镁对方解石相变压力和拉曼光谱影响的实验研究

doi:10.3964/j.issn.1000-0593(2019)07-2053-06

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Abstract 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 (Mg_0.03Ca_0.97)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⁺-CO^2-₃ 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.

Key words：Calcite; Phase transition; Raman spectroscopy; Magnesium concentration

Received: 2018-05-22 Accepted: 2018-10-09

ZTFLH:

P574

Corresponding Authors: YUAN Xue-yin E-mail: xueyinyuan@live.com

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2019)07-2053-06 OR https://www.gpxygpfx.com/EN/Y2019/V39/I07/2053

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