-(Y), kenoplumbopyrochlore, fluorcalciomicrolite, oxycalciomicrolite, kenoplumbomicrolite, hydromicrolite, hydrokenomicrolite, oxycalciobetafite, oxyuranobetafite, fluornatroroméite, fluorcalcioroméite, oxycalcioroméite, and oxyplumboroméite. For these, there are only chemical or crystalstructure data. Type specimens need to be defined. Potential candidates for several other species exist, but are not sufficiently well characterized to grant them any official status. Ancient chemical data refer to wet-chemical analyses and commonly represent a mixture of minerals. These data were not used here. All data used represent results of electron-microprobe analyses or were obtained by crystal-structure refinement. We also verified the scarcity of crystal-chemical data in the literature. There are crystalstructure determinations published for only nine pyrochlore-supergroup minerals:

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Electron-microprobe analyses of Russian and Mongolian chevkinite-group minerals from little-known host lithologies, including various metasomatic rocks, quartzolites and an apatite deposit, are presented. The mineral species analysed include chevkinite-(Ce), perrierite-(Ce), polyakovite-(Ce) and Sr- and Zr-rich perrierite-(Ce). Compositional variation in the Sr-rich members of the group is broadly represented by the exchange vector (Fe + Mn + Al + REE) ↔ (Ca + Sr + Ti + Zr). Despite the varied parageneses, the chevkinite-(Ce) compositions are similar to previously published data. Many crystals have strong internal compositional variations, partly produced during primary crystallization and partly during low-temperature hydrothermal alteration.

The crystal structure of annealed chevkinite-(Ce), ideally (REE,Ca) Infrared spectra of unannealed crystals suggest significant radiation-induced damage. The structures of two unannealed crystals were refined, but there are major discrepancies (~13%) between the sums of the site-scattering values at the octahedrally coordinated M sites and the analogous values calculated from the chemical compositions determined by electron-microprobe analysis. These discrepancies were not present when data were collected on annealed crystals, indicating that the original crystals are partly metamict and that this can significantly affect the results of site-scattering refinement. In the crystal structure of chevkinite, there are two distinct ( sur une base de 13 cations par unité formulaire. Les spectres infrarouges des échantillons non chauffés indiquent la présence de dommage important dû à la radiation. Les structures de deux cristaux naturels ont été affinées, mais révèlent des différences importantes (~13%) entre les sommes des valeurs de la dispersion aux divers sites M à coordinence octaédrique et les valeurs analogues calculées à partir des compositions chimiques déterminées par analyse à la microsonde électronique. Ces différences ne sont pas évidentes quand les données sont prises des mêmes cristaux recuits, indication que les cristaux étaient métamictes à l'origine et que cet état peut affecter largement les résultats de l'affinement de la dispersion associée aux divers sites. Dans la structure de la chevkinite, deux tétraèdres ( Mots-clés: chevkinite-(Ce), silicate de Ti, structure cristalline, Mongolie, dommage dû à la radiation.

Abstract:The new mineral novograblenovite, (NH 4 ,: 3.330 (100) (2 2 0), 2.976 (45) ( ̅ ), 2.353 (29) ( ̅ ), 3.825 (26) (2 0 2), 1.997 (25) ( ̅̅̅̅̅ 2). The density calculated from the empirical formula and the X-ray data is 1.504 g·cm -3 . The mineral is biaxial (+) with α = 1.469(2), β = 1.479(2), γ = 1.496(2) (λ = 589 nm); 2V (meas.) = 80(10) º and 2V (calc.) = 75.7º. The crystal structure (solved and refined using single-crystal X-ray diffraction data, R1 = 0.0423) is based on the perovskite-like network of (NH 4 ,K)Cl 6 -octahedra sharing chlorine vertices, and comprises [Mg(H 2 O) 6 ] 2+ groups in framework channels. The positions of all independent H atoms were obtained by difference Fourier techniques and refined isotropically. All oxygen, nitrogen and chlorine atoms are involved in the system of hydrogen bonding, acting as donors or acceptors. The formula resulting from the structure refinement is [(NH 4 ) 0.7 K 0.3 ]MgCl 3 ·6H 2 O. The mineral is named after Prokopiy Trifonovich Novograblenov, one of the researchers of Kamchatka Peninsula, a teacher, naturalist, geographer and geologist.

Samples from Russia and Scotland are used to examine the interaction of the REE-Ti silicate chevkinite-(Ce) with hydrothermal fluids. Altered zones in crystals are distinguished by using areas of low intensity on backscattered-electron images, low analytical totals, increasingly large departures from stoichiometry and, in some cases, the presence of micropores. Initial alteration of the chevkinite results in strong Ca enrichment. With increasing degrees of alteration, Ca abundances drop sharply, as do those of the REE, Fe and Si. In contrast, Ti levels increase strongly, usually accompanied by higher Nb ± Th levels. The most altered zones contain up to 36 wt.% TiO2 and the formula cannot be expressed in the standard chevkinite formula. In detail, samples follow different alteration trends, presumably reflecting different P, T, fO2 and fluid composition. The Ti enrichment may have been related to a reaction front of dissolution-reprecipitation passing through the outer zones of the original chevkinite, leaving behind a reprecipitated Ti-enriched phase which may or may not be chevkinite.

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