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常温高压条件下硬石膏相变的原位拉曼光谱研究
引用本文:熊欣,袁学银.常温高压条件下硬石膏相变的原位拉曼光谱研究[J].光谱学与光谱分析,2019,39(4):1075-1079.
作者姓名:熊欣  袁学银
作者单位:中国地质大学(北京)地球科学与资源学院 ,北京 100083;中国地质科学院矿产资源研究所 ,自然资源部成矿作用与资源评价重点实验室 ,北京 100037;中国地质科学院矿产资源研究所 ,自然资源部成矿作用与资源评价重点实验室 ,北京 100037
基金项目:中央级公益性科研院所基本科研业务费专项基金项目(YYWF201520)和国家自然科学基金项目(41702039)资助
摘    要:硬石膏(CaSO4)是地球上分布最广的硫酸盐矿物之一,为研究硬石膏向高压硬石膏转变的压力条件和相变机理、确定硬石膏拉曼光谱压标的适用范围,实验结合水热金刚石压腔和激光拉曼光谱实验技术,研究了常温高压条件下硬石膏的相变过程以及硬石膏和高压硬石膏的拉曼光谱特征。实验结果显示,常温条件下硬石膏向高压硬石膏发生相变的压力在2.3 GPa左右,但是该相变压力在增压和降压过程中存在较大差异,表明硬石膏与高压硬石膏的转变过程存在明显滞后性,证实了该相变过程属于重建型相变。由于重建型相变的控制因素除了温度和压力之外,还包括相变的速率以及矿物结构的亚稳定性等,从而很好地解释了不同实验者获得的硬石膏与高压硬石膏的相变压力之间存在的巨大差异。与硬石膏相比,高压硬石膏的拉曼光谱特征表现为SO4对称伸缩振动(ν1)从1 128.28 cm-1突然下降至1 024.39 cm-1,同时对称弯曲振动(ν2)分裂为441,459和494 cm-1三个峰,反对称伸缩振动(ν3)分裂为1 136,1 148,1 158和1 173 cm-1四个峰,反对称弯曲振动(ν4)也分裂为598,616,646和671 cm-1四个峰,可以作为判定硬石膏进入高压相态的有效标志。与硬石膏相比,高压硬石膏SO4振动产生的拉曼峰数量更多、强度更低,表明影响SO4振动的原子更多、分布更加复杂,这与高压硬石膏晶体结构(独居石结构,单斜晶系)的对称性比硬石膏(斜方晶系)更低相吻合。在硬石膏结构稳定的压力范围内(常压至2.3 GPa),硬石膏SO4拉曼振动中除了ν2,416的振动频率变化不显著以外,其余振动均随着压力的升高以稳定的速率向高波数方向移动,同时谱峰的强度、形态和半高宽没有明显改变,从而保证了不同压力下硬石膏的拉曼峰具有一致的拟合误差和压力标定精度。同时,还通过方解石ν1,1 085拉曼峰随压力的变化速率、方解石向CaCO3-Ⅱ以及CaCO3-Ⅱ向CaCO3-Ⅲ的相变压力对硬石膏压力标定结果进行检验,确定了硬石膏压标的可靠性。

关 键 词:硬石膏  拉曼光谱  相变  高压
收稿时间:2018-01-24

An In-Situ Raman Spectroscopic Study of the Phase Transition of Anhydrite under High Pressures
XIONG Xin,YUAN Xue-yin.An In-Situ Raman Spectroscopic Study of the Phase Transition of Anhydrite under High Pressures[J].Spectroscopy and Spectral Analysis,2019,39(4):1075-1079.
Authors:XIONG Xin  YUAN Xue-yin
Institution:1. School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China 2. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
Abstract:Anhydriteis one of the most widely distributed sulfite on the earth. Inorder to investigate the phase transition pressure and transformation mechanism between anhydrite and high pressure anhydrite, and to constrain the p-T area where the anhydrite Raman pressure sensor is applicable, in this paper the phase transition between anhydrite and high pressure anhydrite and the Raman spectra of both polymorphs were investigated by using a hydrothermal diamond anvil cell and laser Raman spectroscopy. Our results showed that the phase transition from anhydrite to a high pressure monazite structure occurred at pressures around 2.3 GPa, and that the phase transition pressure varied during the compressing and decompressing processes, which suggested the transformation between anhydrite and high pressure anhydrite was reconstructive process with significant hysteresis. As reconstructive transformations were controlled not only by pressure and temperature, but also by kinetics and metastability of the structure, hence explaining the discrepancy among the phase transition pressures between anhydrite and high pressure anhydrite. In contrast to those of anhydrite, the Raman vibrations of high pressure anhydrite were characterized by shifting of the ν1 mode from 1 128.28 to 1 024.39 cm-1, and by splitting of the ν2 mode into 441, 459 and 494 cm-1, ν3 into 1 136, 1 148, 1 158 and 1 173 cm-1, and ν4 into 598, 616, 646 and 671 cm-1, which ca be used as identifications for the transformation from anhydrite to high pressure anhydrite. The splitting of the ν2~ν4 vibrations into more bands indicated that the SO4 vibrations in high pressure anhydrite were affected by more nearby atoms, which was consistent with the high pressure anhydrite crystal symmetry (monoclinic) being lower than that of anhydrite (orthorhombic). Within the stability pressure range of anhydrite, All observed Raman bands of the SO4 vibrations, except for the ν2, 416, shifted to higher frequencies with constant ?ν/?p rates, mean while the Raman peak intensities and shapes remained stable, which meant that the Raman peak fitting and pressure calibration results could be equally precise under different pressures. In addition, we also verified the reliability of the anhydrite Raman pressure sensor by measuring the shifting rate of the ν1, 1 085 Raman peak position of calcite with pressure, and the phase transition pressures from calcite to CaCO3-Ⅱ and from CaCO3-Ⅱ to CaCO3-Ⅲ.
Keywords:Anhydrite  Raman spectroscopy  Phase transition  High pressure  
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