Hydroxycalciomicrolite, Ca1.5Ta2O6(OH), a new member of the microlite group from Volta Grande Pegmatite, Nazareno, Minas Gerais, Brazil

Hydroxycalciomicrolite, Ca1.5Ta2O6(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 yellow and 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, ncalc. = 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, (Ca1.48Na0.06Mn0.01)Σ1.55(Ta1.88Nb0.11Sn0.01) Σ2.00O6.00[(OH)0.76F0.20O0.04]. Hydroxycalciomicrolite is cubic, with unit-cell parameters a = 10.4205(1) A, V = 1131.53(2) A3 and Z = 8. It represents a pyrochlore supergroup, microlite-group mineral exhibiting P4332 symmetry, instead of Fd-3m 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.