Theoretical study of the local charge compensation and spectroscopic properties of B-type carbonate defects in apatite

The structure and spectroscopic properties of selected models of B-type carbonate defects in apatite locally compensated by fluoride or hydroxyl ions are investigated using first-principles quantum mechanical calculations. Theoretical infrared absorption spectra and ¹³C, and ¹⁹F nuclear magnetic resonance chemical shifts are determined. Among the investigated models, only the clumped (CO₃ ^2?, F^?) defect, with the carbonate group close to the sloping face of the tetrahedral site and the F^? ion at the remaining apex, corresponds to previous experimental observations performed on carbonate-fluorapatite samples. Although the substitution of hydroxyl by fluoride ions is commonly observed in minerals, the clumped (CO₃ ^2?, OH^?) defects are unlikely to occur in apatite, considering both their theoretical spectroscopic properties and relative stability. Anionic F^? for OH^? exchange between channel and B sites displays a preference of ~20 kJ/mol for the local charge compensation by fluoride ions at the B-site, pointing to a significantly different behavior of F^? and OH^? ions in the charge compensation mechanism. This difference is ascribed to the poor H-bond acceptor character of available oxygen atoms surrounding the apex of the tetrahedral site. The explicit calculation of the infrared absorption spectra of the defect models is also used to interpret the significant difference observed in the linewidth of the ν₂ and ν₃ CO₃ infrared powder absorption bands of carbonated apatite samples. It is shown that for a concentration of 4.4 wt% of CO₂, long-range electrostatic effects already significantly contribute to the broadening of the ν₃ CO₃ bands in apatite.     

Experimental and theoretical study of the vibrational properties of diaspore (α-AlOOH)

Vibrational properties of diaspore, α-AlOOH, have been re-investigated using room-temperature single-crystal Raman spectroscopy and low-temperature powder infrared (IR) transmission spectroscopy. First-principles harmonic calculations based on density functional theory provide a convincing assignment of the major Raman peaks and infrared absorption bands. The large width of the Raman band related to OH stretching modes is ascribed to mode–mode anharmonic coupling due to medium-strength H-bonding. Additional broadening in the powder IR spectrum arises from depolarization effects in powder particles. The temperature dependence of the IR spectrum provides a further insight into the anharmonic properties of diaspore. Based on their frequency and temperature behavior, narrow absorption features at ~2,000 cm⁻¹ and anti-resonance at ~2,966 cm⁻¹ in the IR spectrum are interpreted as overtones of fundamental bending bands.     

Surface modes in the infrared spectrum of hydrous minerals: the OH stretching modes of bayerite

The theoretical infrared (IR) and Raman spectra of bayerite (β-Al(OH)₃) are computed in the density functional theory framework, using the linear response theory. The results are consistent with the occurrence of six non-equivalent OH groups in a bayerite structure with space group P2₁/n. Similar to gibbsite, the transmission powder IR spectrum of bayerite in the region of the OH stretching bands is found to depend on the shape of particles. In particular, the broadening of the strong band observed at about 3,460 cm⁻¹ in the spectrum of Al hydroxides is related to the electrostatic charges occurring at the surface of the polarized dielectric particles. The experimental correlation observed between the shape of this band and morphological parameters has therefore a physical, instead of chemical, origin.     

