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

C.-C. Lin  L.-G. Liu  T. P. Mernagh  T. Irifune 《Physics and Chemistry of Minerals》2000,27(5):320-331

 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  相似文献   

Thermodynamic properties and hydrogen speciation from vibrational spectra of dense hydrous magnesium silicates     

H. Cynn  A. M. Hofmeister  P. C. Burnley  A. Navrotsky 《Physics and Chemistry of Minerals》1996,23(6):361-376

Infrared absorption measurements were taken from 100 to 5000 cm^?1 of a natural chondrodite and three dense hydrous magnesium silicates: phase A, phase B, and superhydrous phase B (shy-B). Raman spectra were also acquired from phase B and the chondrodite. Roughly half of the lattice modes are represented and our data are the first report of the low frequency modes. Comparison of our new spectra to symmetry analyses suggests that multiple sites for hydrogen exist for all the phases. The shy-B we examined crystallizes in P2₁ nm with two OH sites. Models for the density of states are constructed based on band assignments for the lattice modes and for the OH stretching vibrations. Heat capacity C_P and entropy S calculated using Kieffer's formulation should be accurate within 3% from 200 to 800 K. Model values for C_P at 298 K are 299.6 J/mol-K for chondrodite, 421.5 J/mol-K for phase A, 529.4 J/mol-K for shy-B, and 618.9 J/mol-K for phase B. Model values for S₂₉₈ ⁰ are 234.2 J/mol-K for chondrodite, 303.5 J/ mol-K for phase A, 377.9 J/mol-K for shy-B, and 473.3 J/mol-K for phase B. Debye temperatures are near 1000 K.  相似文献   

Single-crystal elastic properties of chondrodite: implications for water in the upper mantle     

S. V. Sinogeikin  J. D. Bass 《Physics and Chemistry of Minerals》1999,26(4):297-303

Chondrodite, a member of the humite group of minerals, forms by hydration of olivine and is stable over a range of temperatures and pressures that includes a portion of the uppermost mantle. We have measured the single crystal elastic properties of a natural chondrodite specimen at ambient conditions using Brillouin spectroscopy. The isotropic aggregate bulk (K) and shear (μ) moduli calculated from the single-crystal elastic moduli, C_ij, are: K_S=118.4(16) GPa and μ=75.6(7) GPa. A comparison of the structures and elasticity of olivine and chondrodite indicate that the replacement of O with (OH,F) in M²⁺O₆ octahedra has a small effect on the elasticity of humite-group minerals. The slightly diminished elastic moduli of humite-group minerals (as compared to olivine) are likely caused by a smaller ratio of strong structural elements (SiO₄ tetrahedra) to weaker octahedra, and perhaps a more flexible geometry of edge-sharing MO₄(O,OH,F)₂ octahedra. In contrast to the humite-olivine group minerals, the incorporation of water into garnets and spineloids leads to a more substantial decrease in the elastic properties of these minerals. This contrasting behavior is due to formation of O₄H₄ tetrahedra and vacant hydroxyl-bearing octahedra in the garnets and spineloids, respectively. Therefore, the mechanism of incorporation of H/OH into mineral phases, not only degree of hydration, should be taken into account when estimating the effect of water on the elastic properties of minerals. The bulk elastic wave velocities of chondrodite and olivine are very similar. If humite-like incorporation of OH is predominant in the upper mantle, then the reaction of OH with olivine will have a minor or possibly no detectable effect on seismic velocities. Thus, it may be difficult to distinguish chondrodite-bearing rocks from “anhydrous” mantle on the basis of seismically determined velocities for the Earth. Received: 25 February 1998 / Revised, accepted: 18 August 1998  相似文献   

