Taipingite-(Ce), (Ce73+, Ca2)9Mg(SiO4)3[SiO3(OH)]4F3, a new mineral from the Taipingzhen REE deposit, North Qinling Orogen, Central China

Abstract A new cerite group mineral species, taipingite-(Ce), ideally ( C e 7 3 + , Ca2)9Mg(SiO4)3[SiO3(OH)]4F3, has been found in the Taipingzhen rare earth element (REE) deposit in the North Qinling Orogen (NQO), Central China. It forms subhedral grains (up to approximately 100 μm×200 μm) commonly intergrown with the REE mineral assemblages and is closely associated with allanite-(Ce), gatelite-(Ce), tornebohmite-(Ce), fluocerite-(Ce), fluocerite-(La), fluorite, bastnasite-(Ce), parisite-(Ce) and calcite. Taipingite-(Ce) is light red to pinkish brown under a binocular microscope and pale brown to colorless in thin section, and it is translucent to transparent with a grayish-white streak and vitreous luster. This mineral is brittle with conchoidal fracture; has a Mohs hardness value of approximately 5½ and exhibits no cleavage twinning or parting. The calculated density is 4.900(5) g/cm3. Optically, taipingite-(Ce) is uniaxial (+), with ω = 1.808(5), e = 1.812(7), c = e, and a = b = ω. Furthermore, this mineral is insoluble in HCl, HNO3 and H2SO4. Electron microprobe analysis demonstrated that the sample was relatively pure, yielding the empirical formula (with calculated H2O): (Ce4.02La1.64Nd1.49Pr0.41Sm0.10Gd0.02Ho0.02Tm0.01Lu0.02Y0.03Ca0.66Mg0.05Th0.01–0.51)Σ9(Mg0.75 F e 0.25 3 + )Σ1(SiO4)3{[SiO3(OH)]3.98[PO3(OH)]0.02}Σ4(F1.81OH1.17Cl0.02)Σ3. Taipingite-(Ce) is trigonal and exhibits space group symmetry R3c with unit cell parameters a = 10.7246(3) A, c = 37.9528(14) A, V = 3780.39(20) A3 and Z = 6. The strongest eight lines in the X-ray diffraction pattern are [d in A(I)(hkl)]: 4.518(50)(202), 3.455(95)(122), 3.297(85)(214), 3.098(35)(300), 2.941(100)(02,10), 2.683(65)(220), 1.945(40)(238) and 1.754(40)(30,18). The crystal structure has been refined to a R1 factor of 0.025, calculated for the 2312 unique observed reflections (Fo ≥ 4σ). The mineral is named after its discovery locality and is characterized as the F-dominant analogue of cerite-(Ce).