Location and quantitative analysis of OH in coesite

 The incorporation of hydrogen (deuterium) into the coesite structure was investigated at pressures from 3.1 to 7.5 GPa and temperatures of 700, 800, and 1100 °C. Hydrogen could only be incorporated into the coesite structure at pressures greater 5.0 GPa and 1100 °C . No correlation between the concentration of trace elements such as Al and B and the hydrogen content was observed based on ion probe analysis (1335 ± 16 H ppm and 17 ± 1 Al ppm at 7.5 GPa, 1100 °C). The FTIR spectra show three relatively intense bands at 3575, 3516, and 3459 cm⁻¹ (ν₁ to ν₃, respectively) and two very weak bands at 3296 and 3210 cm⁻¹ (ν₄ and ν₅, respectively). The band at 3516 cm⁻¹ is strongly asymmetric and can be resolved into two bands, 3528 (ν_2a) and 3508 (ν_2b) cm⁻¹, with nearly identical areas. Polarized infrared absorption spectra of coesite single-crystal slabs, cut parallel to (0 1 0) and (1 0 0), were collected to locate the OH dipoles in the structure and to calibrate the IR spectroscopy for quantitative analysis of OH in coesite (ɛ _{i ,tot}=190 000 ± 30 000 l mol⁻¹ _H2O cm⁻²). The polarized spectra revealed a strong pleochroism of the OH bands. High-pressure FTIR spectra at pressures up to 8 GPa were performed in a diamond-anvil cell to gain further insight into incorporation mechanism of OH in coesite. The peak positions of the ν₁, ν₂, and ν₃ bands decrease linearly with pressure. The mode Grüneisen parameters for ν₁, ν₂, and ν₃ are −0.074, −0.144 and −0.398, respectively. There is a linear increase of the pressure derivatives with band position which follows the trend proposed by Hofmeister et al. (1999). The full widths at half maximum (FWHM) of the ν₁, ν₂, and ν₃ bands increase from 35, 21, and 28 cm⁻¹ in the spectra at ambient conditions to 71, 68, and 105 in the 8 GPa spectra, respectively. On the basis of these results, a model for the incorporation of hydrogen in coesite was developed: the OH defects are introduced into the structure by the substitution Si^4+(Si2)+4O²⁻= ^[4]□^(Si2) + 4OH⁻, which gives rise to four vibrations, ν₁, ν_2a, ν_2b, and ν₃. Because the OH(D)-bearing samples do contain traces of Al and B, the bands ν₄ and ν₅ may be coupled to Al and/or B substitution. Received: 19 December 2000 / Accepted: 23 April 2001     

Coordination of water molecules with Na+ cations in a beryl channel as determined by polarized IR spectroscopy

Subtle variations of frequencies in the infrared (IR) absorption spectra of beryl have been predicted based on the coordination between extra-framework cations and water molecules in two orientations (referred to as type I and type II) trapped within the channel. In this study, the polarized IR spectra of hydrated synthetic beryl and natural beryl were measured to clarify the relationships between the frequencies of the absorption bands and the coordination states of type II water. Na⁺ was assumed to be the predominant cation coordinated to type II water in our samples, as determined by chemical analyses. These measurements revealed a clear quantitative linear relationship in absorbance between bands at 3,602 and 1,619 and at 3,589 and 1,631 cm⁻¹. On the basis of experimental and theoretical studies, we assigned these pairs of bands to the ν₁ and ν₂ modes of doubly coordinated type II H₂O and to singly coordinated type II H₂O, respectively. These assignments were supported by IR measurements of annealed natural beryl. We also conducted dehydration studies of natural beryl, in which two observed dehydration peaks, at 600 and 750°C, suggested the dehydration of type I and type II water, respectively.     

Thermoluminescence, electron paramagnetic resonance and optical absorption in natural and synthetic rhodonite crystals

Thermoluminescence, electron paramagnetic resonance and optical absorption properties of rhodonite, a natural silicate mineral, have been investigated and compared to those of synthetic crystal, pure and doped. The TL peaks grow linearly for radiation dose up to 4 kGy, and then saturate. In all the synthetic samples, 140 and 340°C TL peaks are observed; the difference occurs in their relative intensities, but only 340°C peak grows strongly for high doses. Al₂O₃ and Al₂O₃ + CaO-doped synthetic samples presented several decades intenser TL compared to that of synthetic samples doped with other impurities. A heating rate of 4°C/s has been used in all the TL readings. The EPR spectrum of natural rhodonite mineral has only one huge signal around g = 2.0 with width extending from 1,000 to 6,000 G. This is due to Mn dipolar interaction, a fact proved by numerical calculation based on Van Vleck dipolar broadening expression. The optical absorption spectrum is rich in absorption bands in near-UV, visible and near-IR intervals. Several bands in the region from 540 to 340 nm are interpreted as being due to Mn³⁺ in distorted octahedral environment. A broad and intense band around 1,040 nm is due to Fe²⁺. It decays under heating up to 900°C. At this temperature it is reduced by 80% of its original intensity. The pink, natural rhodonite, heated in air starts becoming black at approximately 600°C.     

