Marc Blanchard

First-principles calculation of the infrared spectrum of hematite

Abstract The theoretical infrared spectrum of hematite (α-Fe2O3) was computed using ab initio quantum mechanical calculations. Frequencies of the normal vibrational modes and Born effective charges were computed using the density functional theory (DFT) with and without the addition of a Hubbard U correction. The infrared reflection spectra of a single crystal of hematite were calculated as well as the infrared powder absorption spectrum using an electrostatic model that takes into account the shape of hematite particles. The theoretical behavior of the absorption bands is in agreement with experimental observations and provides a firm basis for the interpretation of the bands in term of vibrational modes. Overall, results suggest that the use of DFT + U, which is necessary to describe correctly the electronic and magnetic properties of hematite, does not improve noticeably the prediction of vibrational properties.

Theoretical infrared absorption coefficient of OH groups in minerals

Abstract The integrated molar absorption coefficient of isolated and localized OH groups in selected minerals is theoretically investigated within the density functional theory framework. The overall decrease in absorption coefficient of stretching modes observed with increasing frequency is consistent with the experimental observations. It is related to a decrease in the magnitude of the hydrogen Born effective charge tensor projected along the OH bond as a function of increasing H-bonding. The scatter of theoretical data shows that the use of a general calibration of infrared absorbances in minerals cannot lead to accurate water contents. In contrast, the combination of theoretical modeling and experimental measurements should improve the determination of the hydrogen distribution among structurally distinct OH defects in nominally anhydrous minerals.

A carbonate-fluoride defect model for carbonate-rich fluorapatite

Abstract We propose a microscopic model of the dominant carbonate for phosphate substitution in fluorapatite. A well-crystallized sedimentary fluorapatite sample containing ~2.3 ± 0.8 wt% of carbonate was investigated using Fourier transform infrared spectroscopy (FTIR) and 13C and 19F magic angle spinning nuclear magnetic resonance (MAS NMR). About 75% of the carbonate groups replace the phosphate group (“B-site”), whereas a lesser contribution from carbonate groups located in the structural channels (“A-site”) is observed. Beside the dominant 19F NMR signal of channel ions at ~ -102 ppm, an additional signal corresponding to ~8% of fluoride ions is observed at -88 ppm. 19F double quantum-single quantum (DQ-SQ) MAS NMR and 13C{19F} frequency-selective Rotational Echo DOuble Resonance (REDOR) experiments prove that this additional signal corresponds to isolated fluoride ions in the apatite structure, located in close proximity of substituted carbonate groups. Density functional theory (DFT) calculations allow us to propose a composite carbonate-fluoride tetrahedron defect model accounting for these experimental observations. The planar carbonate ion lies in the sloping face of the tetrahedron opposite a fluoride ion occupying the remaining vertex, together replacing the tetrahedral phosphate ion. This “francolite-type” defect leads to a diagnostic narrow IR absorption band at 864 cm-1 that could be used as a guide to, e.g., detect the incipient transformation of fossil bone and teeth samples.

Incorporation of water in iron-free ringwoodite: A first-principles study

Abstract The structures, infrared active OH stretching modes, and relative energies of OH-defects in ringwoodite (γ-Mg2SiO4) have been studied by first-principles calculations based on density functional theory (DFT). Two types of fully protonated cationic defects in normal spinel were considered at 0 and 20 GPa, i.e., [VMg(OH)2]x, [VSiOH)4]x defects. In addition, two defects associated with the partial inversion of the spinel structure have been investigated. The first one corresponds to two protons compensating a Mg substituted for Si in tetrahedral site, [MgSi(OH)2]x, whereas the second defect corresponds to a Mg vacancy located nearby a Mg-Si substitution, [VMg(OH)2MgSiSiMg]x. The infrared spectrum and evolution with pressure of these OH-defects make it possible to interpret the major IR absorption bands experimentally observed. The main absorption band at ~3150 cm-1 corresponds to protons located between the O-O pairs shared by 16c and 16d octahedra, instead of OH along the tetrahedral edges as usually proposed in the literature. The large width of this band is most likely related to the association of OH defects with the various cationic configurations related to the partial inversion of a vacancy-bearing spinel structure. The less intense band at ~3675 cm-1 is assigned to hydrogarnet-type defects with a protonation of the tetrahedral edges. This interpretation is consistent with an Mg/Si ratio lower than 2 and its weak variation as a function of water concentration, as experimentally observed. These results emphasize the importance of taking into account the structural relaxation experienced by defects, instead of using empirical correlation, to assign OH stretching bands to specific O-O pairs of the structure.