Raman spectrum of natural and synthetic stishovite     

R. J. Hemley  H -K. Mao  E. C. T. Chao 《Physics and Chemistry of Minerals》1986,13(5):285-290

Raman spectra of natural and synthetic samples of stishovite have been measured with a micro-optical spectrometer system. These spectra have a pattern that is characteristic of rutile-structured oxides. The spectrum of synthetic stishovite is characterized by well-resolved bands at 231, 589, 753, and 967 cm^?1, which are assigned as theB _1g,E _g,A _1g, andB _2g fundamentals, respectively, of the first-order Raman spectrum of the ideal, ordered structure. Natural stishovite obtained from Meteor Crater, Arizona has a first-order Raman spectrum that is fully consistent with that of the synthetic material. The observed spectrum of the natural sample, however, is weaker and has bands in addition to those identified as fundamentals in the spectrum of the synthetic material. A broad band at ～475 cm^?1 may be indicative of glass or contaminants derived from the extraction procedure. Alternatively, this band may arise from multiphonon scattering that is enhanced by poor crystallinity or structural disorder in the natural shocked sample.  相似文献   

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

C. C. Lin 《Physics and Chemistry of Minerals》2001,28(4):249-257

 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  相似文献   

Raman spectra of silicate garnets   总被引：2，自引：1，他引：1  

B. A. Kolesov  C. A. Geiger 《Physics and Chemistry of Minerals》1998,25(2):142-151

The single-crystal polarized Raman spectra of four natural silicate garnets with compositions close to end-members almandine, grossular, andradite, and uvarovite, and two synthetic end-members spessartine and pyrope, were measured, along with the powder spectra of synthetic pyrope-grossular and almandine-spessartine solid solutions. Mode assignments were made based on a comparison of the different end-member garnet spectra and, in the case of pyrope, based on measurements made on additional crystals synthesized with ²⁶Mg. A general order of mode frequencies, i.e. R(SiO₄)>T(metal cation)>T(SiO₄), is observed, which should also hold for most orthosilicates. The main factors controlling the changes in mode frequencies as a function of composition are intracrystalline pressure (i.e. oxygen-oxygen repulsion) for the internal SiO₄-vibrational modes and kinematic coupling of vibrations for the external modes. Low frequency vibrations of the X-site cations reflect their weak bonding and dynamic disorder in the large dodecahedral site, especially in the case of pyrope. Two mode behavior is observed for X-site cation vibrations along the pyrope-grossular binary, but not along the almandine-spessartine join. Received: 3 December 1996 / Revised, accepted: 13 April 1997  相似文献   

Single crystal Raman spectrum of uvarovite, Ca3Cr2Si3O12     

Anastasia Chopelas 《Physics and Chemistry of Minerals》2005,32(8-9):525-530

Single-crystal Raman spectra of synthetic end-member uvarovite (Ca₃Cr₂Si₃O₁₂) and of a binary solution (59% uvarovite, 41% andradite) have been measured using single crystal techniques. For each of these garnets, 22 and 21 of the 25 Raman modes were located, respectively. The spectra for uvarovite garnets closely resemble those of the other calcic garnets, grossular, and andradite. The modes for uvarovites do not fit into the same trends as established by the other five anhydrous end-member garnets: the high energy “internal” Si–O modes do not depend on lattice constant in uvarovite. They exceed frequencies for both andradite and grossular. This is likely due to the large crystal field stabilization energy of trivalent chromium. The low energy and midrange modes are at similar frequencies to the other calcic garnets.  相似文献   

Phase equilibria involving humite minerals in impure dolomitic limestones     

Jack M. Rice 《Contributions to Mineralogy and Petrology》1981,75(3):205-223

The natural occurrence of critical assemblages among the phases clinohumite, chondrodite, norbergite, tremolite, forsterite, brucite, periclase, diopside, calcite and dolomite, together with experimental and thermochemical data, permits the calculation of phase equilibria governing the stability of the humite group minerals in impure dolomitic limestones. The phase relations are described by 29 divariant (OH-F) continuous reactions, and 11 univariant discontinuous reactions. The equilibrium conditions for these reactions have been calculated and plotted in isobaricT-X(OH-F) andT- phase diagrams. Continuous reactions govern the compositions of (OH-F) solid solutions and the consequent movement of three-phase triangles on the chemographic diagram. Discontinuous reactions result in the appearance or disappearance of a distinct phase assemblage. The pure OH-humite minerals are metastable relative to forsterite+brucite. With increasing fluorine content, clinohumite, followed in turn by chondrodite and norbergite, becomes stable. The stability fields for the individual humite minerals expand to more CO₂-rich fluid compositions with increasing fluorine content and decreasing total pressure. At 1,000 bars, clinohumite can contain a maximum of 58 mole percent fluorine before reacting discontinuously to form chondrodite (X _F=0.61) and forsterite. The stability field for clinohumite+calcite is restricted to fluids with <0.40. At temperatures less than 700°C, the minimum fluorine mole fractions required to stabilize chondrodite and norbergite are 0.31 and 0.62 respectively. At the same conditions, chondrodite can contain a maximum of approximately 85 mole % F. The calculated phase equilibrium boundaries, the range of stable humite compositions and the compositions of coexisting (OH-F) phases are in good overall agreement with natural assemblages. Owing to steepdT/d slopes for several of the humite continuous reactions, the F/(F+OH) ratio of a given humite mineral is a useful indicator of the CO₂/H₂O ratio of the fluid phase.  相似文献   

