高压下白云石的原位拉曼光谱研究

利用热液金刚石压腔研究了白云石在温度298 K、压力100～1000 MPa下C-O键弯曲振动峰V1097的拉曼变化特征.结果表明,在实验的压力范围内白云石稳定,其拉曼位移和压力具有很好的线性关系,拟合后得出压力与白云石1097 cm-1拉曼线频率位移的关系为:P=143.47(△Vp)1097 102.67(1097＜Vp＜1103).因此白云石非常适合作为热液金刚石压腔的压标.     

高温高压条件下水镁石相变的拉曼光谱研究

本文应用金刚石压腔装置结合拉曼光谱分析技术原位观测了高温高压条件下水镁石的结构变化特征。结果表明：常温升至300℃过程中,随温度升高,水镁石中表征H-O对称伸缩振动的3652 cm-1拉曼特征峰向低波数移动,拉曼特征峰波数与对应的体系温度呈现良好的线性相关关系。常温条件下体系加压过程中,当压力升高至1.19 GPa时,水镁石中表征H-O振动的3652 cm-1拉曼峰向高波数移动并逐渐消失,同时产生表征方镁石的1078 cm-1拉曼特征峰,表明水镁石脱水相变为方镁石。随后压力降低过程中,表征水镁石H-O振动的3652 cm-1拉曼特征峰没有重新出现,脱水相变过程不可逆。     

流体包裹体压力计研究之二:高温高压下碳氢化合物的拉曼光谱

在温度23～315℃、压力高达近2000MPa下用金刚石压腔下研究了正庚烷、环己烷及其混合物的拉曼光谱特征。结果表明：环己烷和正庚烷混合,只是改变了环己烷及正庚烷的平均C-H伸缩振动的拉曼位移,但是并不影响其P-△^-vp关系式。另外,经过数据拟合,得到了平均C-H伸缩振动的拉曼位移与压力的关系为：P=78．21（△^-vp）＋71．56。该公式可以用来作为流体包裹体尤其是油气包裹体的压力计。     

异极矿热相变过程的高温原位拉曼光谱

利用高温拉曼光谱技术,对异极矿进行了原位拉曼光谱的测试和研究。结果表明,异极矿加热至800 K时,与结晶水(H₂O)中O-H伸缩振动对应的3 470 cm^-1特征拉曼谱峰消失,但标志硅酸盐骨架[Si₂O^₇] (Q1)结构的特征谱峰926 cm^-1未受影响,表明结晶水的丢失并不影响异极矿的整体结构。当加热至1 050 K,反映结构水O-H伸缩振动的特征峰3 580 cm^-1消失,与Q1结构单元Si-Onb对称伸缩振动相对应的特征峰926 cm^-1强度逐渐减小,并出现与Q0相对应的852.4 cm^-1特征峰。这表明加热到1 050 K时,异极矿开始出现相变。当升温达1 100 K以上,结构水(OH)的特征谱峰(3 580 cm^-1)消失,与Si-Onb对称伸缩振动对应的特征拉曼谱峰变为855 cm^-1,这标志着异极矿原有的Q1结构已完全转变为硅锌矿的Q0结构 ([SiO₄]结构),也就是说异极矿已完成向硅锌矿的转变。     

流体包裹体压力计研究之一:高温高压下Na2SO4溶液的拉曼光谱

用金刚石压腔研究了Na2SO4溶液在温度30~400℃、压力90~2400 MPa条件下的拉曼光谱特征。结果表明:温度相同时,SO42-的对称伸缩振动随压力增大向高频方向偏移;而在压力基本相同时,随温度升高则向低频方向偏移。经过数据拟合得到了SO42-对称伸缩振动的拉曼位移与温度、压力的关系为:ρ=190.44Δυp 0.002 7t2 2.901 9t-111.68。该公式可以用来作为流体包裹体的压力计。     