Kinetics of deuteration in pyrope

Hydrogen-deuterium exchange experiments were performed at temperatures between 973 and 1223 K on a Dora Maira pyrope and on two pyrope samples from mantle xenoliths. The Fourier transform infrared (FTIR) spectra of hydrogen in the natural Dora Maira and xenolith pyropes are different, with several OH bands between 3600 and 3650 cm-1 and between 3510 and 3575 cm -1 , respectively. The self-diffusion of deuterium in Dora Maira pyrope is given by D = D 0 exp [-140 ± 38 kJmol -1 /RT] with log D 0 (m 2 /s) = -5.8 ± 1.9. This activation energy for diffusion is identical to those measured in diopside and olivine, but the pre-exponential term D 0 is one to two orders of magnitude smaller. This suggests that the mechanism of hydrogen self-diffusion in the pyrope structure is similar to that for other upper mantle mineral structures, such as olivine and pyroxene. In contrast to the Dora Maira pyrope, for the two pyrope crystals extracted from xenoliths, the total concentration of hydrous species is not preserved during H-D exchange experiments; dehydration occurs concurrently with deuteration. This renders the analysis of the kinetics of H-D exchange difficult. In the pyropes from mantle xenoliths, the kinetics of dehydration under reducing conditions is close to that for hydrogen dehydrogenation from experiments performed in air by Wang et al. (1996). This suggests that hydrogen exchange in mantle pyropes may be independent of the oxygen fugacity.

Kinetics of hydrogen extraction and deuteration in grossular

Abstract The kinetics of hydrogen mobility in grossular with a chemically homogeneous composition of Gr83.2 And14.3 Py2.2 were studied by sequential annealing experiments monitored by Fourier transform infrared spectroscopy. Slices of single crystals <0.5 mm thick were annealed at temperatures in the range 1073-1323 K at ambient pressure in air and in gas mixtures of Ar(90%)/D2(10%) and Ar(90%)/H2(10%). The change of total infrared (IR) absorbance in the OH-stretching region (3700-3500 cm-1) and the OD-stretching region (2750-2580 cm-1) was used to calculate the diffusion coefficients. The law for diffusion of deuterium is given by D = D0 exp[-102±45 kJ mol-1/RT] with log D0 (m2/s) = -7.6. For hydrogen extraction in air the diffusion law is expressed by D = D0 exp[-323±46 kJ mol-1/RT] with log D0 (m2/s) = 1.0. This activation energy agrees with the values found for Dora Maira pyrope and for other pyropes from mantle xenoliths, but the diffusivity is slower for the grossular. A detailed investigation of the decrease in individual OH bands during hydrogen extraction in air revealed two different kinds of kinetics behaviour, suggesting that at least two different types of OH defects are present in this grossular.

Effect of iron and trivalent cations on OH defects in olivine

Abstract Hydrogen incorporation in olivine involves many OH defects, which will control the hydrogen solubility at mantle conditions. Several of these OH defects are identified from the investigation of forsterite (the olivine Mg end-member). We study here the effect of Fe2+, Fe3+, Al3+, and Cr3+ on OH defects to improve our understanding of the hydrogen speciation in natural olivine. Low-temperature infrared spectra (−194 °C) are collected on synthetic and natural olivines. These spectra are then interpreted in the light of the theoretical determination of the structural, vibrational, and infrared spectroscopic properties of Fe-related OH defects, using first-principles calculations based on density functional theory. The presence of Fe2+ changes the cationic environment around the fully protonated vacancies in forsterite, leading to a slight modification of their infrared signatures. In particular, the presence of Fe2+ in an octahedral site adjacent to a hydrogarnet-type defect is likely responsible for the additional bands observed at 3599 cm−1 and around 3520–3550 cm−1 in Fe-doped olivines. Results show that the OH bands between 3310 and 3380 cm−1 are associated with the presence of trivalent cations. Specifically, two bands at 3323 and 3358 cm−1, commonly observed in natural olivine, are associated with the substitution of Mg2+ by Cr3+ while two similar bands at 3328 and 3353 cm−1 are associated with the substitution of Mg2+ by Fe3+. The presence of these defects and the “titanoclinohumite” defect in natural olivine clearly underlines the prominent role of trace elements on the hydrogen incorporation in lithospheric olivine.