Electronic absorption by Ti3+ ions and electron delocalization in synthetic blue rutile

Polarized absorption spectra, σ and π, in the spectral range 30000–400 cm⁻¹ (3.71–0.05 eV) were obtained on crystal slabs // [001] of deep blue rutile at various temperatures from 88 to 773 K. The rutile crystals were grown in Pt-capsules from carefully dried 99.999% TiO₂ rutile powder at 50 kbar/1500 °C using graphite heating cells in a belt-type apparatus. Impurities were below the detection limits of the electron microprobe (about 0.005 wt% for elements with Z≥13). The spectra are characterized by an unpolarized absorption edge at 24300 cm⁻¹, two weak and relatively narrow (Δν_1/2≈3500–4000 cm⁻¹), slightly σ-polarized bands ν₁ at 23500 cm⁻¹ and ν₂ at 18500 cm⁻¹, and a complex, strong band system in the NIR (near infra red) with sharp weak peaks in the region of the OH stretching fundamentals superimposed on the NIR system in the σ-spectra. The NIR band system and the UV edge produce an absorption minimum in both spectra, σ and π, at 21000 cm⁻¹, i.e. in the blue, which explains the colour of the crystals. Bands ν₁ and ν₂ are assigned to dd transitions to the Jahn-Teller split upper E_g state of octahedral Ti³⁺. The NIR band system can be fitted as a sum of three components. Two of them are partly π-polarized, nearly Gaussian bands, both with large half widths 6000–7000 cm⁻¹, ν₃ at 12000 cm^–1 and the most intense ν₄ at 6500 cm⁻¹. The third NIR band ν₅ of a mixed Lorentz-Gaussian shape with a maximum at 3000 cm⁻¹ forms a shoulder on the low-energy wing of ν₄. Energy positions, half band widths and temperature behaviour of these bands are consistent with a small polaron type of Ti³⁺Ti⁴⁺ charge transfer (CT). Polarization dependence of CT bands can be explained on the basis of the structural model of defect rutile by Bursill and Blanchin (1983) involving interstitial titanium. Two OH bands at 3322 and 3279 cm⁻¹ in σ-spectra show different stability during annealing, indicating two different positions of proton in the rutile structure, one of them probably connected with Ti³⁺ impurity. Total water concentration in blue rutile determined by IR spectroscopy is 0.10 wt-% OH. The EPR spectra measured in the temperature interval 20–295 K show the presence of an electron centre at temperatures above 100 K and Ti³⁺ ions in more than one structural position, but predominantly in compressed interstitial octahedral sites, at lower temperatures. These results are in good agreement with the conclusions based on the electronic absorption data. Received: 24 March 1997 / Revised, accepted: 14 October 1997     

Structural and vibrational behaviour of fluorapatite with pressure. Part II: in situ micro-Raman spectroscopic investigation

