Raman spectra from the Al2SiO5 polymorphs at high pressures and room temperature

Raman spectra of the three Al₂SiO₅ polymorphs; andalusite, sillimanite and kyanite were recorded as a function of pressure at room temperature. All the Raman active bands which could be observed from the high-pressure cell showed a linear pressure dependence for each of the three Al₂SiO₅ polymorphs and no phase changes were observed over the pressure ranges used in this study. In andalusite and to a lesser extent in sillimanite, vibrations which could be correlated with internal motions of the SiO₄ tetrahedra were generally well separated from the lattice modes and showed a greater pressure dependence than that observed for other modes. The distinct pressure dependence of the internal SiO₄ modes is less evident in kyanite, probably due to the lack of continuous tetrahedral chains and the fact that the rigid SiO₄ tetrahedra now form an integral part of the structural network. At ambient pressure, kyanite also exhibits two fluorescence bands at 705 and 706.2 nm which are due to small amounts of Cr³⁺ in the kyanite crystals. These fluorescence bands showed a non-linear frequency shift as pressure was increased.     

Raman spectroscopic study of PbCO3 at high pressures and temperatures

Cerussite (PbCO₃) has been investigated by high-pressure and high-temperature Raman spectroscopy up to pressures of 17.2 GPa and temperatures of 723 K. Two pressure induced phase transitions were observed at about 8.0(2) and 16.0(2) GPa, respectively. The post-aragonite transition (PbCO₃-II) at 8.0(2) GPa is accompanied by softening of the v ₂-out-of-plane mode of the CO ₃ ^2? group and disappearance of the B_1g (v ₄-in-plane band of the CO ₃ ^2? group) mode. Stronger shifts of the carbonate group modes after the phase transition suggest that the new structure is more compressible. The formation of a second high-pressure polymorph begins at about 10 GPa. It is accompanied by the occurrence of three new bands at different pressures and splitting of the v ₁-symmetric C–O stretching mode of the CO ₃ ^2? group. The transitions are reversible on pressure release. A semi-quantitative phase diagram for PbCO₃ as a function of pressure and temperature is proposed.     

Raman spectra of hydrous γ -Mg2SiO4 at various pressures and temperatures

 One well-defined OH Raman band at 3651 ± 1 cm⁻¹ and one weak feature near 3700 ± 5 cm⁻¹ are recognized for the hydrous γ-phase of Mg₂SiO₄. Like the hydrous β-phase, the H₂O content in the γ-phase shifts most of the corresponding silicate modes towards lower frequencies. Variations in Raman spectra of the hydrous γ-phase were investigated up to about 200 kbar at room temperature and in the range 81–873 K at atmospheric pressure. Unlike the anhydrous γ-phase, which remains intact up to at least 873 K, the hydrous γ-phase sometimes converts to a defective forsterite structure above 800 K. Although the hydrous γ-phase remains intact up to at least 800 K, Raman signals of the OH bands disappear completely above 423 K. The Raman frequency of the well-defined OH band decreases linearly with increasing temperature between 81 and 423 K. In the region of the silicate vibrations, the Raman frequencies of the two most intense bands increase nonlinearly with increasing pressure, and decrease with increasing temperature. The frequencies for all other weak bands, however, decreased linearly with increasing temperature. The latter most likely reflects the larger scatter of the data for the weak bands. Received: 27 April 2001 / Accepted: 12 September 2001     

Raman spectra of MgSiO3·10% Al2O3-perovskite at various pressures and temperatures

Variations of Raman spectra of MgSiO₃·10% Al₂O₃-perovskite were investigated up to about 270 kbar at room temperature and in the range 108–425 °K at atmospheric pressure. Like MgSiO₃-perovskite, the Raman frequencies of MgSiO₃·10% Al₂O₃-perovskite increase nonlinearly with increasing pressure and decrease linearly with increasing temperature within the experimental uncertainties and the range investigated. A comparison of these data with those of MgSiO₃-perovskite suggests that MgSiO₃·10% Al₂O₃-perovskite is slightly more compressible than MgSiO₃-perovskite, and that the volume thermal expansion for MgSiO₃·10% Al₂O₃-perovskite is also slightly greater than that for MgSiO₃-perovskite.     