常温和压力0.1～1300 MPa下硬石膏的拉曼光谱研究

利用立方氧化锆压腔装置测量了高压下硬石膏中硫酸根离子内部S-O键的四个振动（对称伸缩振动、弯曲振动、反对称伸缩振动和变形弯曲振动）的拉曼位移,研究结果表明：在25℃和0．1～1300MPa压力范围内硬石膏未发生相变,所观察到的硬石膏各个拉曼峰的位移值随压力的增加而线性增加,它们与压力之间的关系可分别表达为：υ1018=0．0053p＋1016.8,υ417=0．0013p＋416．82,υ498=0．0044P＋499．25,υ1129=0．0052p＋1128．5,υ1160=0．0067p＋1159．5,υ608=0．0028p＋608．76,υ627=0．0036p＋627．01,υ675=0．0039p＋675．32,且伸缩振动受压力的影响比弯曲振动大。其1018cm^-1峰强度大,是石英464cm^-1峰的6倍,因此硬石膏也是宝石压腔良好的压力标定物质。     

流体包裹体压力计研究之二:高温高压下碳氢化合物的拉曼光诸

在温度23～315℃、压力高达近2000MPa下用金刚石压腔下研究了正庚烷、环己烷及其混合物的拉曼光谱特征.结果表明:环已烷和正庚烷混合,只是改变了环已烷及正庚烷的平均C-H伸缩振动的拉曼位移,但是并不影响其P-△vp关系式.另外,经过数据拟合,得到了平均C-H伸缩振动的拉曼位移与压力的关系为:P=78.21(△vp)+71.56.该公式可以用来作为流体包裹体尤其是油气包裹体的压力计.     

金刚石压腔结合拉曼光谱技术进行氧同位素分馏的实验研究

应用水热金刚石压腔结合拉曼光谱技术进行石英和方解石间氧同位素的分馏研究。金刚石压腔装置实现体系的高温高压条件,重氧水为提供18O的中间介质。首先给体系加压至500 MPa左右,然后升温至300℃使石英、方解石发生部分溶解,随后快速降至常温使溶解部分重新结晶,从而使矿物和重氧水之间发生同位素交换。拉曼光谱中,物质特征峰的峰强度比值与相应物质的量的比值呈线性相关,结合同位素分馏系数公式计算得出300℃时石英、方解石之间的氧同位素分馏系数为1.0016。此方法操作简便,实验过程采用原位测量方法,不破坏样品,避免了污染,提高了实验精度。实验数据的准确度证明了金刚石压腔结合拉曼光谱法进行稳定同位素分馏的实验研究是可行的。     

橄榄石原位高温拉曼光谱研究

利用微型电阻丝加热装置产生高温, 采用镍铬热电偶探测样品的温度, 在273～1 113 K温度范围,对橄榄石晶体进行了原位高温拉曼光谱研究。拉曼光谱数据显示：Si-O伸缩振动模高于800 cm^-1,Si-O弯曲振动模为400～700 cm^-1,SiO₄四面体旋转振动模和金属-氧的平移振动模低于400 cm^-1;得到了橄榄石拉曼振动频率随温度的变化关系,首次发现了橄榄石常温频率为541 cm^-1的Si-O反对称弯曲振动的A_g模,比其它的振动频率有较弱的温度效应。     

高压下多硅白云母的拉曼光谱学研究

在金刚石压腔中,通过原位拉曼光谱研究了多硅白云母在常温高压下的稳定性。实验获得了多硅白云母从常压到20GPa的拉曼光谱数据,研究了多硅白云母的266、708和3618cm^-1叫谱峰与压力的相关性。研究发现,多硅白云母的708cm^-1叫谱峰随压力增加有规律地向高频方向偏移,与压力的增加呈明显的正相关性,即y（拉曼位移,cm^-1）=0．5238x（压力,GPa）＋712．31,相关系数R^2=0．9656,并且该谱峰在压力4．7GPa时消失,这可能与多硅白云母中的Si、Al替代有关。羟基3618cm^-1谱峰则随压力增加向低频方向移动,谱峰的降低与压力的增加呈明显的线性关系变化（y=-0．3402X＋3617．8,R^2=0．9662）,并且强度随着压力的增加也在逐渐减弱,在压力达18GPa时开始消失,推测该压力可能为多硅白云母在常温下脱羟基的极限压力。     

