Hydroxycalciomicrolite, Ca_1.5Ta₂O₆(OH), a new member of the microlite group from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

Hydroxycalciomicrolite, Ca_1.5Ta₂O₆(OH) is a new microlite-group mineral found in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. It occurs as isolated octahedral and as a combination of octahedral and rhombic dodecahedral crystals, up to 1.5 mm in size. The crystals are yellowand translucent, with a white streak and vitreous to resinous lustre. The mineral is brittle, with a Mohs hardness of 5-6. Cleavage is not observed and fracture is conchoidal. The calculated density is 6.176 g cm^-3. Hydroxycalciomicrolite is isotropic, n_calc. = 2.010. The infrared and Raman spectra exhibit bands due to O-H stretching vibrations. The chemical composition determined from electron microprobe analysis (n = 13) is (wt.%): Na₂O 0.36(8), CaO 15.64(13), SnO₂ 0.26(3), Nb₂O₅ 2.82(30), Ta₂O₅ 78.39(22), MnO 0.12(2), F 0.72(12) and H₂O 1.30 (from the crystal structure data), O = F -0.30, total 99.31(32), yielding an empirical formula, (Ca_1.48Na_0.06Mn_0.01)_Σ1.55(Ta_1.88Nb_0.11Sn_0.01)_Σ2.00O_6.00[(OH)_0.76F_0.20O_0.04]. Hydroxycalciomicrolite is cubic, with unit-cell parameters a = 10.4205(1) Å, V = 1131.53(2) ų and Z = 8. It represents a pyrochlore supergroup, microlite-groupmineral exhibiting P4₃32 symmetry, instead of Fd3m. The reduction in symmetry is due to long-range ordering of Ca and vacancies on the A sites. This is the first example of such ordering in a natural pyrochlore, although it is known from synthetic compounds. This result is promising because it suggests that other species with P4₃32 or lower-symmetry space group can be discovered and characterized.