Abstract

Fluornatromicrolite is a new mineral species of the pyrochlore supergroup (microlite group), with fluorine dominant at the second anion site. The type locality is the Alto Quixaba granitic pegmatite near Quixaba village, municipality of Frei Martinho, Paraíba, Brazil. Fluornatromicrolite forms adamantine green regular to flattened octahedral crystals in albite. The streak is white, the Mohs hardness is 5, and the calculated density is 6.568 g/cm3. The chemical composition is (Na1.10Ca0.64Bi0.15) Ta2.00O5.91F1.02, or, ideally (Na,Ca,Bi)2Ta2O6F. The mineral is cubic, space group Fd3m, with a 10.4451(2) Å, V 1139.56(8) Å3 and Z = 8. The crystal structure was refined to R = 0.034 using 2861 reflections obtained with MoKα radiation on a four-circle automatic diffractometer. The structure is built up of TaO6 octahedra and (Na,Ca,Bi)O6F2 polyhedra. The (Na,Ca,Bi)–F distances are significantly shorter than the (Na,Ca,Bi)–O distances.

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