Constraints on the spin-state transition of siderite from laboratory-based Raman spectroscopy and electrical conductivity under high temperature and high pressure

The vibrational and electrical transport properties of natural siderite are systematically investigated by means of in-situ Raman spectroscopy and alternating current impedance spectroscopy under conditions of 0.6-55.6 GPa, 298-873 K and different hydrostatic environments using a diamond anvil cell (DAC). Upon non-hydrostatic compression, all of these observable characteristic variations of siderite including the appearance of three absolutely new Raman peaks (L’, v₄' and v₁'), the disappearance of Raman peaks (T, L and v₄) and the discontinuity in the pressure-dependent electrical conductivity can provide robust evidence of electronic spin transitions of Fe²⁺ from high-spin to mixed-spin to low-spin states at the respective pressures of 42.5 GPa and 48.5 GPa. As far as hydrostatic condition, the electronic spin states from high-spin to mixed-spin to low-spin states occurred at the higher pressures of 45.7 GPa and 50.4 GPa, respectively, which implied the highly sensitive hydrostaticity of electronic spin transition pressures. Upon decompression, the reverse electronic spin transitions from low-spin to mixed-spin to high-spin states were detected at the respective pressures of 47.2 GPa and 28.7 GPa under non-hydrostatic condition, and as well as at the pressures of 49.4 GPa and 25.1 GPa under hydrostatic condition, respectively. The huge pressure hysteresis of 13.8 GPa and 20.6 GPa for the electronic spin state transition was revealed under non-hydrostatic and hydrostatic environments, respectively. In order to explore the effect of temperature on the electronic spin transition, a series of electrical conductivity experiments on siderite were performed over the temperature range of 323-873 K under conditions of three typical pressures of 47.7, 49.8 and 51.6 GPa. Furthermore, the functional relationships between the temperature and pressure describing the high-spin to mixed-spin to low-spin transitions for siderite were successfully established: P₁ (GPa) = 39.318 + 0.015 T (K) and P₂ (GPa) = 41.277 + 0.018 T (K), respectively. In conclusion, our acquired phase diagram of the electronic spin transition on siderite is beneficial to deep insight into the electronic spin behavior for those of iron-bearing carbonate minerals under high-temperature and high-pressure conditions.     

Raman spectroscopic study of K-lingunite at various pressures and temperatures

K-lingunite is a high-pressure modification of K-feldspar that possesses the tetragonal hollandite structure. Variations of the Raman spectra of K-lingunite were studied up to ~31.5 GPa at room temperature, and in the range 79–823 K at atmospheric pressure. The Raman frequencies of all bands were observed to increase with increasing pressure, and decrease with increasing temperature for K-lingunite. This behavior is in line with those observed for most of other materials. New sharp Raman bands appear at pressures greater than 13–15 GPa, suggesting a phase transition in K-lingunite with increasing pressure. The transition is reversible when pressure was released. The appearance of these new Raman bands may correspond to the phase transition revealed earlier at around 20 GPa by X-ray diffraction studies. Instead of transforming back to its stable minerals, such as orthoclase, microcline or sanidine, K-lingunite became amorphous in the temperature range 803–823 K at atmospheric pressure.     