In situ Raman spectroscopy on kerogen at high temperatures and high pressures

Kerogen samples were treated at temperatures and pressures up to 25–600°C and ~9 GPa, respectively. In situ micro-Raman spectroscopy was used to measure the systematic changes in the first-order Raman spectral features during the process of temperature or pressure increment. Three Raman bands, D1, D2, and G bands, were examined to characterize the structural and chemical changes of kerogen at high temperatures and pressures. We found that the wavenumbers of D1, D2 and G bands showed a linear variation with both temperature and pressure. Therefore, a correlation between R1 and R2 and the peak temperature in regionally metamorphosed rocks cannot be applied to this work. This result implies that the G band may serve as a temperature or pressure indicator during the promotion of maturation of kerogen. Kerogen possesses reversible properties in contrast with the natural samples recovered from the field suffered from prolonged thermal history during regional metamorphism.     

An infrared and Raman spectroscopic study of gypsum at high pressures

 We present Raman and infrared spectra of gypsum to 21 GPa at 300 K. Our measurements encompass the internal modes of the (SO₄)⁻⁴ group that lie between 400 and 1150 cm⁻¹, hydroxyl-stretching vibrations between 3200 and 3600 cm⁻¹, and a libration and bending vibrations of the molecular H₂O group. All vibrations of the sulfate group have positive pressure shifts, while the hydroxyl-stretching and -bending vibrations have a mixture of positive and negative pressure shifts: the effect of pressure on the hydrogen bonding of the water molecule thus appears to be complex. Near 5 GPa, the two infrared-active bending vibrations of the water molecule coalesce, and the morphology of the hydroxyl-stretching region of the spectrum shifts dramatically. This behavior is consistent with a pressure-induced phase transition in gypsum in the vicinity of 5–6 GPa, which is observed to be reversible on decompression to zero pressure. The spectral observations are consistent with the onset of increased disorder in the position of the water molecule in gypsum: the sulfate vibrations are largely unaffected by this transition. The Raman-active symmetric stretch of the sulfate group undergoes an apparent splitting near 4 GPa, which is interpreted to be produced by Fermi resonance with an overtone of the symmetric bending vibration. The average mode Grüneisen parameter of the 20 vibrational modes we sample is less than 0.05, in contrast to the bulk thermal Grüneisen parameter of 1.20. Accordingly, the vibrations of both water and sulfate units within gypsum are highly insensitive to volumetric compaction. Therefore, in spite of the changes in the bonding of the water unit near 5 GPa, metastably compressed gypsum maintains strongly bound molecular-like units to over 20 GPa at 300 K. Received: 31 July 2000 / Accepted: 5 April 2001     

The crystal structure of Ca5(PO4)2SiO4 (Silico-Carnotite)

Summary The crystal structure of Ca₅(PO₄)₂SiO₄ (silico-carnotite) has been determined from 3358 x-ray diffraction data collected by a counter method and has been refined toR _w =0.038,R=0.045, in space group Pnma. The unit cell parameters area=6.737 (1) Å,b=15.508 (2) Å andc=10.132 (1) Å at 24°C;Z=4. The observed density is 3.06 and the calculated density is 3.03 g · cm^–3. The crystal contains about 2.5% V₂O₅ as an impurity. The bond lengths within the tetrahedral anions suggest that substitution or disorder of PO₄ ^3–, SiO₄ ^4– and possibly VO₄ ^3– occurs among the anion sites. The structure has some relationship to that of Ca₅(PO₄)₃OH, the predominant inorganic phase in the human body, but suggests that the Ca₅(PO₄)₃OH type structure may not be stable without some of the OH positions being filled. Ca₅(PO₄)₂SiO₄ is more closely related to K₃Na(SO₄)₂ (glaserite) if it is considered that there are systematic cation vacancies in Ca₅(PO₄)₂SiO₄.This type of structure is consistent with the view that cation vacancies in the glaserite-type structure account for solid solutions between Ca₂SiO₄ and Ca₃(PO₄)₂ and between Ca₃(PO₄)₂ and CaNaPO₄.