天然与合成紫晶的红外和偏振拉曼光谱鉴定特征     

黄文清  金绪广  左锐  晁东娟  杨桂群  薛盼  陈小军  张锦雯 《岩矿测试》2019,38(4):403-410

天然紫晶与合成紫晶的鉴别是国内外珠宝鉴定实验室的一个难题,前人主要从双晶、色带、包裹体、红外吸收光谱特征等方面开展了研究。在利用红外光谱鉴别天然紫晶与合成紫晶时,不同的学者尚对3595cm-1或3543cm-1吸收峰作为诊断性还是指示性的判据存在不同认识。本文系统采集了典型的天然紫晶与合成紫晶样品,研究了利用红外光谱测试技术鉴别天然紫晶与合成紫晶的局限性,并尝试将偏振拉曼光谱应用于紫晶成因鉴别。结果表明:利用3595cm-1、3543cm-1红外吸收峰进行紫晶鉴别仅具有指示性意义,不能作为决定性的判定依据,偏振拉曼光谱可作为重要的补充。天然紫晶的偏振拉曼光谱(偏振方向:HH)均出现400cm-1的拉曼峰,而该峰在合成紫晶偏振拉曼光谱中缺失;合成紫晶的偏振拉曼光谱(偏振方向:HH)均具有795cm-1、448cm-1的拉曼峰,而这两个峰在天然紫晶偏振拉曼光谱中缺失。偏振拉曼光谱产生差异的原因可能与天然紫晶和合成紫晶内部晶格变形程度的不同有关。本文揭示的400cm-1、448cm-1和795cm-1偏振拉曼峰可作为鉴别紫晶成因的新依据。  相似文献   

High-pressure behaviour of serpentine minerals: a Raman spectroscopic study   总被引：5，自引：2，他引：3  

A.-L.?Auzende  I.?DanielEmail author  B.?Reynard  C.?Lemaire  F.?Guyot 《Physics and Chemistry of Minerals》2004,31(5):269-277

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.  相似文献   

Vibrational spectroscopy of aluminate spinels at 1 atm and of MgAl2O4 to over 200 kbar     

A. Chopelas  A. M. Hofmeister 《Physics and Chemistry of Minerals》1991,18(5):279-293

Single-crystal Raman and infrared reflectivity data including high pressure results to over 200 kbar on a natural, probably fully ordered MgAl₂O₄ spinel reveal that many of the reported frequencies from spectra of synthetic spinels are affected by disorder at the cation sites. The spectra are interpreted in terms of factor group analysis and show that the high energy modes are due to the octahedral internal modes, in contrast to the behavior of silicate spinels, but in agreement with previous data based on isotopic and chemical cation substitutions and with new Raman data on gahnite (～ ZnAl₂O₄) and new IR reflectivity data on both gahnite and hercynite (～Fe_0.58Mg_0.42Al₂O₄). Therefore, aluminate spinels are inappropriate as elastic or thermodynamic analogs for silicate spinels. Fluorescence sideband spectra yield complementary information on the vibrational modes and provide valuable information on the acoustic modes at high pressure. The transverse acoustic modes are nearly pressure independent, which is similar to the behavior of the shear modes previously measured by ultrasonic techniques. The pressure derivative of all acoustic modes become negative above 110 kbar, indicating a lattice instability, in agreement with previous predictions. This lattice instability lies at approximately the same pressure as the disproportionation of spinel to MgO and Al₂O₃ reported in high temperature, high pressure work.  相似文献   

