Tantalaeschynite-(Ce), a new Ta- dominant member of aeschynite- group from Ta-enriched pegmatite at Huangshan, South China

Abstract Tantaleaschynite-(Ce), ideally Ce(TiTa)O6, is newly discovered and approved by the IMA-CNMNC (IMA 2023-058), as Ta-dominant member of aeschynite-group mineral in Huangshan pegmatite, South China. Tantaleaschynite-(Ce) was discovered at the border of pegmatite dyke, and occurs as subhedral or anhedral granular, tabular crystals and radiating clusters, 10 to 50 μm in size, and paragenetically associated with Ta-rich titanite, and fluorcalciomicrolite. Tantaleaschynite-(Ce) is translucent to opaque and yellowish-orange color in thin section scale, conchoidal fracture and resinous lustre. The density is calculated 6.88 g·cm–3 on the basis of empirical formula of Ce(TiTa)O6. The Raman spectrum of tantalaeschynite–(Ce) shows the bending of O−Ta−O, O−Ti−O and the stretching Ti−O vibrations in the range of 200−450 cm–1, the bands of Ta−O stretching vibrations at 659 and 856 cm–1. The aeschynite–group minerals have the general formula AB2O6, and electron microprobe analyses yield an empirical formula for tantaleaschynite-(Ce) of (Ce0.38Ca0.38Nd0.12La0.07Pr0.04Sm0.01Eu0.01Fe0.01)Σ1.02(Ti0.94Ta0.74Nb0.21Si0.08W0.02 Al0.01)Σ2.00(O5.69OH0.22F0.09)Σ6.00, calculated on the basis of two B-site cations per formula unit. The ideal formula is Ce(TiTa)O6. The crystal structure, determined by transmission electron microscopy (TEM) and aberration-corrected transmission electron microscopy (AC-SEM), is consistent with that of synthetic Ce(TiTa)O6. Structural refinement confirms that tantalaeschynite-(Ce) adopts the aeschynite-type orthorhombic structure, space group Pnma (#62), with unit-cell parameters a = 10.969(4) Å, b = 7.551(9) Å, c = 5.421(7) Å, V = 449.1(3) Å3. This structure consists of chains of [CeO8] polyhedron and edge-sharing [Ti(Ta)O6] octahedron. Tantalaeschynite-(Ce) co-occurs with other Ta-rich minerals, including fluorcalciomicrolite and Ta-rich titanite, in a narrow zone adjacent to the blocky K-feldspar unit within the Huangshan Ta-rich pegmatite. The constitutional zone refining model provides the most robust explanation for the sharp, localized Ta enrichment observed in this boundary layer, consistent with its classification as a highly evolved pegmatite. The presence of tantalaeschynite-(Ce), together with widespread Ta-metasomatic minerals, underscores the critical role of this boundary layer in the petrogenesis of highly evolved pegmatites at Huangshan.