Raman spectroscopic study of hydroxyl-clinohumite at various pressures and temperatures

 Variations of Raman spectra of hydroxyl-clinohumite were studied up to ∼370 kbar at room temperature, and in the range 81–873 K at atmospheric pressure. With the exception of the symmetric OH-stretch bands, the Raman frequencies of all bands were observed to increase monotonically with increasing pressure, and decrease with increasing temperature. This behavior is in line with those observed for other humite members (norbergite and chondrodite) so far studied. The symmetric OH-stretching band shows a mode softening with increasing pressure, and splits into two bands at either high pressure or low temperature. In the quasihydrostatic experiment, the compression and decompression paths of one of the asymmetric OH-stretch bands form a hysteresis loop, but the same behavior was not observed in the nonhydrostatic experiment. These results indicate that the two kinds of OH groups in hydroxyl-clinohumite have nonequivalent movement paths on compression, and with one OH group experiencing a release of spatial hindrance during compression. This behavior appears to be modified by shear stress. The same complication of the OH groups was not observed in the temperature variation study. The pressure and temperature variations of the Raman frequencies for the various vibrations involving the SiO₄ tetrahedra and MgO₆ octahedra below ∼1000 cm⁻¹ for clinohumite behave similarly to other hydrous magnesium silicates. On the basis of the relationship between isothermal bulk modulus and Raman data, it is suggested that the linear pressure dependences of vibrational frequencies of various Raman bands reported in the literature are inadequate. Received: 20 March 1999 / Revised, accepted: 24 August 1999     

High-pressure Raman spectroscopic study of chondrodite

Variation of Raman spectra of both natural (F-bearing) and synthetic (F-free) chondrodite samples were studied up to 400 kbar at room temperature. Ambient Raman frequencies for the synthetic sample are in general lower than those for the natural one. This is correlated with a slight expansion of the volume of the synthetic sample due to substitution of OH for F. The frequencies of all Raman bands for both samples increase monotonically with increasing pressure. The positive pressure dependences in the O−H stretch frequencies for both F-free and F-bearing samples are contrary to those for other dense hydrous magnesium silicates. A mechanism involving both the hydrogen-hydrogen repulsion and hydrogen bondings is proposed to explain the abnormal behavior. The effects of substitution of F for OH on both the ambient and high-pressure Raman spectra of chondrodite are also discussed. Received: 19 February 1998 / Revised accepted: 26 June 1998     

High-pressure Raman spectroscopic study of Co- and Ni-olivines

 The Raman spectra of synthetic α-Co₂SiO₄ and α-Ni₂SiO₄ olivines have been studied at room temperature and various pressures. All the Raman frequencies of the two olivines increase with increasing pressure, and most of the frequency–pressure plots obtained under both quasi- and nonhydrostatic conditions are nonlinear. It has been found that the average pressure derivative of Raman frequencies of the lattice modes in both Co- and Ni-olivines is smaller than that of the internal modes of SiO₄, indicating that the distortion of SiO₄ tetrahedra under static compression may be more severe than that of MO₆ octahedra. In addition, four new Raman bands were observed in Ni-olivine under nonhydrostatic compression and above 30 GPa. This result suggests that a new phase of Ni-olivine should be formed at 30 GPa or amorphization may occur at still higher pressure. Received: 11 July 2000 / Accepted: 19 December 2000     

Phase transition of synthetic zinc ferrite spinel (ZnFe2O4) at high pressure, from synchrotron X-ray powder diffraction

 Synthetic Zn-ferrite (ideally ZnFe₂O₄; mineral name: franklinite) was studied up to 37 GPa, by X-ray powder diffraction at ESRF (Grenoble, France), on the ID9 beamline; high pressure was achieved by means of a DAC. The P-V equation of state of franklinite was investigated using the Birch-Murnaghan function, and the elastic properties thus inferred [K₀ = 166.4(±3.0) GPa K₀ ^′ = 9.3(±0.6) K₀ ^″ = −0.22 GPa⁻¹] are compared with earlier determinations for MgAl-spinel and magnetite. The structural behaviour of Zn-ferrite as a function of pressure was studied by Rietveld refinements, and interpreted in the light of a phase transition from spinel to either CaTi₂O₄- or MnFe₂O₄-like structure; this transformation occurs above 24 GPa. Received: 15 March 1999 / Accepted: 22 April 2000     