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Die Kristallstruktur vonCa ₅(PO ₄)₂ SiO ₄ (Silicocarnotit)

Zusammenfassung Die Kristallstruktur von Ca₅(PO₄)₂SiO₄ (Silicocarnotit) wurde aus 3358 Röntgendiffraktometer-Daten bestimmt und in Raumgruppe Pnma aufR _w =0,038,R=0,045 verfeinert. Die Gitterkonstanten (bei 24° C) sind:a=6,737 (1) Å,b=15,508 (2) Å undc=10,132 (1) Å,Z=4; D_obs.=3,06 g · cm^–3, D_exp.=3,03 g · cm^–3. Der Kristall enthält etwa 2,5% V₂O₅ als Verunreinigung. Die Bindungslängen in den tetraedrischen Anionen legen nahe, daß unter den Anionenplätzen gegenseitige Vertretung oder Unordnung von PO₄ ^3–, SiO₄ ^4– und möglicherweise VO₄ ^3– auftritt. Die Struktur zeigt einige Verwandtschaft zu der von Ca₅(PO₄)₃OH, der wichtigsten anorganischen Substanz im menschlichen Körper, weist aber darauf hin, daß eine Struktur vom Ca₅(PO₄)₃OH-Typ ohne Besetzung eines Teiles der OH-Position nicht stabil ist. Ca₅(PO₄)₂SiO₄ zeigt engere Beziehungen zu K₃Na(SO₄)₂ (Glaserit), wenn man berücksichtigt, daß in Ca₅(PO₄)₃SiO₄ systematische Kationen-Leerstellen sind. Dieser Strukturtyp ist mit derAuffassung in Übereinstimmung, daß Kationenleerstellen für die festen Lösungen zwischen Ca₂SiO₄ und Ca₃(PO₄)₂ und zwischen Ca₃(PO₄)₂ und CaNaPO₄ verantwortlich sind.

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With 9 Figures     

Thermodynamic properties of Zn(Al,Cr)2O4 spinels at high temperatures and pressures

Three Al-Cr exchange isotherms between Zn(Al, Cr)₂O₄ spinel and (Al, Cr)₂O₃ corundum crystalline solutions have been studied experimentally at 900°, 1100°, and 1300° C, at a total pressure of 25 kbar. Employing data on the equation of state of corundum (Chatterjee et al. 1982), the experimental results were evaluated thermodynamically. It was found that the thermodynamic mixing properties of Zn(Al, Cr)₂O₄ spinels are best described in terms of a symmetric Margules equation. The T- and P- dependence of the Margules Parameter, W _G ^Sp , and of ΔG^* of the exchange reaction, 1/2 ZnAl₂O₄ + 1/2 Cr₂O₃ = 1/2 ZnCr₂O₄+1/2 A1₂O₃, are found to be ΔG ^*=1493?2.869·T+0.0081·P and W _G ^Sp (J/mol)=23456+0.0386·P, with T given in K and P in bar.     

