方解石-白云石-菱镁矿的中远红外光谱学特征研究

利用拉曼光谱和红外光谱研究了方解石、白云石和菱镁矿的光谱学特征,探究了影响三种矿物红外辐射性能的因素。三种矿物的拉曼光谱(Raman)、中红外吸收光谱(MIR)、远红外吸收光谱(FIR)显示随着矿物中镁含量的增大将会影响CO₃^2-的面外弯曲振动(ν₂)、反对称伸缩振动(ν₃)和平面内弯曲振动(ν₄),使各光谱特征峰均向高频端迁移。基于黑体辐射定律以及在80 ℃、400~2 000 cm^-1矿物的辐射能量谱,结果显示方解石、白云石、菱镁矿的发射率依次减少(0.951,0.938,0.895)。三种矿物的红外吸收光谱和发射光谱中的振动位置均受CO₃^2-基频的显著影响,在1 300~1 650 cm^-1均产生宽的低吸收带,该吸收带与CO₃^2-的反对称伸缩振动相关,且吸收带范围(202,236,272 cm^-1)与发射率之间呈负相关关系。因此,当最强化学键的振动出现在发射光谱窄的吸收带范围内会产生相对较高的辐射能和发射率。此外,矿物的晶体结构也会影响发射率,大的离子半径、键长和晶胞体积将降低辐射过程中能量的吸收,增强辐射特性。综上研究结果,方解石、白云石和菱镁矿的拉曼光谱和红外光谱揭示了金属原子的相对质量对光谱学特征的显著影响,其发射率可能受到C—O键的反伸缩振动范围、最强吸收带控制的最低发射率以及矿物晶体结构的共同影响。这项研究呈现了必要的光谱信息和热发射率数据以识别特定的碳酸盐矿物,为类似矿物的光谱特征研究奠定了基础;同时为进一步认识地壳中大量的碳酸盐矿物提供了研究方法,也为地外勘探的深入研究给予相关的理论基础。     

俯冲带温度条件下橄榄石中硅氧四面体的非谐振动:透射红外光谱研究

橄榄石既是上地幔的主要矿物,又是俯冲板块的主要矿物。在正常地幔温度条件下,橄榄石中原子之间的振动是非谐振动,这已成为学界共识;然而,对于俯冲带的较低温情况,橄榄石中原子之间的振动的非谐性是否重要,目前还没有比较一致的结论。本研究利用透射红外光谱的方法,系统研究了室温下不同厚度橄榄石中硅氧四面体的红外光谱特征,并着重对其在2100~1500cm-1区域形成的二倍频峰/合频峰进行了指认。在此基础上,本研究通过采集不同温度下的透射红外光谱数据(最高温度达约450℃),确立了橄榄石中硅氧四面体的相应基频峰及二倍频峰峰位对温度的依赖性,进而得出其相应红外振动的非谐系数(χ)随温度(T)的变化关系,具体为:χ838=6.37(2)×10-7×T+0.0014(1)及χ993=7.86(3)×10-7×T+0.0015(1),其中温度的单位为℃。外推到600℃时,有χ838=0.0018(1)及χ993=0.0020(2)。这些结果表明,在俯冲带的较低温条件下,橄榄石中硅氧四面体的内部振动的非谐性可能不重要。     

An X-ray photoelectron spectroscopic study of shock-loaded quartz

X-ray photoelectron spectra (XPS) for the Si 2p and O 1s signals of quartz recovered after shock-loading at pressures up to 55 GPa revealed the presence of stishovite in the pressure region between 10 and 34 GPa. The stishovite binding energy for both the Si 2p and O 1s is found to be independent of the shock stress level from which it is recovered. Moreover, the binding energy values obtained from 0.5 mm thick samples shocked in the laboratory for times of ca. 1 μs are equal, within experimental uncertainty, to stishovite produced by the Ries impact event. Variations of binding energies observed for the other phases (residual quartz and glass formed simultaneously with or by decomposition of stishovite) are discussed in the framework of previous results obtained by other methods such as X-ray diffraction and infrared spectroscopy. Although unequivocal interpretation of the variation in binding energy with exposure to different shock pressure is not always possible, the XPS method proves to be very well suited for recognition of high pressure phases and for distinction of pressure regions dominated by various shock or post-shock events.     

