Stibiotantalite from the Nanyangshan LCT Pegmatite, North Qinling Orogen, Central China: Occurrence and New Data on its Crystal Chemistry

ABSTRACT

Stibiotantalite, SbTaO4, from the Nanyangshan LCT pegmatite, North Qinling Orogen, Central China is described. The mineral is associated with oxystibiomicrolite, oxycalcioroméite, hydrokenomicrolite, hydroxynatromicrolite, hydromicrolite, stibnite, bismuthinite, tantalite-(Mn), columbite-(Mn), spodumene, polylithionite, trilithionite, luanshiweiite, fluorluanshiweiite, elbaite, albite, and quartz. Results from electron microprobe data analysis (n = 4) yield the average empirical formula (based on O = 4): (Sb0.96Bi0.04)Σ1.00(Ta0.85Nb0.15)Σ1.00O4. The seven strongest X-ray powder diffraction lines are [d in Å (I)(hkl)] 3.126(100)(121), 2.964(81)(040), 5.976(27)(020), 3.515(22)(111), 1.891(22)(240), 1.736(19)(161), and 2.693(16)(131). Single-crystal X-ray diffraction (SXRD) patterns show strong reflections consistent with space group Pna21. The refined unit-cell parameters from the SXRD data are a 5.5419(2) Å, b 4.9200(2) Å, c 11.8003(5) Å, and V 321.75(2) Å3 with Z = 4. The structure has been refined to a final R1 = 0.0173 on the basis of 766 reflections with Fo > 4σ(Fo). In the previous study, the AO6 octahedron of stibiotantalite was close to an ideal octahedron. The new structure refinement of stibiotantalite from the Nanyangshan pegmatite indicates that the AO6 octahedron has a larger distortion due to the lone-pair effect of Sb3+. The high-quality structural data presented in this study gives a more accurate portrayal of the crystal structure of stibiotantalite than provided in the original description of the mineral.