Phase relations of a F-enriched peraluminous granite: an experimental study of the Kymi topaz granite stock,southern Finland     

S. Lukkari  F. Holtz 《Contributions to Mineralogy and Petrology》2007,153(3):273-288

Phase relations of F-rich Kymi equigranular topaz granite have been investigated experimentally at 100–500 MPa as a function of water activity and F content. Fluorite and topaz can crystallize as liquidus phases in F-rich peraluminous systems, but the F content of the melt should exceed 2.5–3.0 wt% for the crystallization of topaz. In peraluminous F-bearing melts containing more than 1 wt% F, topaz and muscovite are expected to be the first F-bearing phases to crystallize at high pressure, whereas fluorite and topaz should crystallize first at low pressure. Following reaction models the saturation of fluorite and topaz: CaAl₂Si₂O₈ (plagioclase) + 2[AlF₃]_melt = CaF₂ (fluorite) + 2Al₂SiO₄F₂ (topaz). The obtained partition coefficient for F between biotite and glass D(F)^Bt/glass ranges from 1.89 to 0.80 (average value 1.29) and can be used as an empirical fluormeter to determine the F content of coexisting melts. Combined petrological and experimental studies of the Kymi equigranular topaz granite indicate that plagioclase was the liquidus phase at nearly water-saturated (fluid-saturated) conditions and that the F content of the melt was at least 2 wt%. The mean F content of natural biotite (3.92 wt%) suggests that the late-stage crystallization of biotite occurred in melts containing about 3 wt% F. The early crystallization of biotite and the presence of muscovite in our experiments at 200 MPa contrasts with the late-stage crystallization of biotite and the absence of muscovite in the natural assemblage, indicating that crystallization pressure may have been lower than 200 MPa for the granite. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.  相似文献   

水晶类宝石的热释光特征及其宝石学意义     

吕璐  ;袁心强  ;陈美华  ;杨晓蓉 《地质科学译丛》2009,(1):31-34

利用组装的热释光光谱测量装置测试了天然与合成无色水晶、茶晶和紫晶样品的热释光,分析对比了其热释光图谱。结果显示,天然无色水晶样品有较弱的热释光特征,合成无色水晶样品无热释光,两者加热后颜色皆无明显变化;天然茶晶样品无热释光,加热后颜色不改变,合成茶晶样品的热释光强烈,其图谱清晰,加热后茶色退为无色,但如仅加热120S,则仍为茶色;天然紫晶样品无热释光,合成紫晶样品有热释光,加热后颜色均无变化。结果表明,热释光对解决珠宝鉴定中的某些疑难问题具有独到的作用。  相似文献   

Structure and lattice vibrations of Mg-Al spinel solid solution     

M. Ishii  J. Hiraishi  T. Yamanaka 《Physics and Chemistry of Minerals》1982,8(2):64-68

X-ray structure refinements have been made for nonstoichiometric (MgO · 3Al₂O₃) and stoichiometric Mg-Al spinels. Several structure variations with chemical composition have been observed and are discussed in relation to Al substitution in tetrahedral sites. Infrared reflection and Raman spectra of the single crystal of the nonstoichiometric spinel (MgO · 3Al₂O₃) have been measured and analyzed. The results obtained are compared with those reported for the stoichiometric sample. From the infrared and Raman frequencies reported for the stoichiometric Mg-Al spinel, which are partly complemented with our results, the effective ionic charges of the ions in MgAl₂O₄ have been estimated on the basis of the rigid ion model.  相似文献   

OH incorporation in nearly pure MgAl₂O₄ natural and synthetic spinels     

Davide Lenaz  Henrik Skogby  Francesco Princivalle 《Geochimica et cosmochimica acta》2008,72(2):475-479