A temperature-dependent single-crystal Raman spectroscopic study of fayalite: evidence for phonon-magnetic excitation coupling

The polarized single-crystal Raman spectrum of synthetic fayalite, Fe₂SiO₄, was recorded between 5 and 773 K in order to investigate its lattice dynamic behavior. A broad absorption envelope is observed at wavenumbers between 800 and 960 cm^–1 and it contains two intense bands at 816 and 840 cm^–1 at 293 K in the (cc) spectrum. The integral area of the envelope decreases upon cooling from 293 K and reaches a minimum around 55 K. It then increases again with a further decrease in temperature down to 5 K. It is proposed that the envelope in the (cc) spectra consists of seven different modes, some of which are symmetry-forbidden, that arise from combination scattering of nonsymmetric internal SiO₄-stretching modes of B_ig symmetry (i = 1, 2, 3) and low-energy excitations. The individual modes can be observed under different polarizations and agree in number and wavenumber with those obtained by fitting the broad envelope with Lorentzians. An analysis of the Raman spectrum as a function of temperature, using the known magnetic properties of fayalite, allows the assignment of the low-energy excitations to short-range magnetic interactions. Modulation of the Fe²⁺(1)–Fe²⁺(2) exchange energy leads to phonon-magnetic excitation coupling and the main role in the Fe²⁺(1)–Fe²⁺(2) magnetic interaction occurs via superexchange through the oxygens. The magnetic excitations are not magnons in the usual sense, that is as quasiparticles having a long wavelength in an ordered system. The degree of observed broadening of the SiO₄-stretching modes is consonant with a Fe²⁺(1)–Fe²⁺(2) exchange energy of 4.7 cm^–1 presented by Schmidt et al. (1992). At temperatures above 300 K the line width of the mode at 840 cm^–1 decreases slightly, whereas those of low energy lattice modes increase. This suggests that a decrease in mode broadening due to weakened magnetic interactions compensates any thermally related broadening. Complete Fe²⁺ spin disorder may not be reached until at least 530 K. Results from this study show that estimates of third-law entropies for silicates using simple crystal-chemical considerations that do not account for magnetic properties cannot give accurate values for many transition-metal-containing phases.     

Water in silicate glasses: An infrared spectroscopic study

Infrared and near-infrared transmission spectra have been taken on 19 volcanic and synthetic silicate glasses with known H₂O contents (0.06–6.9 wt. %). Absorption peaks were observed at wavelengths of 1.41 m, 1.91 m, 2.22 m, 2.53 m, and 2.8 m. These peaks have been attributed to the first overtone of the OH stretching vibration, the combination stretching+bending mode of H₂O molecules, the combination stretching+bending mode of X-OH groups, a combination mode of the fundamental OH stretch+a low energy lattice vibration, and the fundamental OH stretching mode, respectively. Molar absorptivities of the peaks have been determined to be 0.2, 1.8, 1.0, 0.9, and 67 l/mol-cm. These values apply over the full range of glass compositions studied (albite, rhyolite, basalt).Quantitative determinations of total H₂O contents and of the concentrations of molecular water and hydroxyl groups in silicate glasses are possible using these molar absorptivities, although they are limited in their accuracy by the accuracy of the reported water contents of the glasses used to calibrate these molar absorptivities. The most important uses of this technique may stem from its applicability to microsamples (100 m) and to the determination of the concentrations of hydroxyl groups and molecular water in quenched silicate melts.Hydroxyl groups are the dominant hydrogen-bearing species in water-bearing glasses at low total water contents, but molecular H₂O was detected in all samples with 0.5 weight percent total water. The concentration of hydroxyl groups increases rapidly with total water content at low total water contents, but more slowly at higher (>3 wt. %) total water contents; it may level off or even decrease at high total water contents. The concentration of molecular water increases slowly at low total water contents and more rapidly at high total water contents. More water is dissolved as molecular water than as hydroxyl groups at total water contents greater than 4 wt. %. Molecular water in these glasses is probably structurally bound rather than present as fluid inclusions as a separate phase, since ice bands were not observed in spectra taken at 78K and since samples were free of visible bubbles.It is proposed that the speciation of water in silicate glass formed by rapid quenching from melt equilibrated at high temperatures reflects that of the melt. According to this hypothesis, neither high water contents nor high pressures are needed to stabilize substantial quantities of molecular water in melts. This hypothesis, that water dissolves in silicate melts as both molecular water and hydroxyl groups in proportions similar to those measured in waterbearing glasses, can explain the variations in viscosity, electrical conductivity, diffusivity of water, diffusivity of cesium, and phase relationships that are observed in melts as functions of total water content. It also explains the observation that at vapor-saturation at high pressures, where most of the dissolved water is expected to be present as molecular water, water solubilities are similar for all melts but that at low pressures and water contents, where most dissolved water is present in dissociated form as hydroxyl groups, vapor-saturated water solubilities differ for different melt compositions. The linear relationship between water fugacity and the square of the mole fraction of total dissolved water observed for silicate melts at low water contents and the observed deviations from this linear relationship at high total water contents can be accounted for by this hypothesis.     