 High-pressure Raman investigations were carried out on a synthetic fluorapatite up to about 7 GPa to analyse the behaviour of the phosphate group's internal modes and of its lattice modes. The Raman frequencies of all modes increased with pressure and a trend toward reduced splitting was observed for the PO₄-stretching modes [(ν_3a(A_g) and ν_3b(A_g); ν_3a(E_2g) and ν_3b(E_2g)] and the PO₄ out-of-plane bending modes [ν_4a(A_g) and ν_4b(A_g)]. The pressure coefficients of phosphate modes ranged from 0.0047 to 0.0052 GPa⁻¹ for ν₃, from 0.0025 to 0.0044 GPa⁻¹ for ν₄, from 0.0056 to 0.0086 GPa⁻¹ for ν₂ and 0.0046 for ν₁ GPa⁻¹, while the pressure coefficients of lattice modes ranged from 0.0106 to 0.0278 GPa⁻¹. The corresponding Grüneisen parameters varied from 0.437 to 0.474, 0.428, 0.232 to 0.409 and 0.521 to 0.800 for phosphate modes ν₃, ν₁, ν₄, ν₂, respectively, and from 0.99 to 2.59 for lattice modes. The vibrational behaviour was interpreted in view of the high-pressure structural refinement performed on the same crystal under the same experimental conditions. The reduced splitting may thus be linked to the reduced distortion of the environment around the phosphate tetrahedron rather than to the decrease of the tetrahedral distortion itself. Moreover, the amount of calcium polyhedral compression, which is about three times the compression of phosphate tetrahedra, may explain the different Grüneisen parameters. Received: 25 April 2000 / Accepted: 20 December 2000     

Distinguishing natural from synthetic amethyst: the presence and shape of the 3595 cm−1 peak

Summary The infrared absorption spectrum of amethyst in the region of stretching vibrations of X–OH groups reveals several bands that have been used for the separation of natural from synthetic amethyst. The intensity and shape of these bands have been measured as a function of crystallographic orientation. Using a resolution of 0.5 cm⁻¹ the 3595 cm⁻¹ band is present in all infrared spectra of natural amethyst and in some rare synthetic ones. If present in synthetic amethyst, its full width at half maximum (FWHM) is about 7 cm⁻¹ whereas it is about 3 cm⁻¹ in all natural samples. This new criterion, unlike the previous ones, seems appropriate to separate natural from synthetic amethyst in all cases.     

Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: III. An analysis of IR line broadening and heat of mixing behavior

A series of synthetic Ca-Tschermak–diopside (CaAlAlSiO₆–CaMgSi₂O₆) clinopyroxenes were investigated by powder infrared spectroscopy at room temperature in the wavenumber range 80–2,000 cm⁻¹. Measurable local structural heterogeneities in the crystals are suggested by the line broadening parameter, Δcorr that are observed for intermediate solid-solution compositions. The broadening is most pronounced in the high wavenumber region of the IR spectra that contains stretching modes involving the TO₄ polyhedra. The effective line widths for three selected wavenumber regions deviate positively from linear behavior. This is also observed for the enthalpy of mixing of this solid solution. The relationship between “excess Δcorr”, δΔcorr, and heat of mixing, ΔH _mix, behavior was investigated for this clinopyroxene series and for several other binary silicate solid solutions. The ΔH _mix versus δΔcorr slope values show a linear relationship with respect to the integrated excess volume of the various solid solutions.     

Solid solutions between lead fluorapatite and lead fluorvanadate apatite: compressibility determined by using a diamond-anvil cell coupled with synchrotron X-ray diffraction

The synthetic solid solutions between lead fluorapatite and lead fluorvanadate apatite, Pb₁₀[(PO₄)_6−x (VO₄) _x ]F₂ with x equal to 0, 1, 2, 3, 4, 5, and 6, were compressed up to about 9 GPa at ambient temperature by using a diamond-anvil cell coupled with synchrotron X-ray radiation. A second-order Birch–Murnaghan equation of state was used to fit the data. As the substitution of the PO₄ ³⁻ cations by the VO₄ ³⁻ cations progresses, the isothermal bulk modulus steadily decreases, with a maximum reduction of about 16% (from 68.4(16) GPa for Pb₁₀(PO₄)₆F₂ to 57.2(28) GPa for Pb₁₀(VO₄)₆F₂). For the entire composition range, the a-axis dimension remains more compressible than the c-axis dimension, with the ratio of the axial bulk moduli (K _T−c :K _T−a ) larger than 1. The ratio of K _T−c to K _T−a increases from about 1.04(4) to 1.23(14) as the composition parameter x increases from 0 to 6, suggesting that the apatite solid solutions Pb₁₀[(PO₄)_6−x (VO₄) _x ]F₂ become more elastically anisotropic.     

