Hydroxykenopyrochlore, (□,Ce,Ba)2(Nb,Ti)2O6(OH,F), a new member of the pyrochlore group from Araxá, Minas Gerais, Brazil

Abstract

ABSTRACT Hydroxykenopyrochlore, (□,Ce,Ba)2(Nb,Ti)2O6(OH,F), occurs in a weathered Nb-ore from alkaline-carbonatite complexes and pegmatites of the Brazilian shield mined by Compania Brasileira de Metalurgia e Mineração (CBMM), Araxá, Minas Gerais, Brazil. The mineral is a product of alkali metasomatism. It occurs as parts of granular grains up to 0.1 mm in size in association with Ba-bearing hydrokenopyrochlore. Hydroxykenopyrochlore is lemon yellow to yellow in color, non-fluorescent, and brittle. The hardness is 4½ on the Mohs scale. The calculated density is 4.36 g/cm3. It is cubic, Fd–3m, with cell parameters a 10.590(5) Å, V = 1187.6(10) Å3, and Z = 2. The strongest seven lines in the powder XRD pattern [d in Å (I/I0) hkl] are 6.06 (49) 111, 3.18 (27) 311, 3.05 (100) 222, 2.64 (29) 400, 1.870 (56) 440, 1.594 (50) 622, 1.213 (15) 662, and 1.182 (13) 840. The empirical formula derived from electron-microprobe analyses is [□1.117Ce0.532Nd0.035La0.021Pr0.010Sm0.003Y0.002Ba0.101Ca0.030Pb0.004Th0.061U0.007K0.040Na0.036]Σ2(Nb1.368Ti0.325P0.095Fe0.091Al0.082Zr0.039)Σ2[O4.719(OH)1.281]Σ6[(OH)0.846F0.154]. Hydroxykenopyrochlore is a member of the pyrochlore supergroup (class 4.DH.15 of Strunz & Nickel; class 8.2.1. of Dana). It is the vacancy-dominant analogue of hydroxycalciopyrochlore, (Ca,Na,U,□)2(Nb,Ti)2O6(OH), and the Nb-dominant analogue of hydroxykenomicrolite, (□,Na,Sb3+)2Ta2O6(OH), and of hydroxykenoelsmoreite, (□,Pb)2(W,Fe3+,Al)2(O,OH)6(OH).