Crystal Structure of Natural Non-metamict Ti- and Fe^(2+)-rich Chevkinite-(Ce)

The crystal structure of non-metamict Ti- and Fe2+-rich chevkinite-(Ce) has been redetermined with the single -crystal sample collected from Bayan Obo, Inner Mongolia, China. The chemical formula of the sample is Ce4Fe2Ti3Si4O22. The crystals are monoclinic with the unit cell parameters a = 13.4656(15) ?, b = 5.7356(6) ?, c = 11.0977(12) ?, β= 100.636(2)o, V = 842.39 (16) ?3 and Z = 2. The structures of Ti- and Fe2+-rich chevkinite-(Ce) were refined with space groups P21/a and C2/m. Least-squares refinement results show that both structural models of Ti- and Fe2+-rich chevkinite-(Ce) are very good, R[F2>2σ(F2)] =0.027 with P21/a and R[F2>2σ(F2)] =0.021 with C2/m. In order to illustrate the relationship between the two space groups P21/a and C2/m, the distribution of diffraction intensities was inspected. Pseudo extinction was found, i.e., reflections with h+k=2n are systematically strong, while those with h+k=2n+1 are weak. By neglecting the systematically weak (h+k=2n+1) reflections the space group becomes C2/m. There is a mirror plane in the C2/m perpendicular to the b axis. However, oxygen atoms in the P21/a model are of a symmetrical relationship with the corresponding pseudo mirror plane. It is concluded that the crystal structure of non-metamict Ti- and Fe2+-rich chevkinite-(Ce) is a superstructure with the space group of P21/a, which is of pseudo symmetry corresponding to the space group C2/m.

Hydroxyl groups in nonmetamict chevkinite-(Ce): a crystal chemical discussion

The minerals of chevkinite group were commonly considered to be anhydrous minerals. The infrared absorption spectrum of natural nonmetamict chevkinite-（Ce） from the aegirine-alkali granite, Miannlng, Sichuan Province, China, exhibited two broad peaks in the 3600-2800 cm^-1 region owing to the OH stretching. The corresponding H20 content required for the charge balance in formula was 1.27%. The O-H. … O bond lengths maight cover from 0.2658 to 0.2794 nm by the correlated OH stretching energies. An electrostatic charge balance for chevkinite-（Ce） based on the assigned site-population from chemical data was calculated without the hydrogen contribution. The resulting empirical bond-valence sum on 06, 08, 02, 03, 05, and 04 ranged from 1.73 to 1.95 vu. The partial substitution of O by OH may occur in four atom sites： 06, 02, 04, and 05. The small differences in the bond-valence sums between the supposed donors and acceptors may mean a mixed donor/acceptor role of the involved oxygen atoms. The IR spectral features between 3394 and 3035 cm-1 consisted of various hydrous species at different structural sites and orientations. The OH groups in the chevkinlte-（Ce） appeared to be involved in local charge imbalance in the structure and to be present when the mineral crystallized hydrothermally.

Crystal Chemistry of Iron in Non-Metamict Chevkinite-(Ce): Valence State and Site Occupation Proportions

The ideal formula for chevkinite can be expressed as A4BC2D2Si4O22. It is important to determine the valence state and site occupation proportions for Fe among the B, C, and D octahedral sites as it may help to identify different species in the chevkinite group. Non-metamict chevkinite-（Ce） from Mianxi alkali feldspar-granite, Sichuan Province, China, was investigated using Moessbauer spectroscopy. The Fe^3＋/∑Fe ratio was 39.2%. A significant increase of Fe^3＋ occured during metamictization and annealing for chevkinite-group minerals. In metamict samples Fe tended to lower coordination, According to the correlation between bond length and isomer shift （IS）, the quadrupole doublets with IS = 1.10 and 0.94 mm·s^-1 can be assigned to Fe^2＋ in the B and C octahedral sites, respectively. Based on the correlation between octahedral distortion and quadrupole splitting （QS）, the quadrupole doublets with QS = 0.86 and 0.77 mm·s^-1 can be assigned to Fe^3＋ in the C and D sites, respectively. The simplified formula can be revised as： Ce4Fe^2＋ （Ti, Fe^2＋, Fe^3＋ ）2（Ti, Fe^3＋ ）2Si4（O,OH）22. It indicated that the non-metamict chevkinite-（Ce） belonged to Fe^2＋ end member of the chevkinite group because Fe^2＋ was the predominant component in the B site.