地幔温压条件下铜的晶体化学行为研究

The physical and chemical properties of copper are related to its electronic structure. The possible change of copper’s electronic structure on the conditions of high pressure and high temperature of the Earth’s mantle would be an important factor to affect its distribution and differentiation between the layers of the Earth. At atmospheric and relative low high-pressures, the valence states of copper occur as +1 and +2 with electronic configurations of 3d10 and 3d9, respectively, which make the crystal structures of copper minerals and compounds with low coordinated number of Cu+ (controlled by covalent nature) or special square planar coordination of Cu2+ (resulted by Jahn-Teller effect). Such kinds of crystal chemical properties of copper minerals and compounds are disadvantaged to form the dense structures at high pressures; and thus, we predict that the electronic structure of copper would possibly modified to fit the high pressure environments in the deep Earth. In this project, we intend to carry out the experimental study on some of the representative copper minerals and compounds at high pressures and high temperatures by using the diamond anvil cell (DAC) techniques. Through the in situ measurements of Raman spectroscopy, synchrotron X-ray diffraction and X-ray absorption on the sample systems at high pressure, we can probe the high pressure structures of these copper minerals and compounds, their stability with pressure, and the possible chemical reactions in the samples. In the process, we will focus especially on the possible modifications of the valence state and coordinated polyhedrons of copper atom to uncover the nature of its electronic structure changes with pressure. Based on the experimental results, we would discuss the possible models of occurrence of copper in the deep mantle, and furthermore, to understand the copper’s geochemical behaviors such as distribution and differentiation in the process of core-mantle segregation and in the environments of the whole mantle convection of the silicate Earth.

铜的物理化学性质与其电子结构相关，它在地幔温压条件下电子结构的可能变化是决定铜在地球各层圈分配分异行为的重要因素。常压和较低压力下铜的价态主要为+1价和+2价，价电子构型分别为3d10和3d9，导致各种铜矿物/化合物晶体结构中铜采取低配位（Cu+，共价键性所致）、或者四方面状配位（Cu2+，Jahn-Teller效应所致）的晶体化学特征，不利于高压下致密结构的形成，据此推测高压下铜的电子结构有发生变化的可能。本申请拟利用金刚石压腔高压装置对一些具代表性的铜矿物/化合物开展高温高压实验研究，通过拉曼光谱、同步辐射X射线衍射和X射线吸收等高压原位测试手段，探测这些铜矿物/化合物在地幔温压条件下的高压结构、稳定性和可能的化学反应，特别关注铜的价态和配位多面体形式的可能变化及其电子结构变化本质，探讨地幔深处铜的可能赋存形式，进一步理解核-幔分离过程和全地幔物质对流环境中铜的分配分异等地球化学行为。

铜(Cu)是一种亲硫性和中等亲铁性元素，可应用于地球早期演化历史重要地质事件的探讨。目前对铜的晶体化学性质和行为的认识仍基于常压或较低压力的观察，对铜的高压晶体化学性质尚缺乏了解。自然界含铜矿物中铜为+1和+2价，它们独特的电子构型（分别为3d10和3d9）使其在各种矿物中表现出低配位数或特别的CuO4四方面状配位，这种晶体化学特征不利于下地幔高压环境对矿物致密化的要求，很可能其高压晶体化学性质将发生显著变化。基于此，我们利用金刚石对顶砧装置，对典型的含铜氧化物矿物（如赤铜矿、黑铜矿、铜铁矿等）和含铜硫化物矿物（如黄铜矿、铜蓝、辉铜矿、CuS2等）开展了大量的高温高压实验研究工作，发现下地幔温压条件下Cu+离子不能稳定存在，Cu2+离子可形成CuO6八面体配位，Cu2+离子亲硫性较Cu+和Fe2+降低等认识。铜铁矿(CuFeO2)在30-54GPa、经高温处理后，转变成与下地幔主要矿物方镁铁矿一样的无序岩盐结构(Fm3m)的(Cu0.5,Fe0.5)O固溶体，表明高温高压导致Cu与Fe之间发生了电荷转移：Cu+ + Fe3+ → Cu2+ + Fe2+，结合赤铜矿(Cu2O)高温高压下转变成黑铜矿(CuO)与金属铜的实验结果，反映下地幔压力下Cu+阳离子不稳定、Cu2+阳离子是下地幔中Cu的主要存在形式。无序岩盐结构(Cu0.5,Fe0.5)O固溶体中Cu2+离子出现在规则的O6八面体位置，反映其低压下Jahn-Teller畸变在高压下被完全抑制，其性质变得类似于其它二价过渡金属离子。(Cu0.5,Fe0.5)O固溶体的状态方程参数显示它具有比FeO更大的晶胞体积，这与已有离子半径数据(Cu2+和Fe2+在八面体配位时离子半径分别为0.73Å和0.78Å)相矛盾，很可能现有Cu2+离子半径需要修正。(Cu0.5,Fe0.5)O固溶体在卸压过程中发生非晶化，反映随着压力降低，Cu2+离子的Jahn-Teller畸变效应故里以恢复。对黄铜矿(CuFeS2)的高温高压实验研究，检测到FeS2和富Cu的Cu-Fe-S相（成份近似Cu4FeS3），而对铜兰(CuS)的实验结果，转变成黄铁矿结构的CuS2和富Cu的CuxS相，反映硫在高压下有形成S22-离子对的趋势，而且Cu对S的争夺能力比Fe低，反映Cu2+离子亲硫性不如Fe2+离子。