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

利用光谱学和x射线粉末衍射方法研究了天然硅酸盐矿物和合成硅酸盐矿物中Fe、Mn、Cu和Ti的分布。合成Na-melilite的加入NaCaFe^<3+>Si_2O_<7->Ca2MgSi_2O_7组分的穆斯堡尔谱由单重态组成，而含有Ca_2Fe^3+AlSiO_<7->组分的ca -melilite和sr -melilite的穆斯堡尔谱由双重态组成。这一结果与Akasaka et al.（1985,1986）一致，并支持了他们关于铁在千粒石中分布的理论，即铁在千粒石四面体位置的分布受T-O键共价的影响。虽然成功地合成了NaAlSiO_<4->NaFe^<3+>SiO^4中的霞石，但x射线粉末衍射图显示，在冷却过程中，它们的晶体结构发生了变形，偏离了理想结构。它们的穆斯堡尔谱由双重态和千英石组成，表明铁在两种四面体位上的偏好不同。然而，由于晶体结构在室温下的变形，铁在霞石中的位置占用随样品的不同而变化。因此，进一步的现场观测研究是必要的。本研究确定了ca - schermak型斜辉石中铁、铬和钒在八面体和四面体之间的占位，并提出了控制这些元素分布的规律。利用Rietveld方法对含catial_2o_6组分的斜辉石进行了结构分析，发现Ti主要分布在四面体位，表明在斜辉石中存在al回避规律。

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.