High-pressure crystal chemistry of chromous orthosilicate, Cr2SiO4. A single-crystal X-ray diffraction and electronic absorption spectroscopy study

The high-pressure behaviour of chromous orthosilicate, Cr₂SiO₄, has been studied by means of single-crystal X-ray diffraction and electronic absorption spectroscopy. X-ray diffraction data show that the structure remains orthorhombic to the highest pressure reached of 9.22?GPa. The compressibility of the unit-cell is strongly anisotropic with the c axis approximately six times more compressible than the a and b axes. A third-order Birch-Murnaghan equation of state fitted to the volume-pressure data yields V ₀?=?610.10(3)?ų, K = 94.7(4)?GPa, K′?=?8.32(14). Cr₂SiO₄ is therefore more compressible than the isostructural Cd analogue, even though its molar volume is smaller. This unusual behaviour can be attributed to the fact that the Cr atom is too small for the six-coordinated site that it occupies, and the site is therefore strongly distorted. Structure refinements indicate that under high pressures the Cr atom remains strongly displaced from the central position of the octahedron. Polarized and unpolarized electronic absorption spectra include a strong absorption band occuring at 18.300 cm^?1 for E//c (which is parallel to the shortest Cr-Cr vector in the structure) which has an unusually large half width (5000?cm^?1), indicative of electronic interaction between metal centres. Deconvolution of unpolarized high-pressure spectra show that the relative integrated intensity of this component increases linearly from 40% at 1?bar to 60% at 11.2?GPa. Both the structural changes and the absorption spectra at high pressures suggest that pairs of adjacent Cr atoms in chromous orthosilicate form chromium dimers with a weak metal-metal bond, which is consistent with the diamagnetic response found at ambient pressure.     

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     

Tilting and distortion of CaSnO<Subscript>3</Subscript> perovskite to 7 GPa determined from single-crystal X-ray diffraction

The structural changes of CaSnO₃, a GdFeO₃-type perovskite, have been investigated to 7 GPa in a diamond-anvil cell at room temperature using single-crystal X-ray diffraction. Significant changes are observed in both the octahedral Sn–O bond lengths and tilt angles between the SnO6 octahedra. The octahedral (SnO6) site shows anisotropic compression and consequently the distortion of SnO6 increases with pressure. Increased pressure also results in a decrease of both of the inter-octahedral angles, Sn–O1–Sn and Sn–O2–Sn, indicating that octahedral tilting increases with increasing pressure, chiefly equivalent to rotation of the SnO6 octahedra about the pseudocubic <001>_p axis. The distortion in the CaO12 and SnO6 sites, along with the octahedral SnO6 tilting, is attributed to the SnO6 site being less compressible than the CaO12 site.Acknowledgments The authors acknowledge with gratitude the financial support for this work from NSF grant EAR-0105864. Ruby pressure measurements were conducted with the Raman system in the Vibrational Spectroscopy Laboratory in the Department of Geosciences at Virginia Tech with the help of Mr. Charles Farley.     

<Superscript>57</Superscript>Fe Mössbauer spectroscopy,X-ray single-crystal diffractometry,and electronic structure calculations on natural alexandrite

Natural alexandrite Al₂BeO₄:Cr from Malyshevo near Terem Tschanka, Sverdlovsk, Ural, Russia, has been characterized by ⁵⁷Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine oxidation state and location of iron. The sample contains 0.3 wt% of total iron oxide. The ⁵⁷Fe Mössbauer spectrum can be resolved into three doublets. Two of them with hyperfine parameters typical for octahedrally coordinated high-spin Fe³⁺ and Fe²⁺, respectively, are assigned to iron substituting for Al in the octahedral M2-site. The third doublet is attributed to Fe³⁺ in hematite. Electronic structure calculations in the local spin density approximation are in reasonable agreement with experimental data provided that expansion and/or distortion of the coordination octahedra are presumed upon iron substitution. The calculated hyperfine parameters of Fe³⁺ are almost identical for the M1 and M2 positions, but the calculated ligand-field splitting is by far too large for high-spin Fe³⁺ on M1.     