Die Kristallstruktur des Finnemanits,Pb5Cl(AsO3)3, mit einem Vergleich zum Strukturtyp des Chlorapatits,Ca5Cl(PO4)3

Zusammenfassung Die Neubestimmung der Struktur des Finnemanits, Pb₅Cl(AsO₃)₃, (a ₀=10,322(7) Å,c ₀=7,055(6) Å RaumgruppeC _6h ² -P6₃/m) ergab nach einer Verfeinerung der Atomkoordinaten und der anisotropen Temperaturparameter von Pb, As und Cl bzw. der isotropen für die Sauerstoffe für 463 beobachtete Reflexe einen konventionellenR-Wert von 0,076. Die Struktur stellt ein aus Pb(1)–O und Pb(2)–O–Cl Polyeder sowie [AsO₃]-Pyramiden dreidimensional verknüpftes Gerüst dar. Dieses Gerüst steht in enger Beziehung zur Struktur des Chlorapatits.

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The crystal structure of finnemanite, Pb₅Cl(AsO₃)₃, with a comparison to the structure-type of chlorapatite, Ca₅Cl(PO₄)₃

Summary The redetermination of the crystal structure of finnemanite, Pb₅Cl(AsO₃)₃, (a ₀=10.322(7) Å,c ₀=7.055(6) Å; space groupC _6h ² -P6₃/m) converged for the refinement of the atomic coordinates, the anisotropic temperature parameters for Pb, As and Cl and isotropic for the O-atoms to a final conventionalR-value of 0.076 for 463 observed reflections. The crystal structure consists of a three-dimensional network built up of Pb(1)–O and Pb(2)–O–Cl polyhedra as well as [AsO₃]-pyramides. This network is closely related to the structure of chlorapatite.

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Mit 3 Abbildungen     

Instability of sapphirine at high pressures

The stability field of Mg-sapphirines is limited at high pressures through the solid-solid breakdown reaction sapphirine?pyrope = corundum+spinel, the univariant curve originating from an invariant point located at 22 kb, 880°C to 30 kb, 1350°C. Under water pressures less than 22 kb sapphirines exhibit the same low-temperature breakdown into the assemblage chlorite+corundum+spinel as determined by Seifert (1974) between 1 kb and 7 kb thus resulting in one continuous univariant lower stability limit extending from 1 kb, about 650°C through 10 kb, 770°C to the invariant point at 22 kb, 880°C. If \(P_{{\text{H}}_{\text{2}} {\text{0}}} < P_{{\text{total}}} \) , the stability field of sapphirine will expand towards lower temperatures. The occurrence of sapphirine in mantle depths requires rather aluminous bulk compositions, high geothermal gradients and/or \(P_{{\text{H}}_{\text{2}} {\text{0}}} < P_{{\text{total}}} \) , with total pressures not exceeding 30 kb. Thus sapphirine is probably not a stable phase in the lower portions of lithospheric plates and the underlying asthenosphere.     

Thermodynamic mixing properties of Mg(Al,Cr)2O4 spinel crystalline solution at high temperatures and pressures

Three Al-Cr exchange isotherms at 1,250°, 1,050°, and 796° between Mg(Al, Cr)₂O₄ spinel and (Al, Cr)₂O₃ corundum crystalline solutions have been studied experimentally at 25 kbar pressure. Starting from gels of suitable bulk compositions, close approach to equilibrium has been demonstrated in each case by time studies. Using the equation of state for (Al, Cr)₂O₃ crystalline solution (Chatterjee et al. 1982a) and assuming that the Mg(Al, Cr)₂O₄ can be treated in terms of the asymmetric Margules relation, the exchange isotherms were solved for Δ G ^*, and . The best constrained data set from the 1,250° C isotherm clearly shows that the latter two quantities do not overlap within three standard deviations, justifying the choice of asymmetric Margules relation for describing the excess mixing properties of Mg(Al, Cr)₂O₄ spinels. Based on these experiments, the following polybaric-polythermal equation of state can be formulated: , P expressed in bars, T in K, G _m ^ex and W _G,i ^Sp in joules/mol. Temperature-dependence of G _m ^ex is best constrained in the range 796–1,250° C; extrapolation beyond that range would have to be done with caution. Such extrapolation to lower temperature shows tentatively that at 1 bar pressure the critical temperature, T _c, of the spinel solvus is 427° C, with dT_c/dP≈1.3 K/kbar. The critical composition, X _c, is 0.42 , and changes barely with pressure. Substantial error in calculated phase diagrams will result if the significant positive deviation from ideality is ignored for Al-Cr mixing in such spinels.     

