黄河矿晶体结构的重新测定及空间群对称的讨论

黄河矿 BaCe(CO_3)_2F(a=0.5072(1)nm,c=3.846(1)nm;V=856.8×10^(-3)nm^3;Z=6)晶体结构的重新测定,采用中心对称空间群 R(?)m—D_(3d)~5,基于337个可观察的单晶 X 射线数据(I>2σ_I)和30个可变参数,R=0.031。原子坐标和均方根振幅(据各向异性位移参数计算,O.013—0.025nm)表明,没有必要降低对称性,即没有必要采用相应的非中心对称空间群或其它次群。黄河矿的结构中,原子形成似层状排列,且垂直于 c 轴;碳酸根通过 Ba 与 Ce 的联结形成三维结构。C(1)O_3,和 C(2)O_3,中的 C 原子偏离三个氧组成的平面,其偏离的距离分别为0.0066(15)nm 和 0.0029(15)nm。

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.