Nomenclature and classification of the aeschynite-group minerals

Abstract The nomenclature and classification of the aeschynite-group minerals were reassessed in accordance with the latest IMA−CNMNC rules governing end-member formulae. The aeschynite group minerals have the general formula AB2O6, where the A site is occupied by the rare earth elements (REEs; mainly lanthanides and Y), Ca, Th, and U, and the B site is occupied by Ti, Nb, or Ta. These minerals have isostructural crystal structures, which are currently assigned to the orthorhombic crystal system with the standard space group Pnma (#62) and H-M symbol 2/m 2/m 2/m. Based on the newly adopted IMA-CNMNC guidelines (i.e. valency-imposed double-site occupancy), the substitution mechanism in the aeschynite group can be expressed as: AREEs3+ + BTi4+ ↔ ACa2+ + B(Nb,Ta)5+. Consequently, to preserve electroneutrality, the end-member compositions of the aeschynite group correspond to vigezzite CaNb2O6, rynersonite CaTa2O6, aeschynite REE(TiNb)O6 and tantalaeschynite REE(TiTa)O6. The “aeschynite” and “nioboaeschynite” series (e.g., -[Ce]) have identical end-member components [Ce(TiNb)O6 and Ce(NbTi)O6], and do not form a distinct solid-solution series. Therefore, nioboaeschynite is discredited and subsumed into the aeschynite, as the root-name aeschynite was established earlier. In total, seven valid end-members of the aeschynite group are proposed here: rynersonite CaTa2O6, vigezzite CaNb2O6, aeschynite-(Ce) Ce(TiNb)O6, aeschynite-(Nd) Nd(TiNb)O6, aeschynite-(Y) Y(TiNb)O6, tantalaeschynite-(Y) Y(TiTa)O6 and tantalaeschynite-(Ce) Ce(TiTa)O6. The IMA‑CNMNC‑endorsed nomenclature and classification scheme for the aeschynite group fully capture the chemical diversity of the aeschynite‑group minerals and convey the key chemical insights of the group to mineralogists, economic geologists and industrial practitioners.