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Dingdaohengite-(Ce) from the Bayan Obo REE-Nb-Fe Mine, China: Both a true polymorph of perrierite-(Ce) and a titanic analog at the C1 site of chevkinite subgroup

¹ Chengdu lnstitute of Geology and Mineral Resources, 610082 Chengdu, China
² China University of Geosciences (Wuhan), 430071 Wuhan, China
³ China University of Geosciences (Beijing), 100083 Beijing, China
⁴ Technologic Managerial Department, Bayan Obo Iron Mine, 014080 Baotou, China

Correspondence: ^* E-mail: xujinsha117{at}yahoo.com.cn

Dingdaohengite-(Ce), ideally Ce₄Fe²⁺Ti₂Ti₂(Si₂O₇)₂O₈, is a new member of the chevkinite group minerals from the world-famous Bayan Obo REE-Nb-Fe Mine near Baotou city, Inner Mongolian Autonomous Region, North China. It occurs in the magnesian skarn in the excontact of granite within dolomitic marble. Most individual crystals vary from 0.2 to 1.0 cm in length. Associated minerals are diopside, tremolite, richterite, allanite-(Ce), magnetite, ilmenite, spinel, titanite, pyrochlore, F-rich phlogopite, fluorapatite, quartz, and fluorite, etc. Dingdaohengite-(Ce) is probably of bimetasomatic origin among Ca-Mg-carbonate rock and/or carbonatite, and REE-, F-rich postmagmatic hydrothermal solutions. The mineral is black and becoming brown black in thin fragments. It is translucent to opaque with a submetallic-metallic luster, and a brown streak. It is brittle with conchoidal fracture. No cleavage or parting is observed. Its hardness is VHN_25g 606.0–717.4 kg/mm² (Mohs hardness near 5.9). The measured density is 4.83(7) g/cm³ and the calculated density is 4.88(0) g/cm³. Its reflectance values (for = 589 nm) are 11.4–12.5%. It is biaxial negative. The strongest six X-ray diffraction lines in the powder pattern [d in Å (I) (hkl)] are 2.7524(100)(21), 2.7263(98)(313), 3.1978(68)(212), 2.5460(54)(04), 2.8702(52)(020), and 3.1622(46)(12).

An electron-microprobe analysis on the crystal used to collect X-ray intensity data for crystal-structure refinement gives SiO₂ 19.29, TiO₂ 18.26, Al₂O₃ 0.04, FeO 8.49, Fe₂O₃ 1.67, ThO₂ 0.16, MgO 1.32, CaO 2.17, Nb₂O₅ 0.47, Ta₂O₅ 0.00, La₂O₃ 19.53, Ce₂O₃ 28.08, Nd₂O₃ n.d., Y₂O₃ 0.00, Na₂O 0.00, sum 99.48 wt%; the Fe³⁺ /Fe²⁺ ratio was converted by Mössbauer spectroscopy. The empirical formula is (Ce_2.13La_1.49Ca_0.48Th_0.01)_4.11Fe²⁺(Ti_0.88Fe²⁺_0.47Mg_0.41Fe³⁺_0.26 Al_0.01) _2.03(Ti_1.96Nb_0.04)_2.00(Si₂O₇)₂O₈, based on 22 O atoms with prevalence of Ti in the C1 site of the structure. Dingdaohengite-(Ce) is monoclinic, a = 13.4656(15) Å, b = 5.7356(6) Å, c = 11.0977(12) Å, β = 100.636(2)°, V = 842.39(46) ų, and Z = 2.

The crystal structure of dingdaohengite-(Ce) was refined with space groups P2₁/a and C2/m. Pseudo-extinction was found, i.e., reflections with h + k = 2n are systematically strong, while those with h + k = 2n + 1 are weak, which show that the true space group of dingdaohengite-(Ce) is P2₁/a (pseudo-C2/m).

Key Words: Dingdaohengite-(Ce) • P2₁/a (pseudo-C2/m) space group • chevkinite subgroup • new mineral • Bayan Obo • China