On the symmetry and crystal chemistry of britholite: New structural and microanalytical data
On the symmetry and crystal chemistry of britholite: New structural and microanalytical data
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关于长沸石的对称性和晶体化学：新的结构和微量分析数据
DOI：
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发表时间：
2001
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影响因子：
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通讯作者：
G. Parodi
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文献类型：
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作者：
R. Oberti;L. Ottolini;G. Della Ventura;G. Parodi
本文对两个成分为（Mn 0.04Ca 4.75RE 4.37Th 0.72U 0.12）<$10.00（Si5.57P0.25B0.16）<$5.98O 24（OH0.23F1.77）<$2.00（产自意大利马塞卢姆）和（Na0.98Ca2.01REE6.97）<$9.96（Si 5.07P 0.75）<$5.82O 24（OH0.53F1.47）<$2.00（产自几内亚洛斯群岛）的硬硅钙石样品进行了完整的结构和微观化学表征。晶体化学式的计算结合电子探针分析的中间Z元素（Na，Ca，Mn，Si，P），离子探针分析的低Z（H，Li，Be，B，F）和高Z元素（Ba，Y，REE和锕系元素），和高品质（Robs 1.2-2.6）单晶结构的细化。结构优化表明，最好的近似真实的对称性是P63空间群。在硬硅钙石中，相对于磷灰石的P63/m空间群的对称性降低意味着O3和O3 a原子不再等价，并且允许四面体围绕Si-O 1键旋转高达~4°。因此，O3 a原子移动得更接近RE 1a位点，而O3原子移动得更远离RE 1位点并更接近RE 2位点，从而呈现[7+1]配位。红外光谱的Britholite从Alzheimer的OH伸缩区域在3437 cm-1，这是一致的三价稀土阳离子在REE 2网站的顺序显示了一个独特的和非常宽的带。摩尔吸光系数εi = 23600 L · mol-1·cm-2。一个显着的恒常性在晶胞体积沿着整个磷灰石britholite组成范围内观察到的值的总离子半径在稀土元素的网站短于1.12和长于1.15毫米，而观察到的中间值大的变化，这种行为表明，由于四面体组的刚性的限制，进一步提高在高对称性。
Abstract We provide in this paper complete structural and micro-chemical characterization of two britholite samples with compositions (Mn0.04Ca4.75REE4.37Th0.72U0.12)Σ10.00(Si5.57P0.25B0.16)Σ5.98O24(OH0.23F1.77)Σ2.00 (from Latium, Italy) and (Na0.98Ca2.01REE6.97)Σ9.96(Si5.07P0.75)Σ5.82O24(OH0.53F1.47)Σ2.00 (from Los Islands, Guinea). The crystal-chemical formulae were calculated by combining electron-microprobe analyses for intermediate-Z elements (Na, Ca, Mn, Si, P), ion-microprobe analyses for low-Z (H, Li, Be, B, F) and high-Z elements (Ba, Y, REE and actinides), and high-quality (Robs 1.2-2.6) single-crystal structure refinements. Structure refinements indicate that the best approximation to the real symmetry is the P63 space group. In britholite, the lowering of symmetry with respect to the P63/m space group of apatite means that the O3 and O3a atoms are no longer equivalent and allows the tetrahedron to rotate up to ~4° around the Si-O1 bond. Consequently, the O3a atom moves closer to the REE1a site, whereas the O3 atom moves farther from the REE1 site and closer to the REE2 site, which thus assumes a [7+1] coordination. The infrared spectrum of britholite from Latium shows a unique and very broad band in the OH-stretching region at 3437 cm-1, which is consistent with the ordering of trivalent REE cations at the REE2 site. The calculated integral molar absorptivity εi is 23600 L · mol-1⋅cm-2. A remarkable constancy in the unit-cell volume along the whole apatite-britholite compositional range is observed for values of the aggregate ionic radius at the REE sites shorter than 1.12 and longer than 1.15 Å, whereas large variations are observed for intermediate values; this behavior suggests constraints due to the rigidity of the tetrahedral group, that are further enhanced at high symmetry.