Modelling the OH stretch envelope shape and temperature effects in infrared spectra of hydrous silica and silicate glasses

Simple model calculations show that the reversible temperature effects reported for broad OH-stretching bands in infrared spectra of silica, aluminosilicate, and similar glasses can be explained, in essentials, by homogeneous thermal broadening of the ν(OH) envelope constituents and a decrease in intensity with increasing temperature taking place uniformly across the band. This means that these effects are reasonably consistent with the temperature behaviour of narrow ν(OH) bands of crystalline OH-bearing minerals. These findings leave little room for the previously agreed interpretation in terms of a change in hydrogen-bonding strength, although the dependence of integrated intensity on temperature still remains to be understood. Received: 16 April 1999 / Accepted: 4 April 2000     

Solid solution between lead fluorapatite and lead fluorvanadate apatite: mixing behavior,Raman feature and thermal expansivity

The solid solution between lead fluorapatite and lead fluorvanadate apatite, Pb₁₀[(PO₄)_6-x (VO₄) _x ]F₂ with x equal to 0, 1, 2, 3, 4, 5 and 6, was synthesized by solid-state reaction at 1 atm and 700°C for 72 h and characterized by scanning electronic microprobe, electronic microprobe analysis, micro-Raman spectroscopy, and powder X-ray diffraction. The volume-composition relationship at ambient temperature does not show significant deviation from the Vegard’s Law. The Raman spectrum data suggest that both P and V are identical on a C _s site and both end-members show no apparent factor-group effect. The Raman frequency shift of the symmetric stretching vibration is linearly dependent on the composition. High temperature X-ray diffraction data, up to 600°C, suggest that the thermal expansion coefficients α _a , α _c , and α _V also vary linearly with the compositions of the apatites.     

Infrared spectroscopic properties of goethite: anharmonic broadening,long-range electrostatic effects and Al substitution

The infrared spectrum and its temperature dependence (20–320 K) were collected on a synthetic goethite sample (α-FeOOH). In addition, the infrared powder absorption spectrum of goethite and aluminum-substituted goethite was computed using ab initio quantum mechanical calculations based on density functional theory. This combined experimental and theoretical approach allows (1) the unequivocal assignment of absorption bands to the corresponding vibrational modes, (2) separate identification of the effects of the particle shape and of the aluminum substitution on the infrared spectrum, and (3) a discussion of the anharmonic properties and the origin of the line broadening in goethite. In particular, the two well-resolved OH bending absorption bands show different temperature evolution. Their detailed analysis suggests that the broadening of the band at ~800 cm^?1 cannot be described solely by a usual three-phonon process. The strong anharmonic behavior of this mode implies the addition of a four-phonon process, such as a pure dephasing process. In our calculations, the effect of the Hubbard U correction is also investigated and found to be most visible on the OH stretching and bending modes, in relation to the associated structural relaxation. The OH stretching frequencies decrease, leading to a better agreement with experimental frequencies, while the OH bending frequencies increase.     

IR spectroscopic characterization of NH4-analcime

 The IR spectrum of ammonium-exchanged natural analcime (basalt, Nidym River, Siberian platform) exhibits several features that suggest a lowered symmetry for the NH₄ ⁺ ion and that this is influenced by hydrogen bonding within the framework. These features are: the pronounced splitting into three components and high-frequency shift of the ν₄-bending mode; appearance of the ν₁-stretching mode which is predicted to become IR-active when the ideal T _d symmetry of NH₄ ⁺ ion is violated, and the low-frequency shift of the ν₁- and ν₃-stretching modes. The absence of absorption lines in the 1800–2400-cm⁻¹ region indicates that hydrogen bonding between the framework and the NH⁺ ₄ ion is very weak. The three-component splitting of the ν₄-bending mode indicates that the symmetry of NH⁺ ₄ ion is lower than C_3v. This implies that at least two N–H bonds of the NH⁺ ₄ ion are disturbed by hydrogen bonding. Computer analysis of the normal vibrations of the NH⁺ ₄ molecule for different symmetry types (using harmonic approximation) indicates that the best fit to the observed ν₄ triplet frequencies for C₁ symmetry implies a deviation of the valent angle ∠H–N–H from ideal T _d symmetry of around ±2.5°. The factors governing the behaviour of the NH⁺ ₄ ion in the analcime structure are discussed. The geometry of the nearest environment of the NH⁺ ₄ ion in the analcime structure is analyzed with respect to the present IR data. Received: 2 January 2002 / Accepted: 26 June 2002 Acknowledgements We thank Dr. D. Harlov and an anonymous reviewer for their helpful comments, as well as Dr. I.A. Belitsky and Dr. S.V. Goryainov for discussion of the material. This work is supported by RFBR grants 01-05-65414, 00-05-65305 and 02-05-65313.     