Structural and vibrational behaviour of fluorapatite with pressure. Part I: in situ single-crystal X-ray diffraction investigation

Using single-crystal X-ray diffraction from a diamond anvil cell, the compressibility of a synthetic fluorapatite was determined up to about 7?GPa. The compression pattern was anisotropic, with greater change along a than c. Unit cell parameters varied linearly with β _a =3.32(8)?10^?3 and β _c =2.40(5)?10^?3 GPa^?1, giving a ratio β _a :β _c =1.38:1. Data fitted with a third-order Birch-Murnaghan EOS yielded a bulk modulus of K ₀=93(4)?GPa with K′=5.8(1.8). The evolution of the crystal structure of fluorapatite was analysed using data collected at room pressure, at 3.04 and 4.72?GPa. The bulk modulus of phosphate tetrahedron is about three times greater than the bulk modulus of calcium polyhedra. The values were 270(10), 100(4) and 86(3) GPa for P, Ca1 (nine-coordinated) and Ca2 (seven-coordinated) respectively. While the calcium polyhedra became more regular with pressure, the distortion of the phosphate tetrahedron remained unchanged. The size of the channel extending along the [001] direction represented the most compressible direction. The Ca2–Ca2 distance decreased from 3.982 to 3.897?Å on compression from 0.0001 to 4.72?GPa. The anisotropic compressional pattern may be understood in terms of the greater compressibility of the channel size over the polyhedral units. The reduction of the channel volume was measured by the evolution of the trigonal prism, having the Ca2–Ca2–Ca2 triangle as its base and the c lattice parameter as its height. This prism volume changed from 47.3?ų at room pressure to 44.78?ų at 4.72?GPa. Its relatively high bulk moduli, 86(3) GPa, indicated that the channel did not collapse with pressure and the apatite structure could remain stable at very high pressure.     

Phase transition(s) in titanite CaTiSiO5: An infrared spectroscopic,dielectric response and heat capacity study

The structural phase transition in titanite near 500 K (averaged symmetries A2/aP2 ₁/a) and a second anomaly around 900 K have been studied using infrared spectroscopy on single crystals aqnd powder samples, measurements of the dielectric properties and the specific heat. The same synthetic single crystal was used in all experiments.The phase transition near 500 K is associated with a break in the temperature evolution of phonon frequencies and absorption intensities. Some phonon signals decrease rapidly under further heating and their extrapolated intensities disappear at ca. 850 K. The most dominant temperature effect relates to Ti-O phonons with amplitudes along the crystallographic a axis. These phonons show large LO-To splitting and continue to soften under heating even at temperatures above the transitions point (ca. 500 K).     

Structural states of natural potassium feldspar: An infrared spectroscopic study

Natural samples of K-feldspar representing various states of Al, Si order were characterised using X-ray methods, transmission electron microscopy, and Fourier transform infrared spectroscopy. Line profiles of infrared absorption bands were observed to show strong correlation with the degree of Al, Si order present. In particular, the absorption frequencies of the 540 cm^?1 and 640 cm^?1 bands were seen to vary by ca. 10 cm^?1 between sanidine and microcline, with modulated samples respresenting intermediate behaviour. Linewidths of these modes also decrease by ca. 50% in this series. The experimental results are discussed within the framework of Hard Mode Infrared Spectroscopy (HMIS), and it is shown that the absorption frequencies vary with the short range order parameter τ = (4t₁-1)² and the symmetry breaking order parameter describing Al, Si order, Q _od=(t₁ 0?t₁ m)/Q _od=(t₁ 0+t₁ m), where t₁ is the average Al occupancy on the T₁ sites and t₁ o and t₁ m are the individual site occupancies of the T₁ o and T₁ m sites, respectively. The structural state of orthoclase is characterised by strain-induced modulations with large spatial variations of the modulation wavelength. No such modulations were observed in the degree of local Al, Si order. Sanidine shows mode hardening in excess of the extrapolated effect of symmetry breaking Al, Si order, which is presumably related to nonsymmetry breaking ordering between T₁ and T₂ sites and/or as yet unobserved short range order of the symmetry breaking ordering scheme. The possibility of an additional phase transition in K-feldspar at temperatures above 1300 K is discussed.     