High-pressure behaviour of serpentine minerals: a Raman spectroscopic study

Four main serpentine varieties can be distinguished on the basis of their microstructures, i.e. lizardite, antigorite, chrysotile and polygonal serpentine. Among these, antigorite is the variety stable under high pressure. In order to understand the structural response of these varieties to pressure, we studied well-characterized serpentine samples by in situ Raman spectroscopy up to 10 GPa, in a diamond-anvil cell. All serpentine varieties can be metastably compressed up to 10 GPa at room temperature without the occurrence of phase transition or amorphization. All spectroscopic pressure-induced changes are fully reversible upon decompression. The vibrational frequencies of antigorite have a slightly larger pressure dependence than those of the other varieties. The O–H-stretching modes of the four varieties have a positive pressure dependence, which indicates that there is no enhancement of hydrogen bonding in serpentine minerals at high pressure. Serpentine minerals display two types of hydroxyl groups in the structure: inner OH groups lie at the centre of each six-fold ring while outer OH groups are considered to link the octahedral sheet of a given 1:1 layer to the tetrahedral sheet of the adjacent 1:1 layer. On the basis of the contrasting behaviour of the Raman bands as a function of pressure, we propose a new assignment of the OH-stretching bands. The strongly pressure-dependent modes are assigned to the vibrations of the outer hydroxyl groups, the less pressure-sensitive peaks to the inner ones.     

High pressure elasticity and phase transformation in brucite, Mg(OH)2

The first pressure derivatives of the second-order elastic constants have been calculated for brucite, Mg(OH)₂ from the second- and third-order elastic constants. The deformation theory and finite strain elasticity theory have been used to obtain the second- and third-order elastic constants of Mg(OH)₂ from the strain energy of the lattice. The strain energy ϕ is calculated by taking into account the interactions up to third nearest neighbors in the Mg(OH)₂ lattice. ϕ is then compared with the strain dependent lattice energy from continuum model approximation to obtain the expressions of elastic constants. The complete set of six second-order elastic constants C _IJ of brucite exhibits large anisotropy. Since C ₃₃ (= 21.6 GPa), which corresponds to the strength of the material along the c-axis direction, is less than the longitudinal mode C ₁₁ (= 156.7 GPa), the interlayer binding forces are weaker than the binding forces along the basal plane of Mg(OH)₂. The 14 nonvanishing components of the third-order elastic constants, C _IJK , of brucite have been obtained. All the C _IJK of brucite are negative except the values of C ₁₁₄ (= 230.36 GPa), C ₁₂₄ (= 75.45 GPa) and C ₁₃₄ (= 36.98 GPa). The absolute values of the C _IJK are, in general, one order of magnitude greater than the C _IJ ’s in the Mg(OH)₂ system as usually expected for a crystalline material. To our knowledge, no previous data are available to compare the pressure derivatives of brucite. The pressure derivatives of the two components viz., C ₁₄ and C ₃₃ become negative indicating an elastic instability in brucite while under pressure. This may be related to the phase transition of brucite largely involving rearrangements of H atoms revealed in the Raman spectroscopic, powder neutron diffraction and synchrotron X-ray diffraction studies.     

Raman spectra of methane hydrate up to 86 GPa

High-pressure Raman studies of methane hydrate were performed using a diamond anvil cell in the pressure range of 0.1–86 GPa at room temperature. Raman spectra of the methane molecules revealed that new softened intramolecular vibration mode of ν ₁ appeared at 17 GPa and that the splitting of vibration mode of ν ₃ occurred at 15 GPa. The appearance of these two modes indicates that an intermolecular attractive interaction increases between the methane molecules and the host water molecules and between the neighboring methane molecules. These interactions might result in the exceptional stability of a high-pressure structure, a filled ice Ih structure (FIIhS) for methane hydrate, up to 40 GPa. At 40 GPa, a clear change in the slope of the Raman shift versus pressure occurred, and above 40 GPa the Raman shift of the vibration modes increased monotonously up to 86 GPa. A previous XRD study showed that the FIIhS transformed into another new high-pressure structure at 40 GPa. The change in the Raman spectra at 40 GPa may be induced by the transition of the structure.