Cell parameter correlations in the aluminosilicate-sodalites

Cell parameters, ranging from 8.447 to 9.331 Å, are given for 13 synthetic aluminosilicate-sodalites. Correlations between the cell edge of aluminosilicate-sodalites and the sizes of the cavity ions are established and discussed. The principal infrared absorption bands for 6 synthetic and 4 natural aluminosilicate-sodalites are also correlated with the cell edge and discussed in terms of the bonding theory of linked TO₄ tetrahedra (T=Si, Al).     

Stress-induced proton disorder in hydrous ringwoodite

We have measured in situ high-pressure IR absorption of synthetic hydrous (Mg_xFe_1−x)₂SiO₄ ringwoodites (x = 0.00 to 0.61) up to a maximum pressure of 30 GPa. In our study, we combined the megabar-type diamond-anvil cell (DAC) with conventional and synchrotron FTIR spectroscopy. The high-pressure measurements were performed in three different pressure-transmitting environments: (1) CsI powder, (2) cryogenically loaded liquid argon, and (3) cryogenically loaded liquid argon annealed at 8.6 GPa at temperature of 120°C before further pressure increase. Between 10 and 12 GPa, all the samples loaded with methods (1) and (2), independent on composition, showed a sudden disappearance of the prominent OH-stretching feature and simultaneous discontinuities and/or changes in the pressure dependence of lattice vibrations compared with spectra of samples loaded with method (3). In experiments performed with method (3) the OH-stretching vibrations as well as lattice vibrations could be observed up to 30 GPa and their pressure behavior (dν/dP) can well be described by linear fits. Molecular vibrations, such as the OH stretching, are sensitive to non-hydrostatic conditions, especially in minerals with highly symmetric structures. We interpret the disappearance of the OH bands using methods (1) and (2) as a stress-induced proton disordering in hydrous ringwoodite. Our results confirm that argon pressure medium produces strongly non-hydrostatic conditions comparable to CsI or KBr, if it is not thermally annealed at pressures above 8 GPa. Our results suggest that the transition observed in hydrous Mg-ringwoodite end member is not present in compositions containing Fe. By comparing the behavior of samples compressed in different environments, we suggest that sudden disappearance of the OH-stretching band in hydrous ringwoodite could be driven by deterioration of the quasi-hydrostatic stress condition instead of a pressure-induced effect.     

The single-crystal,polarized-light,FTIR spectrum of stoppaniite,the Fe analogue of beryl

We report here a single-crystal polarized-light study of stoppaniite, ideally (Fe,Al,Mg)₄(Be₆Si₁₂O₃₆)(H₂O)₂(Na,□), from Capranica (Viterbo). Polarized-light FTIR spectra were collected on an oriented (hk0) section, doubly polished to 15 μm. The spectrum shows two main bands at 3,660 and 3,595 cm⁻¹; the former is strongly polarized for E ⊥c, while the latter is polarized for E //c. A sharp and very intense band at 1,620 cm⁻¹, plus minor features at 4,000 and 3,228 cm⁻¹ are also polarized for E //c. On the basis of literature data and considering the pleochroic behavior of the absorptions, the 3,660 cm⁻¹ band is assigned to the ν₃ stretching mode and the 1,620 cm⁻¹ (associated with an overtone 2*ν₂ at 3,230 cm⁻¹) band to the ν₂ bending mode of “type II” water molecules within the structural channels of the studied beryl. The sharp band at 3,595 cm⁻¹ is not associated with a corresponding ν₂ bending mode; thus it is assigned to the stretching vibration of O–H groups in the sample. The minor 4,000 cm⁻¹ feature can be assigned to the combination of the O–H bond parallel to c with a low-frequency metal-oxygen mode such as the Na–O stretching mode. The present results suggest that the interpretation of the FTIR spectrum of Na-rich beryl needs to be carefully reconsidered.     

