Columbite supergroup of minerals: nomenclature and classification

AbstractThe 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 MO2; 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 a0, b0 and c0. Based on the multiplying of the ixiolite-type unit cell the following derivatives are distinguished: ixiolite type [ixiolite-group minerals; a = a0,b = b0 and c = c0; space group Pbcn; the members are ixiolite-(Mn2+), ixiolite-(Fe2+), scrutinyite, seifertite and srilankite]; wolframite type [wolframite-group minerals, ordered analogues of the ixiolite type with a = a0,b = b0 and c = c0; P2/c; the members are ferberite, hübnerite, huanzalaite, sanmartinite, heftetjernite, nioboheftetjernite, rossovskyite and riesite]; samarskite type [samarskite-group minerals; a = 2a0, b = b0 and c = c0; P2/c; the members are samarskite-(Y), ekebergite and shakhdaraite-(Y)]; columbite type [columbite-group minerals; a = 3a0,b = b0 and c = c0; 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 = 2a0,b = 2b0 and c = c0; C2/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-(Mn2+), with the end-member formula (Ta2/3Mn2+1/3)O2. Ta- and Nb-dominant analogues of ixiolite with different schemes of charge balancing have the end-member formulae (M15+0.5M23+0.5)O2, M15+2/3M22+1/3)O2, M15+0.75M2+0.25)O2 or M15+0.8□0.2)O2 and the root name ‘ixiolite’ (for M1 = Ta) or ‘nioboixiolite’ (for M1 = Nb).