Identification of hydrogen defects linked to boron substitution in synthetic forsterite and natural olivine

Abstract Experimental and theoretical evidence for the coupled substitution of B and H in synthetic forsterite and a natural olivine is presented. The intensities of OH bands at 3704 cm-1 (//z), 3598 cm-1 (//x,y), and 3525 cm-1 (//x) in a heterogeneous B-doped synthetic forsterite crystal matches the zoning of B concentration measured by ion probe. The two anti-symmetric stretching vibrations of BO3 groups occur at 1301 cm-1 (//x) and 1207 cm-1 (//z) for the 10B and at 1256 and 1168 cm-1 for the 11B isotope. A microscopic model of the mixed (B,H) defect that accounts for experimental observations is obtained from quantum mechanical calculations. The BO3 group lies on the (O3-O1-O3) face of the vacant Si site and the H atom is bonded to the O2 atom at the remaining apex. The occurrence of the same OH bands associated with v3 BO3 vibrations in a natural olivine sample from Pakistan confirms the occurrence of this defect in nature. The three diagnostic OH bands can be used as a signature of H associated with boron substitution in olivine and forsterite, leading to a quantitative analysis of their contribution to H-defects.

Infrared signatures of OH-defects in wadsleyite: A first-principles study

Abstract The structure and the polarized infrared absorption spectrum of OH-defects in wadsleyite (β-Mg2SiO4) are studied, at 0 and 15 GPa, by first-principles calculations based on density functional theory (DFT). Four types of OH-defects are considered: fully protonated magnesium vacancies, fully protonated silicon vacancies, silicon vacancies compensated by a magnesium cation and two protons, and OH-defects associated with the migration of a silicon cation to a normally vacant site, as reported by Kudoh and Inoue (1999). The results suggest that the main absorption band constituted by a doublet (3326 and 3360 cm-1) corresponds to at least two types of OH-defects involving M3 vacancies with protonation of the O1-type O atoms along the O1...O4 edges. The main contribution of the less intense band at 3581 cm-1 is likely related to the partial protonation of a silicon vacancy (protonation of the O3-type oxygen) associated with the migration of the silicon cation to the Si2 site. This assignment is consistent with several experimental constraints: wavenumber and pleochroism of infrared OH-stretching bands, pressure-dependence of the band wavenumber, evidence from X‑ray diffraction of magnesium vacancies in M3 site, and increase of the b/a axial ratio with water content. The integrated absorption coefficients of the corresponding OH-defects are also calculated and thus complement the set of data obtained previously for forsterite and ringwoodite. Absorption coefficients of wadsleyite computed at 0 and 15 GPa indicate that for a precise quantification of the hydrogen content in in situ experiments, one must consider higher absorption coefficients than those determined at 0 GPa after quench. It is also shown that a single theoretical relation can account for the three Mg2SiO4 polymorphs at 0 GPa: Kint = 278.7 ± 18.1 (3810 ± 465 - x), where Kint is the integrated molar absorption coefficient of the OH stretching modes and x is the average wavenumber in cm-1. Absorption coefficients are significantly lower than the general calibrations, the use of which would lead to an underestimation of the water concentrations

Theoretical infrared spectrum of partially protonated cationic vacancies in forsterite

Hydroxyl defects in pure forsterite are usually ascribed to incorporation of protons fully compensating the electrostatic charge of cationic vacancies. However, partially compensated vacancies have been predicted from theoretical considerations. Here, we theoretically determine the structural, vibrational and infrared spectroscopic properties of partially protonated cationic vacancies in forsterite using a first-principles theoretical modeling approach. The results show that the partial protonation of Si vacancies strongly modifies their spectroscopic properties with respect to those of the fully protonated (4H) x Si defect, leading to a significant downshift of at least one of the OH-stretching absorption bands. Comparison with experimental observations shows that such defects are unlikely to significantly contribute to the speciation of OH groups in pure synthetic forsterite samples. A partial protonation of Mg vacancies has a weak effect on the spectroscopic properties of OH groups, compared with those of the fully protonated defect, making it difficult to assess their occurrence from spectroscopic observations only.