Calcioancylite-(La), (La,Ca)2(CO3)2(OH,H2O)2, a new member of the ancylite group from Gejiu nepheline syenite, Yunnan Province, China

Abstract

AbstractCalcioancylite-(La), ideally (La,Ca)2(CO3)2(OH,H2O)2, has been discovered from nepheline syenite of the Gejiu alkaline complex in the Honghe Hani and Yi Autonomous Prefecture, Yunnan Province, China. The mineral occurs as aggregates of subhedral grains, and the size of single crystals varies between 5–20 μm. Calcioancylite-(La) is colourless to pale pinkish grey and has transparent to translucent lustre. It is brittle with a Mohs hardness of 4. The calculated density is 4.324 g/cm3. The mineral is biaxial (−), with α =1.662, β = 1.730, γ = 1.771, 2Vmeas. = 70°(1) and 2Vcalc. = 73°. Electron microprobe analysis for holotype material yielded an empirical formula of (La0.58Ce0.55Pr0.14Nd0.10Ca0.39Sr0.20K0.04)Σ2.00(CO3)2[(OH)1.25F0.06⋅0.69H2O]Σ2.00. Calcioancylite-(La) is orthorhombic, with space group Pmcn, a = 5.0253(3) Å, b = 8.5152(6) Å, c = 7.2717(6) Å, V = 311.17(4) Å3 and Z = 2. By using single-crystal X-ray diffraction, the crystal structure has been determined and refined to a final R1 = 0.0652 on the basis of 347 independent reflections (I > 2σ). The seven strongest powder X-ray diffraction lines [d in Å (I) (hkl)] are: 2.334 (100) (013), 2.970 (80) (121), 4.334 (75) (110), 3.678 (68) (111), 2.517 (55) (200), 2.647 (47) (031) and 2.077 (44) (221). Calcioancylite-(La) is the La-analogue of calcioancylite-(Ce) and is a new member of ancylite-group minerals. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2021-090).