High-pressure single-crystal X-ray diffraction study of jadeite and kosmochlor

The crystal structures of natural jadeite, NaAlSi₂O₆, and synthetic kosmochlor, NaCrSi₂O₆, were studied at room temperature, under hydrostatic conditions, up to pressures of 30.4 (1) and 40.2 (1) GPa, respectively, using single-crystal synchrotron X-ray diffraction. Pressure–volume data have been fit to a third-order Birch–Murnaghan equation of state yielding V ₀ = 402.5 (4) Å³, K ₀ = 136 (3) GPa, and K ₀ ^′  = 3.3 (2) for jadeite and V ₀ = 420.0 (3) Å³, K ₀ = 123 (2) GPa and K ₀ ^′  = 3.61 (9) for kosmochlor. Both phases exhibit anisotropic compression with unit-strain axial ratios of 1.00:1.95:2.09 for jadeite at 30.4 (1) GPa and 1:00:2.15:2.43 for kosmochlor at 40.2 (1) GPa. Analysis of procrystal electron density distribution shows that the coordination of Na changes from 6 to 8 between 9.28 (Origlieri et al. in Am Mineral 88:1025–1032, 2003) and 18.5 (1) GPa in kosmochlor, which is also marked by a decrease in unit-strain anisotropy. Na in jadeite remains six-coordinated at 21.5 (1) GPa. Structure refinements indicate a change in the compression mechanism of kosmochlor at about 31 GPa in both the kinking of SiO₄ tetrahedral chains and rate of tetrahedral compression. Below 31 GPa, the O3–O3–O3 chain extension angle and Si tetrahedral volume in kosmochlor decrease linearly with pressure, whereas above 31 GPa the kinking ceases and the rate of Si tetrahedral compression increases by greater than a factor of two. No evidence of phase transitions was observed over the studied pressure ranges.     

X-ray single-crystal study of spinels: in situ heating

 Two MgAl₂O₄ stoichiometric spinel crystals, one natural and one synthetic, were heated from 25 to 950 °C and studied in situ by single-crystal X-ray diffraction. The natural crystal, quenched from 850 °C, was further heated and cooled. Thermal expansion was characterized, and cation partitioning at the various temperatures was determined according to a model purposely constructed for high-temperature bond lengths. It was found that the structural evolution of the samples with temperature depended on order–disorder at room temperature. At the temperatures lower than the beginning of cation exchange, thermal expansion was completely reversible and the oxygen coordinate remained stable in spite of varying temperatures. At the temperature at which cation exchange starts, the disordered samples first tend to order and then to disorder at higher temperatures, at variance with the ordered sample, which tends to disorder steadily. In general, the evolution of the spinel structural state on cooling and heating over the same temperature range and the same time intervals does not follow the same path. In particular, in the 600–950 °C range, only partially reversible order–disorder processes occurred in the time span used for the experiments. Received: 16 July 2001 / Accepted: 8 January 2002     

Refined relationship between the position of the fundamental OH stretching and the first overtones for clays

The aim of this paper is to determine a relationship between the wavenumbers of the first OH-stretching overtones (W_2OH) and the wavenumbers of the OH-stretching fundamentals (W_OH) to help to interpret the near-infrared (NIR) spectra. The first overtone (2OH) bands appear at wavenumbers less than twice those of the fundamental bands (OH), due to the anharmonic character of vibrations, X = W_2OH/2 - W_OH, with X being the anharmonicity constant. Talc samples with various crystal chemistries are used to solve the equation and the experimental data are well fitted with X = –85.6 cm^–1. As far as the authors are aware, it is the first time that the anharmonicity constant for the OH-stretching vibrations is determined for phyllosilicates. The anharmonicity constant remains almost unchanged for several types of clay samples. Therefore the relation, established from talc samples because their absorption bands are narrow and their wavenumber range of OH vibrations is wide, can be used for any other clay minerals.     

