Distribution of transition elements in tetrahedral sites of silicates and its effect on change of the crystal structures

用光谱分析和X-射线粉末衍射法研究了天然硅酸盐矿物和合成硅酸盐矿物中铁、锰、铜、钛的分布。NaCaFe^&lt；3+&gt；Si_2O_&lt；7-&gt；Ca2MgSi_2O_7中合成钠黄长石的穆斯堡尔谱由一组二重态组成，而含Ca_2Fe^3+Alsio_&lt；7-&gt；组分的钙黄长岩和锶黄长岩则由二重重态组成。这一结果与Akasaka等人的结果一致。(1985,1986)，并支持他们关于铁在黄长岩中的分布的理论，即铁在黄长岩的四面体位置的分布受T-O键的共价性的影响。虽然成功地合成了NaAlSiO_&lt；4-&gt；NaFe；&lt；3+&gt；SiO^4中的霞石，但其晶体结构在降温过程中发生了形变，偏离了理想结构，X射线粉末衍射谱表明了这一点。它们的穆斯堡尔谱均由二重态和黄长石组成，表明Fe在这两种四面体晶位上具有不同的择优地位。然而，由于室温下的晶体结构发生了变形，Fe在样品中的占位率是不同的。因此，有必要通过现场观测进行进一步的研究。本文测定了Ca-tschermak型单斜辉石中Fe、Cr、V在八面体和四面体之间的占位情况，并提出了控制这些元素分布的规律。通过Rietveld方法的结构分析发现，在含CaTiAl_2O_6组分的单斜辉石中，钛主要分布在四面体位置，这表明铝的回避规则在辉石中适用。

Distribution of Fe, Mn, Cu and Ti in the natural and synthetic silicate minerals has been investigated by using spectroscopic and X-ray powder diffraction methods. The Mossbauer spectrum of synthetic Na-melilite in the join NaCaFe^<3+>Si_2O_<7->Ca2MgSi_2O_7 is composed of one-doublet, whereas Ca-melilites and Sr-melilites containing Ca_2Fe^3+AlSiO_<7-> component composed of two-doublets. This result is consistent with Akasaka et al. (1985, 1986), and supports their theory on the distribution of Fe in melilites that distribution of Fe in the tetrahedral sites of melilites is influenced by the covalency of T-O bonding. Although nephelines in the join NaAlSiO_<4->NaFe^<3+>SiO^4 were synthesized successfully, their csytal structures were deformed during the cooling process and deviated from the ideal structure, which were observed by the X-ray powder diffraction patterns. Their Mossbauer spectra were composed of two-doublets as well as melilite, which suggests the different preferency of Fe in the two kinds of tetrahedral sites. However, as the crystal structure at room temperature is deformed, the site occupancy of Fe in the nephelines were variable sample by sample. Thus, further study by in situ observation is necessary. The site occupancies of Fe, Cr and V between octahedral sites and tetrahedral sites in Ca-tschermak type clinopyroxenes has been determined by the present study, and the rule controlling the distribution of these elements has been proposed. By the structural analyses using Rietveld method, present study revealed that Ti distributes dominantly in the tetrahedral sites in the clinopyroxenes containing CaTiAl_2O_6-component, which suggests the Al-avoidance rule holds for in the pyroxene.