Four nearly pure MgAl₂O₄ spinels, of both natural and synthetic occurrence, have been studied by means of X-ray single crystal diffraction and FTIR spectroscopy in order to detect their potential OH content. Absorption bands that can be assigned to OH incorporated in the spinel structure were only observed in spectra of a non-stoichiometric synthetic sample. The absorption intensity of two bands occurring at 3350 and 3548 cm⁻¹ indicate an OH content of 90 ppm H₂O. Based on correlations of OH vibrational frequencies and O-H?O distances, the observed absorption bands correspond to O-H?O distances of 2.77 and 2.99 Å, respectively, which is close to the values obtained by the structure refinements for ^VIO-O_unsh (2.825 Å) and ^IVO-O (3.001 Å). This indicates that one probable local position for hydrogen incorporation is the oxygens coordinating a vacant tetrahedral site. The present spectra demonstrate that the detection limit for OH in Fe-free spinels is in the range 10-20 ppm H₂O. However, at appreciable Fe²⁺ levels, the detection of OH bands becomes hampered due to overlap with strong absorption bands caused by electronic d-d transitions in Fe²⁺ in the tetrahedral position.  相似文献   

Raman and infrared study of hydroxyl sites in natural uvite,fluor-uvite,magnesio-foitite,dravite and elbaite tourmalines     

C. Fantini  M. C. Tavares  K. Krambrock  R. L. Moreira  A. Righi 《Physics and Chemistry of Minerals》2014,41(4):247-254

We present the Raman and infrared spectra of different tourmaline species in the spectral range associated with the hydroxyl stretching modes, investigated through polarized Raman spectroscopy. Different lineshapes are observed for the OH spectra in uvite, fluor-uvite, magnesio-foitite, dravite and elbaite samples, and can be related to the coordination of OH in the two different structural V[O(3)]- and W[O(1)]-occupied sites. Local arrangements around the two different OH sites were assigned, and different ion substitutions for these five tourmaline species were identified. Our work with polarized Raman spectroscopy revealed that OH-stretching modes are described by totally symmetric, irreducible representations.  相似文献   

The natural occurrence of hydroxide in olivine   总被引：1，自引：0，他引：1  

Gregory H. Miller  George R. Rossman  George E. Harlow 《Physics and Chemistry of Minerals》1987,14(5):461-472

Polarized infrared (IR) spectra of olivine single crystals from 17 different localities show a tremendous variability in both mode and abundance of hydroxide (OH) incorporation. Kimberlitic olivines contain the most total OH at an estimated concentration level of 976 H/10⁶Si, whereas olivines from basalts contain the least at 3 H/10⁶Si. Olivines of metamorphic and hydrothermal origin have widely varying concentration levels intermediate between those of basalts and kimberlites. Over 30 distinct OH absorption bands have been identified. Most of these bands are not unique to individual localities but may be found in samples from several different localities. Pleochroism is consistent among localities, but relative band intensities vary. No evidence is found for molecular H₂ in olivine. Hydrous minerals have been identified in olivine by their characteristic OH absorption bands. Serpentine is commonly found and is clearly distinguishable from intrinsic OH. Talc is present in one sample. Prominent OH bands at 3572 and 3525 cm^?1 are attributed to humite group minerals. San Carlos, Arizona, olivines annealed in the presence of H₂O develop absorption bands which are found in natural samples, however the OH absorption spectra of these annealed olivines are not identical to those of any single natural crystal. Sharp-band OH abundances in annealed samples are an order of magnitude lower than the maximum measured in natural specimens. The mechanical properties determined from these annealed olivines may not be directly applicable to mantle olivine because both the OH sites and concentrations are different.  相似文献   

Phase relations in the phlogopite-apatite system at 20 kbar; implications for the role of fluorine in mantle melting     

D. Vukadinovic  A. D. Edgar 《Contributions to Mineralogy and Petrology》1993,114(2):247-254

Phase relations have been determined at 20 kb in the simple, Fe-, Ti-free systems hydroxyphlogopite-hydroxyapatite and hydroxyfluorphlogopite-hydroxyfluorapatite in order to determine distribution of fluorine between phlogopite, apatite and melt under mantle conditions. No excess H₂O was present in the hydroxyphlogopite-hydroxyapatite system and the F/(OH) ratio was unity in the F-bearing system. Both systems are pseudobinary and contain forsterite at phlogopite-rich compositions. In the F-absent system, the minimum melting occurs at 1225°C and Phl₈₅Ap₁₅, whereas in the F-bearing system this temperature is 1260°C and Phl₆₆Ap₃₄. Phlogopite in the F-absent system has lower Al than in the F-bearing system with both showing Si+Mg=^[IV]Al+^[VI]Al as the principal substitution. Increase in CaO in forsterite increases with increasing apatite in the bulk composition and is more pronounced in the F-absent system. Distribution of fluorine between phlogopite and liquid and apatite and liquid shows that D _F ^(Phl/glass) ranges from 2–1.25 depending on temperature and bulk composition, whereas the D _F ^(Apat/glass) is about unity. These results suggest that fluorine will tend to remain in the solid phases rather than the melt during partial melting in the mantle. Hence the enrichment of fluorine in ultrapotassic magmas and its role in their evolution are constrained.  相似文献   

