QU Kai1, FAN Guang2*, LI Guo-wu3, SIMA Xian-zhang1, CHEN Hua-kai4, LIU Xing1, GE Xiang-kun2, YU A-peng2
(1. Tianjin Center, China Geological Survey, Tianjin 300170, China;
2. Beijing Research Institute of Uranium Geology, CNNC, Beijing 100029, China;
3. Crystal Structure Laboratory, China University of Geosciences, Beijing 100083, China;
4. Henan Nuclear Geological Bureau, Zhengzhou 450044, China)
Abstract: The t?rnebohmite-(Ce), which was discovered in China for the first time, occurred in altered rocks in fracture zones of the Taipingzhen REE deposit, Henan Province. It is closely associated with other REE minerals including allanite-(Ce), gatelite-(Ce), F-rich cerite-(Ce), bastn?esite-(Ce), synchysite-(Ce), fluocerite-(La), and fluocerite-(Ce) in forms of intergrowth. It mostly occurred as fine-grained aggregates in form of allanite-(Ce) pseudomorph, or as the t?rnebohmite-gatelite-(Ce) intergrowth which replaced marginal parts of allanite-(Ce) crystals. It has euhedral-subhedral, granular-tabular crystals with size of generally less than 0.1 mm. It is transparent. It has light green to olive green color, glass luster, irregular fracture, Mohs' hardness of about 4.5, with calculated density of 5.08 g/cm3. It has optical properties of biaxial crystal, optically positive, strong polychromatism, green color under single polarized light, with bright high-level interference color under crossed nicols. Its chemical compositions analyzed using EPMA (with 10 analytical points in 2 grains) are of 32.05%-34.26% Ce2O3, 13.67%-17.66% La2O3, 2.92%-3.68% Pr2O3, 7.52%-10.34% Nd2O3, 8.59%-10.93% Al2O3, and 21.35%-24.97% SiO2. According to the equation of Si+P=2 to calculate the number of cations, and the equation of OH+F=1 to calculate the number of anions, its empirical formula is (Ce1.02La0.49Nd0.27Pr0.10 Sm0.02Gd0.01Th0.01Mg0.06 Ca0.01)Σ2.00 (Al0.99Fe0.03)Σ1.02(SiO4) Σ2.00(OH0.81F0.19)Σ1.00。On the basis of the analysis of X-ray single crystal diffraction, the t?rnebohmite-(Ce) belongs to monoclinic system, with the unit cell parameters of a= 7.4382(3)?, b= 5.6730(2)?, c= 16.9819(8)?; β=118.84(0)°, Z=4, V= 655.13(5)?3, and the space group of P21/c, respectively. Furthermore, characteristic peaks of the laser Raman spectral were observed at 241, 362, 862, 895, 954, 3722 cm-1, respectively. All those features of t?rnebohmite-(Ce) from the Taipingzhen are very similar to those of the reported t?rnebohmite-(Ce) in abroad. This is the first exploration for the molecular vibrational characteristics of t?rnebohmite-(Ce) from the Taipingzhen REE deposit. The genesis and affiliation of t?rnebohmite-(Ce) have also been basically discussed in this paper.
Keywords: tornebohmite-(Ce)；hydrous REE-Al-silicates；mineral characteristics；Taipingzhen REE deposit
ACTA MINERALOGICA SINICA Vol. 39, No.2, 2019, page 133-142
