Columbite supergroup of minerals: nomenclature and classification

Abstract

Abstract The columbite supergroup is established. It includes five mineral groups (ixiolite, wolframite, samarskite, columbite and wodginite) and one ungrouped species (lithiotantite). The criteria for a mineral to belong to the columbite supergroup are: the general stoichiometry M O 2 ; the crystal structure based on the hexagonal close packing ( hcp ) of anions (or close to it); the six-fold coordination number of M -type cations (augmented to eight-fold in the case of slight distortion of hcp ); and the presence of zig-zag chains of edge-sharing M -centred polyhedra. The ixiolite-type structure is considered as an aristotype with the space group Pbcn , the smallest unit cell volume, and the basic vectors a 0 , b 0 and c 0 . Based on the multiplying of the ixiolite-type unit cell the following derivatives are distinguished: ixiolite type [ixiolite-group minerals; a = a 0, b = b 0 and c = c 0 ; space group Pbcn ; the members are ixiolite-(Mn 2+ ), ixiolite-(Fe 2+ ), scrutinyite, seifertite and srilankite]; wolframite type [wolframite-group minerals, ordered analogues of the ixiolite type with a = a 0 , b = b 0 and c = c 0 ; P 2/ c ; the members are ferberite, hübnerite, huanzalaite, sanmartinite, heftetjernite, nioboheftetjernite, rossovskyite and riesite]; samarskite type [samarskite-group minerals; a = 2 a 0 , b = b 0 and c = c 0 ; P 2/ c ; the members are samarskite-(Y), ekebergite and shakhdaraite-(Y)]; columbite type [columbite-group minerals; a = 3 a 0, b = b 0 and c = c 0 ; Pbcn ; the members are columbite-(Fe), columbite-(Mn), columbite-(Mg), tantalite-(Fe), tantalite-(Mn), tantalite-(Mg), fersmite, euxenite-(Y), tanteuxenite-(Y) and uranopolycrase]; and wodginite type [wodginite-group minerals; a = 2 a 0, b = 2 b 0 and c = c 0 ; C 2/ c ; the members are wodginite, ferrowodginite, titanowodginite, ferrotitanowodginite, tantalowodginite, lithiowodginite and achalaite]. Samarskite-(Yb), ishikawaite and calciosamarskite are insufficiently studied, tentatively considered as possible members of the samarskite supergroup. Qitianlingite, yttrocolumbite-(Y), yttrotantalite-(Y) and yttrocrasite-(Y) are questionable and need further studies. Polycrase-(Y) is discredited as identical to euxenite-(Y). Ixiolite has been renamed as ixiolite-(Mn 2+ ), with the end-member formula (Ta 2/3 Mn 2+ 1/3 )O 2 . Ta- and Nb-dominant analogues of ixiolite with different schemes of charge balancing have the end-member formulae ( M 1 5+ 0.5 M 2 3+ 0.5 )O 2 , M 1 5+ 2/3 M 2 2+ 1/3 )O 2 , M 1 5+ 0.75 M 2 + 0.25 )O 2 or M 1 5+ 0.8 □ 0.2 )O 2 and the root name ‘ixiolite’ (for M 1 = Ta) or ‘nioboixiolite’ (for M 1 = Nb).