An infrared and <Superscript>1</Superscript>H MAS NMR investigation of strong hydrogen bonding in ussingite,Na<Subscript>2</Subscript>AlSi<Subscript>3</Subscript>O<Subscript>8</Subscript>(OH)

The mineral ussingite, Na₂AlSi₃O₈(OH), an interrupted tectosilicate, has strong hydrogen bonding between OH and the other nonbridging oxygen atom in the structure. Infrared spectra contain a strongly polarized, very broad OH-stretching band with an ill-defined maximum between 1500 and 1800 cm^–1, and a possible OH librational bending mode at 1295 cm^–1. The IR spectra confirm the orientation of the OH vector within the triclinic unit cell as determined from X-ray refinement (Rossi et al. 1974). There are three distinct bands in the ¹H NMR spectrum of ussingite: a predominant band at 13.5 ppm (TMS) representing 90% of the structural hydrogen, a second band at 15.9 ppm corresponding to 8% of the protons, and a third band at 11.0 ppm accounting for the remaining 2% of structural hydrogen. From the correlation between hydrogen bond length and ¹H NMR chemical shift (Sternberg and Brunner 1994), the predominant hydrogen bond length (H...O) was calculated to be 1.49 Å, in comparison to the hydrogen bond length determined from X-ray refinement (1.54 Å). The population of protons at 15.9 ppm is consistent with 5–8% Al–Si disorder. Although the ussingite crystal structure and composition are similar to those of low albite, the bonding environment of OH in low albite and other feldspars, as characterized through IR and ¹H NMR, is fundamentally different from the strong hydrogen bonding found in ussingite.     

Solar absorption infrared spectroscopic measurements over Mexico City: Methane enhancements

In this work, the experiment for performing solar-absorption infrared measurements from the atmospheric observatory of the Universidad Nacional Autónoma de México (UNAM) located at the university campus in Mexico City is described. The Fourier transform infrared (FTIR) spectrometer and solar-tracking system have been operating since June 2010, and from the recorded spectra the total column amounts of several atmospheric gases can be derived. The current study presents the results obtained for methane (CH₄), an important pollutant involved in ozone production and a rapidly increasing greenhouse gas. The total column amounts, retrieved with high temporal resolution, present a large dispersion and day-to-day variability. A mean value of 2.88 × 10¹⁹ molecules/cm² (1.829 ppm), with a 95% confidence interval between 2.62 and 3.14 × 10¹⁹ molecules/cm², has been obtained for the period from June 2010 to December 2011. No clear annual cycle can be determined from the monthly means due to the large variability in the measurements, suggesting a significant effect of local emissions on the natural background concentrations. Some days with extraordinary enhancements are presented and a simple back trajectory analysis points to a predominant source direction from the northeast of the measurement site. The methane-contaminated air masses passing over the UNAM atmospheric observatory, however, originate presumably not from one but several dispersed sources. A more detailed analysis with modeling of the dynamics of these air masses is required.     

A low temperature X-ray single-crystal diffraction and polarised infra-red study of epidote

The effects of low-temperature on the crystal structure of a natural epidote [Ca_1.925Fe_0.745Al_2.265Ti_0.004Si_3.037O₁₂(OH), a = 8.8924(7), b = 5.6214(3), c = 10.1547(6)? and β = 115.396(8)° at room conditions, Sp. Gr. P2₁ /m] have been investigated with a series of structure refinements down to 100 K on the basis of X-ray single-crystal diffraction data. The reflection conditions confirm that the space group is maintained within the T-range investigated. Structural refinements at all temperatures show the presence of Fe³⁺ at the octahedral M(3) site only [%Fe(M3) = 70.6(4)% at 295 K]. Only one independent proton site was located and two possible H-bonds occur, with O(10) as donor and O(4) and O(2) as acceptors. The H-bonding scheme is maintained down to 100 K and is supported by single crystal room-T polarised FTIR data. FTIR Spectra over the region 4,000–2,500 cm⁻¹ are dominated by the presence of a strongly pleochroic absorption feature which can be assigned to protonation of O(10)–O(4). Previously unobserved splitting of this absorption features is consistent with a NNN influence due to the presence of Al and Fe³⁺ on the nearby M(3) site. An additional relatively minor absorption feature in FTIR spectra can be tentatively assigned to protonation of O(10)–O(2). Low-T does not affect significantly the tetrahedral and octahedral bond distances and angles, even when distances are corrected for “rigid body motions”. A more significant effect is observed for the bond distances of the distorted Ca(1)- and Ca(2)-polyhedra, especially when corrected for “non-correlated motion”. The main low-T effect is observed on the vibrational regime of the atomic sites, and in particular for the two Ca-sites. A significant reduction of the magnitude of the thermal displacement ellipsoids, with a variation of U _eq (defined as one-third of the trace of the orthogonalised U _ij tensor) by ~40% is observed for the Ca-sites between 295 and 100 K. Within the same T-range, the U _eq of the octahedral and oxygen sites decrease similarly by ~35%, whereas those of the tetrahedral cations by ~22%.