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     

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.     

Crystallographic studies of some olivine-related (Ni,Mg)3(PO4)2 solid solutions

Olivine-related (Ni, Mg)₃(PO₄)₂ solid solutions were prepared and equilibrated at 1070 K. Accurate monoclinic unit cell dimensions were determined from Guinier-Hägg photographic data. Structural refinements based on the X-ray profile-fitting technique after Rietveld were carried out for pure nickel (II) orthophosphate and for three Ni/Mg solid solutions. (Ni_1-x Mg _x )₃(PO₄)₂ phases with 0.40≦x≦0.60 are probably isostructural with Ni₃(PO₄)₂ (P2₁/a) while phases with low magnesium contents (<27 atom % Mg) deviate structurally from Ni₃(PO₄)₂. The results also show that Ni²⁺ is partially ordered at the octahedralM(1) sites, withK _D (Ni, Mg)=4.0±0.2     

Chemistry, Raman and infrared spectroscopic characterization of the phosphate mineral reddingite: (MnFe)3(PO4)2(H2O,OH)3, a mineral found in lithium-bearing pegmatite

Detailed investigation of an intermediate member of the reddingite–phosphoferrite series, using infrared and Raman spectroscopy, scanning electron microcopy and electron microprobe analysis, has been carried out on a homogeneous sample from a lithium-bearing pegmatite named Cigana mine, near Conselheiro Pena, Minas Gerais, Brazil. The determined formula is $ ({\text{Mn}}_{1.60} {\text{Fe}}_{1.21} {\text{Ca}}_{0.01} {\text{Mg}}_{0.01} )_{\sum 2.83} ({\text{PO}}_{4} )_{2.12} \cdot ({\text{H}}_{2} {\text{O}}_{2.85} {\text{F}}_{0.01} )_{\sum 2.86} $ , indicating predominance in the reddingite member. Raman spectroscopy coupled with infrared spectroscopy supports the concept of phosphate, hydrogen phosphate and dihydrogen phosphate units in the structure of reddingite-phosphoferrite. Infrared and Raman bands attributed to water and hydroxyl stretching modes are identified. Vibrational spectroscopy adds useful information to the molecular structure of reddingite–phosphoferrite.     

Refinement of the crystal structure of bonshtedtite, Na3Fe(PO4)(CO3)

The crystal structure of bonshtedtite, Na₃Fe(PO₄)(CO₃) (monoclinic, P2₁/m, a = 5.137(4), b = 6.644(4), c = 8.908(6) Å, β = 90.554(14)°, V = 304.0(4) ų, Z = 2) has been refined to R ₁ = 0.041 on the basis of 1314 unique reflections. The structure is similar to that of other minerals of the bradleyite group. It is based on the [Fe(PO₄)(CO₃)]^3? layers oriented parallel to (001). The layers are formed by corner-sharing PO₄ tetrahedra and FeO₄(CO₃) complexes, where FeO₆ tetrahedra and CO₃ triangles are edge-shared. The topology of the octa-tetrahedral layer in bonshtedtite is similar to that of the autunite-group minerals, but it differs from the latter in terms of local topological properties.     

Fugacities of gases and some mineralogical reactions at very high pressures and temperatures (an extrapolation)

Fugacities of H₂, N₂, O₂, CH₄, H₂O, H₂S, NH₃, CO, and CO₂ are calculated on the basis of the extrapolation of the caloric properties, and those of the compressibility of substances into the region of pressures up to 2 Mbar and of temperatures up to 4000K. Several equilibrium mineralogical reactions are calculated. The most probable sequence of decreasing the magnitudes of the equilibrium molar fractions (masses) of these gases in the lower mantle of the earth is predicted.