High-pressure Raman spectroscopic study of Fo90 hydrous wadsleyite     

Annette K. Kleppe  Andrew P. Jephcoat  Joseph R. Smyth 《Physics and Chemistry of Minerals》2006,32(10):700-709

Raman spectra of monoclinic Fo₉₀ hydrous wadsleyite with 2.4 wt% H₂O have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium to 58.4 GPa at room temperature. The most intense, characteristic wadsleyite modes, the Si–O–Si symmetric stretch at 721 cm⁻¹ and the symmetric stretch of the SiO₃ unit at 918 cm⁻¹, shift continuously to 58.4 GPa showing no evidence of a first order change in the crystal structure despite compression well beyond the stability field of wadsleyite in terms of pressure. The pressure dependence of these two modes is nearly identical for Fo₉₀ hydrous and Fo₁₀₀ anhydrous wadsleyite. A striking feature in the high-pressure Raman spectra of Fo₉₀ hydrous wadsleyite is the appearance of new Raman modes above 9 GPa in the mid-frequency range (300–650 cm⁻¹ at 1-bar and shifted to 500–850 cm⁻¹ at 58.4 GPa) accompanied by a significant growth in their intensities under further compression. In the OH stretching frequency range Fo₉₀ hydrous wadsleyite exhibits a larger number of modes than the Mg end-member phase. The higher number of modes may be due to either additional protonation sites or simply that we observe a different subset of all possible OH modes for each sample. The high-pressure behaviour of the OH stretching modes of Fo₉₀ and Fo₁₀₀ hydrous wadsleyite is consistent: OH stretching modes with frequencies <3,530 cm⁻¹ decrease with increasing pressure whereas the higher-frequency OH modes show a close to constant pressure dependence to at least 13.2 GPa. The approximately constant pressure dependence of the OH modes above 3,530 cm⁻¹ is consistent with protons being located at the O1···O edges around M3.  相似文献   

Polymorphic phase transition in Superhydrous Phase B     

M. Koch-Müller  P. Dera  Y. Fei  H. Hellwig  Z. Liu  J. Van Orman  R. Wirth 《Physics and Chemistry of Minerals》2005,32(5-6):349-361

We synthesized superhydrous phase B (shy-B) at 22 GPa and two different temperatures: 1200°C (LT) and 1400°C (HT) using a multi-anvil apparatus. The samples were investigated by transmission electron microscopy (TEM), single crystal X-ray diffraction, Raman and IR spectroscopy. The IR spectra were collected on polycrystalline thin-films and single crystals using synchrotron radiation, as well as a conventional IR source at ambient conditions and in situ at various pressures (up to 15 GPa) and temperatures (down to −180°C). Our studies show that shy-B exists in two polymorphic forms. As expected from crystal chemistry, the LT polymorph crystallizes in a lower symmetry space group (Pnn2), whereas the HT polymorph assumes a higher symmetry space group (Pnnm). TEM shows that both modifications consist of nearly perfect crystals with almost no lattice defects or inclusions of additional phases. IR spectra taken on polycrystalline thin films exhibit just one symmetric OH band and 29 lattice modes for the HT polymorph in contrast to two intense but asymmetric OH stretching bands and at least 48 lattice modes for the LT sample. The IR spectra differ not only in the number of bands, but also in the response of the bands to changes in pressure. The pressure derivatives for the IR bands are higher for the HT polymorph indicating that the high symmetry form is more compressible than the low symmetry form. Polarized, low-temperature single-crystal IR spectra indicate that in the LT-polymorph extensive ordering occurs not only at the Mg sites but also at the hydrogen sites.  相似文献