Local structural heterogeneity in garnet solid solutions

Local heterogeneities in pyrope-almandine, almandine-grossular and pyrope-grossular solid solutions have been investigated using IR-powder absorption spectroscopy. Correlations of the wavenumber shifts and line broadening systematics with the thermodynamic mixing properties were found. Wavenumber shifts of the highest energy modes correlate closely with the Si-O bond distances and give an indirect view of the average distortions across the three solid solutions. They have a linear behaviour for Py-Alm, but show positive variations from linearity for Alm-Gr and Py-Gr systems. An effective line width (Δ_corr) of the absorption bands over a given wavenumber interval was obtained using the autocorrelation function. Line broadening is associated with local heterogeneities arising from cation substitution in the structure of samples at intermediate compositions. Non-linearities of the line broadening were found for Alm-Gr and Py-Gr and have a shape similar to the enthalpy of mixing, ΔH_mix. An empirical analysis was therefore carried out to compare ΔH_mix and Δ_corr quantitatively. Low-temperature far-IR spectra were recorded for the end-members pyrope, almandine and grossular and far-IR and mid-IR low-temperature spectra for Py₆₀Gr₄₀ in the temperature range 292–44 K. Softening of the lowest energy band with decreasing temperature was observed in the spectrum of pyrope and more enhanced in the spectrum of Py₆₀Gr₄₀. The same softening occurs by substitution of grossular component into pyrope. High energy modes of Py₆₀Gr₄₀ show the effect of saturation below 110–130 K, which correlates with the volume saturation at low temperature. This could provide an alternative explanation for the heat capacity anomaly found for Py-Gr solid solution at low-temperatures.     

Synthetic ANaB(Na x Li1 − x Mg1)CMg5Si8O22(OH)2 (with x = 0.6, 0.2 and 0) P21/m amphiboles at high pressure: a synchrotron infrared study

The high-pressure behavior of three synthetic amphiboles crystallized with space group P2₁/m at room conditions in the system Li₂O–Na₂O–MgO–SiO₂–H₂O has been studied by in situ synchrotron infrared absorption spectroscopy. The amphiboles have compositions ^ANa ^B(Na _x Li_1 − x Mg₁) ^CMg₅ Si₈ O₂₂(OH)₂ with x = 0.6, 0.2 and 0.0, respectively. The high-P experiments up to 32 GPa were carried out on the U2A beamline at Brookhaven National Laboratory (NY, USA) using a diamond anvil cell under non-hydrostatic or quasi-hydrostatic conditions. The two most intense absorption bands in the OH-stretching infrared spectra can be assigned to two non-equivalent O–H dipoles in the P2₁/m structure, bonded to the same local environment ^M1M3Mg₃–OH–^ANa, and pointing toward two differently kinked tetrahedral rings. In all samples these bands progressively merge to give a unique symmetrical absorption with increasing pressure, suggesting a change in symmetry from P2₁/m to C2/m. The pressure at which the transition occurs appears to be linearly correlated to the aggregate B-site dimension. The infrared spectra collected for amphibole ^B(Na_0.2Li_0.8Mg₁) in the frequency range 50 to 1,400 cm⁻¹ also show a series of changes with increasing pressure. The data reported here support the inference of Iezzi et al. (Am Miner 91:479–482, 2006a) regarding a new high-pressure amphibole polymorph.