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

Abstract Hydroxycalciomicrolite, Ca 1.5 Ta 2 O 6 (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 to1.5 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 . Hydroxycalciomicroliteis 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.%): Na 2 O 0.36(8), CaO 15.64(13), SnO 2 0.26(3),Nb 2 O 5 2.82(30), Ta 2 O 5 78.39(22), MnO 0.12(2), F 0.72(12) and H 2 O 1.30 (from the crystal structure data), O = F –0.30, total 99.31(32), yielding an empirical formula, (Ca 1.48 Na 0.06 Mn 0.01 ) ∑1.55 (Ta 1.88 Nb 0.11 Sn 0.01 ) ∑2.00 O 6.00 [(OH) 0.76 F 0.20 O 0.04 ].Hydroxycalciomicrolite is cubic, with unit-cell parameters a = 10.4205(1) Å, V = 1131.53(2) Å 3 and Z = 8. It represents a pyrochlore supergroup, microlite-group mineral exhibiting P 4 3 32 symmetry, instead of Fd 3 m . Thereduction 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 P 4 3 32or lower-symmetry space group can be discovered and characterized.