为研究镁对方解石在高压条件下的相变行为和拉曼振动光谱的影响, 探索碳酸盐在地球深部的存在形式和物理化学性质, 结合金刚石压腔和激光拉曼光谱, 对具有不同镁含量的方解石开展高压实验研究。 实验选取天然无色透明冰洲石、 淡黄色半透明方解石脉和白色大理石作为研究对象, 利用ICP-AES测定冰洲石和方解石脉的成分为CaCO3; 大理石中Mg/(Mg+Ca)摩尔比为0.03, 其成分可简化为(Mg0.03Ca0.97)CO3。 每种方解石样品挑选两粒大小约为50~100×50×20 μm的颗粒放入金刚石压腔, 并在不同压力下进行相变过程观察和激光拉曼光谱测量。 实验结果显示, 常压下冰洲石和方解石脉样品的T1, T2, ν4和ν1拉曼振动频率分别为156.82, 283.55, 713.86和1 088.19 cm-1, 大理石样品的拉曼振动频率为158.15, 284.76, 715.07和1 089.20 cm-1, 表明方解石中含有3 mol%的MgCO3时会造成方解石的拉曼振动频率整体升高1 cm-1以上。 但是该变化幅度在不同压力下没有显著差别, 表明镁对方解石的拉曼振动频率随压力的变化速率(ν/p)没有明显影响。 冰洲石和方解石脉样品在1.5 GPa压力附近转变为方解石-Ⅱ, 并在2.0 GPa进一步变为方解石-Ⅲ或Ⅲb; 相比之下含有3 mol%的MgCO3的大理石则是在2.4和3.7 GPa时才转变为方解石-Ⅱ和方解石-Ⅲ。 假设镁对方解石相变压力的影响是线性的, 即方解石向方解石-Ⅱ和方解石-Ⅲ/Ⅲb的相变压力随MgCO3含量的增加以0.30和0.57 GPa·mol%-1的速率升高, 当MgCO3含量达到50 mol%时, 方解石向方解石-Ⅱ和方解石-Ⅲ/Ⅲb的相变压力将分别为16.5和30.5 GPa, 这与白云石向白云石-Ⅱ和白云石-Ⅲ的相变压力吻合。 结合前人关于方解石中MnCO3含量对矿物相变压力和拉曼光谱影响的研究结果, 发现当方解石中部分Ca2+被具有不同半径和质量的离子(如Mg2+, Mn2+等)替代以后, 阳离子与CO2-3之间以及CO2-3内部C—O化学键长度和强度都会发生改变, 从而引起矿物结构稳定性以及拉曼振动频率的明显变化; 并且两种阳离子之间半径差别越大, 该影响效果越明显。 因此, 在研究高温高压条件下方解石的相变行为和拉曼光谱时, 矿物中Mg和Mn等杂质元素对矿物结构稳定性和拉曼振动频率的影响是必须考虑的关键因素之一。

In order to investigate the influence of magnesium on the phase transitions and Raman vibrations of calcite under highpressure conditions, and to explore the stable structure and physic-chemical properties of carbonates in the deep earth, experiments under high-pressure environment were carried out with natural calcite samples containing different magnesium concentrations by using diamond anvil cell and micro-Raman spectroscopy. Colorless transparent Iceland spar, pale yellow translucent calcite vein and white marble were selected as the research objects, results from ICP-AES analysis showed that the chemical compositions of contents of Iceland spar and calcite vein were CaCO3, whereas a Mg/(Mg+Ca) molar ratio of 0.03 and a chemical composition of (Mg0.03Ca0.97)CO3 were determined for the marble. The calcite samples were crushed and fragments of about 50~100×50×20 μm were loaded into the HDAC. In-situ observations and laser Raman measurements were made while the samples were under different pressures. The Raman vibrational frequencies of Iceland spar and calcite vein as measured under ambient pressure were 156.82, 283.55, 713.86 and 1 088.19 cm-1 for the T1, T2, ν4 and ν1 vibrations, respectively, whereasvalues of 158.15, 284.76, 715.07 and 1 089.20 cm-1 were obtained for the marble sample, indicating that the Raman peak positions shifted to higher frequencies by at least 1 cm-1 for the calcite containing 3 mol% MgCO3. Within the stability pressure range of calcite, no significant difference in the shifting rates of the Raman peak positions with pressure (ν/p) was observed among different samples. Both Iceland spar and calcite vein transformed to CaCO3-Ⅱ under 1.5 GPa, and further to CaCO3-Ⅲ and Ⅲb under 2.0 GPa. Whereas for the marble containing 3 mol% MgCO3, the phase transition pressures to CaCO3-Ⅱ and to CaCO3-Ⅲ were 2.4 and 3.7 GPa, respectively. Assuming that the influence of magnesium on the calcite phase transition pressures was linear, the shifting rates of the calcite to CaCO3-Ⅱ and CaCO3-Ⅱ to CaCO3-Ⅲ/Ⅲb phase transition pressures with MgCO3 concentration were determined to be 0.30 and 0.57 GPa·mol%-1, respectively. The shifting rates could be extrapolated to 16.5 and 30.5 GPa for samples containing 50 mol% MgCO3, which is in nice agreement with the transformation pressures from dolomite to dolomite-Ⅱ and dolomite-Ⅲ. By combining our results with those investigating the influence of MnCO3 on the phase transition pressures and Raman vibrations of calcite, it can be concluded that replacement of Ca2+ by smaller and lighter ions (e. g., Mg2+ or Mn2+) will result in changes in the M2+-CO2-3 and C—O chemical bond length and bond strength, and thus, leads to significant increase in the calcite phase transition pressures and shifts of the Raman peak positions. Therefore, it is highly necessary to ascertain the influence of Mg, Mn and Fe on the calcite structure stability and Raman vibrational frequencies while discussing the phase transitions and Raman vibrations of carbonates under high pressure and high temperature conditions.