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Schiavinatoite, (Nb, Ta) BO₄, the Nb analogue of behierite

¹ Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università degli Studi di Milano, Via G. Venezian 21, I-20133 Milan, Italy
² C. N. R., Centro Studio per la geodinamica alpina e quaternaria, Via Botticelli 23, I-20133 Milan, Italy
³ Dipartimento di Scienze della Terra, Sezione di Mineralogia e Petrografia, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milan, Italy
⁴ Museo Civico di Storia Naturale, Corso Venezia 55, I-20121 Milan, Italy

^* e-mail: carlo{at}r10.terra.unimi.it

The new mineral schiavinatoite (Nb, Ta) BO₄, the niobium analogue of behierite, has been found in a boronrich pegmatite at Antsongombato, south of Betafo, in the Malagasy Republic, as portions of a greyish-pink bipyramidal crystal of Nb-rich behierite. Associated minerals are rhodizite, liddicoatite, spodumene, pollucite, danburite, apatite. The mineral is tetragonal, space group I4₁/amd, isostructural with behierite (zircon-type), with a = 6.219(5), c = 5.487(5) Å, V = 212.2(5) ų, Z = 4, D_calc = 6.548 g/cm³. The strongest lines in the X-ray powder diffraction pattern [d in Å (I, hkl)] are: 4.115(100,101), 3.110(84,200), 2.328(49,112), 1.598(42,312), 2.481(36, 211), 1.939(29,301), 1.646(25,321). Mohs's hardness is 8. Luster is vitreous, transparent in thin section, streak is white, the crystal is uniaxial (+) with average n = 2.30(5). The structure has been refined from single-crystal data, using 178 observed unique reflections to R = 0.011, R_w = 0.014.

Key-words: schiavinatoite, new mineral, behierite, niobium, tantalum, borate, crystal structure, chemical composition.

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