Crystallochemical aspects of two microlite-group new mineral species

Fluorcalciomicrolite, Ca₁.₅Ta₂O₆F, and hydroxycalciomicrolite, Ca₁.₅Ta₂O₆(OH), are new microlite-group [1] minerals found in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. Both occur as octahedral and rhombododecahedral crystals. The crystals are colourless, yellow and translucent, with vitreous to resinous luster. The densities calculated for fluorcalciomicrolite [2] and hydroxycalciomicrolite are 6.160 and 6.176 g/cm³, respectively. The empirical formulae obtained from electron microprobe analysis are (Ca₁.₀₇Na₀.₈₁□₀.₁₂)Σ₂ (Ta₁.₈₄Nb₀.₁₄Sn₀.₀₂)Σ₂[O₅.₉₃(OH)₀.₀₇]Σ₆.₀₀[F₀.₇₉(OH)₀.₂₁] for fluorcalciomicrolite and (Ca₁.₄₈Na₀.₀₆Mn₀.₀₁)Σ₁.₅₅(Ta₁.₈₈Nb₀.₁₁Sn₀.₀₁)Σ₂O₆[(OH)₀.₇₆F₀.₂₀O₀.₀₄] for hydroxycalmicrolite. Fluorcalciomicrolite is cubic, space group Fd-3m, a = 10.4191(6) Å, V = 1131.07(11) ų, and Z = 8. Hydroxycalciomicrolite is also cubic; however, the presence of P-lattice is confirmed by the large number of weak reflections observed by X-ray diffraction. As a result, the space group is P4₃32 and unit-cell parameters are a = 10.4211(8) Å, and V = 1131.72(15) ų.