The crystal chemistry of birefringent natural uvarovites: Part II. Single-crystal X-ray structures
The crystal chemistry of birefringent natural uvarovites: Part II. Single-crystal X-ray structures
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DOI：
10.2138/am-2001-1011
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发表时间：
2001-10
影响因子：
3.1
通讯作者：
M. Wildner;M. Andrut
中科院分区：
文献类型：
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作者：
M. Wildner;M. Andrut
摘要：研究了6种不同产地的双折射乌云母榴石的晶体结构(Saranov；维谢洛夫斯克和萨兰卡。乌拉尔山脉。利用单晶x射线CCD衍射数据对其进行了研究。强度和晶格参数数据证明了立方石榴石空间群Ia3 ~（1 ~(1 ~)）的破坏，以及对称约化为三斜（1 ~(1 ~)）、单斜（2 ~(1 ~)）或最多正交对称（Fddd）的亚群。从空间群/I开始仔细的结构改进表明，在八面体位点上的部分长程Cr3+/Al有序是最突出的非立方特征。对于每个晶体，观察到单个八面体尺寸与其Cr占位几乎完美的线性相关。考虑到体积Cr摩尔分数的依赖性，非立方uvarovita -grossular固溶体中个体八面体尺寸为（Å） = 1.9247 + 0.0147 XCr（体积）+ 0.0534 XCr（个体）。这些乌瓦云母在结构上也以非立方方式加入了微量的水成分（<1 wt% H2O）作为O4H4“水榴石”取代。这就导致了对立方对称的进一步微妙的偏离。在这些低含水量的范围内，精炼的Si-O键长，平均在晶体学上不同的四面体上。与总OH吸收系数αi的关系式为≪Si-0》（Å） = 1.6455 + 1.0074·10-7 αi（cm-2）。虽然Si和Ca原子在i1o中占据一般位置。它们几乎没有偏离它们各自在la3°d中的特殊位置。与各自的角晶格畸变相一致，在Saranka样品中，Cr3+/Al位点的精细分布模式绝对是三斜的，在Veselovsk样品中，略微不那么明显的三斜（伪单斜），而在Saranov地区的所有四个样品中，都明显是伪正交或正交的。考虑到晶体化学证据，三个高含水量的伪正交Saranov样品似乎也具有三斜对称性，而一个低含水量的uvarovitite被归类为正交。这些结果与本研究第一部分报告的光学和UV-VIS-IR光谱调查结果很好地一致（Andrut和Wildner 2001）。比较和讨论了以往文献报道的非立方双折射石榴石的结构数据。特别强调的是结构和晶体化学的细节，显示出非理想的混合行为沿着乌氏岩-粗晶接合。讨论了文献中假设的阳离子排序模型的支持和反对证据。
Abstract The crystal structures of six birefringent uvarovite-grossular garnets from three localities (Saranov. Veselovsk, and Saranka. Ural Mountains. Russia) were investigated using single-crystal X-ray CCD diffraction data. The intensity and lattice parameter data attest to the violation of the cubic garnet space group Ia3̅d and the symmetry reduction to subgroups with triclinic (I1̅), monoclinic (I2/a), or at most orthorhombic symmetry (Fddd). Careful structure refinements starting in space group /I reveal that partial long-range Cr3+/Al ordering on the octahedral sites is the most prominent non- cubic feature. For each crystal, a nearly perfect linear correlation of the individual octahedral size with its Cr occupancy is observed. Considering the dependence on the bulk Cr mole fraction, the individual octahedral size in non-cubic uvarovite-grossular solid solutions is represented by (Å) = 1.9247 + 0.0147 XCr(bulk) + 0.0534 Xcr(individual). These uvarovites also structurally incorporate traces of hydrous component (<1 wt% H2O) as O4H4 “hydrogarnet” substitution in a non-cubic way. thus leading to further subtle deviations from cubic symmetry. Within the range of these low water contents, the refined Si-O bond length, averaged over crystallographically different tetrahedra. correlates with the total integral OH absorption coefficient αi through the equation ≪ Si-0≫ (Å) = 1.6455 + 1.0074·10-7 αi(cm-2). Although the Si and Ca atoms occupy general positions in I1̅. they deviate little from their respective special positions in la3̅d. Consistent with the respective angular lattice distortions, the refined Cr3+/Al site distribution pattern is definitely triclinic in the Saranka sample, somewhat less pronounced triclinic (pseudomonoclinic) in the Veselovsk sample, and distinctively pseudo-orthorhombic or orthorhombic in all four samples from the Saranov locality. Considering crystal chemical evidence, three of the pseudo-orthorhombic Saranov samples with elevated water content appear to have triclinic symmetry as well, while one low-water uvarovite is classified as orthorhombic. These results are in good agreement with those of optical and UV-VIS-IR spectroscopic investigations reported in Part I of the present study (Andrut and Wildner 2001). Previous structure data for birefringent garnets reported in the literature as non-cubic are compared and discussed. Special emphasis is also drawn to structural and crystal chemical details exhibiting a non-ideal mixing behavior along the uvarovite-grossular join. Evidence for and against the cation ordering models postulated in the literature are discussed.