Eugen Libowitzky

An IR absorption calibration for water in minerals

Abstract Using IR absorption data from polarized measurements on single-crystal minerals with stoichiometric water contents (in the form of H2O or OH groups in the structure), a linear calibration curve (r2 ≈ 0.98) for water in minerals is established in the form: εi (the integrated molar absorption coefficient in units of cm-2 per mol H2O/L) = 246.6(3753 - ν) (ν = the mean wavenumber of the OH stretching band [in cm-1]). The investigated minerals include hydrogrossular, analcime, hemimorphite and its dehydrated phase, lawsonite, goethite, diaspore, manganite, mozartite, and pectolite. The influence of hydrogen bonding, leading to increased absorption values with lower OH stretching band energies, is confirmed. It is further shown that only the use of integrated absorbance values (band areas) results in a linear correlation with water content, whereas linear absorption data (peak heights) are not correlated. The calibration agrees with previously published quantitative IR data on staurolite and trace H in pyroxenes. It is also close to the frequently used trend of Paterson (1982). However, some of the previous calibrations of trace H in nominally anhydrous minerals, e.g., kyanite and pyrope, differ appreciably from the correlation derived from stoichiometrically hydrous minerals.

Principles of quantitative absorbance measurements in anisotropic crystals

AbstractThe accurate measurement of absorbance (A=-log T; T=I/I0) in anisotropic materials like crystals is highly important for the determination of the concentration and orientation of the oscillator (absorber) under investigation.The absorbance in isotropic material is linearly dependent on the concentration of the absorber and on the thickness of the sample (A=ɛ·c·t). Measurement of absorbance in anisotropic media is more complicated, but it can be obtained from polarized spectra (i) on three random, but orthogonal sections of a crystal, or (ii) preferably on two orthogonal sections oriented parallel to each of two axes of the indicatrix ellipsoid. To compare among different crystal classes (including cubic symmetry) it is useful to convert measured absorbance values to one common basis (the total absorbance Atot), wherein all absorbers are corrected as if they were aligned parallel to the E-vector of the incident light. The total absorption coefficient (atot=Atot/t) is calculated by

FTIR spectroscopy of lawsonite between 82 and 325 K

\left( {\text{i}} \right)a_{{\text{tot}}} = \sum\limits_{i = 1}^3 {(a_{\max ,i} + a_{\min ,i} )} /2, {\text{or}} {\text{by}} {\text{(ii) }}a_{{\text{tot}}} = a_x + a_y + a_z .

Raman spectra of isolated and interconnected pyramidal XS3 groups (X = Sb,Bi) in stibnite, bismuthinite, kermesite, stephanite and bournonite

Only in special circumstances will unpolarized measurements of absorbance provide data useful for quantitative studies of anisotropic material.The orientation of the absorber with respect to the axes of the indicatrix ellipsoid is calculated according to Ax/Atot=cos2 (x < absorber), and analogously for Ayand Az. In this way, correct angles are obtained for all cases of symmetry.The extinction ratio of the polarizer (Pe=Icrossed/Iparallel) has considerable influence on the measured amplitude of absorption bands, especially in cases of strong anisotropic absorbance. However, if Pe is known, the true absorbance values can be calculated even with polarizers of low extinction ratio, according to Amax=−log[(Tmax,obs−0.5·Pe·Tmin,obs)/(1−0.5·Pe)], and similar for Amin.The theoretical approach is confirmed by measurements on calcite and topaz.

Cathodoluminescence, electron microscopy, and Raman spectroscopy of experimentally shock-metamorphosed zircon

Polarized FTIR spectra of near endmember forsterite single crystals from Pamir, Tadzikistan show the existence of sharp strongly pleochroic absorption bands in the region of the OH stretching fundamental. Bands centered at 3674/3624, 3647/3598 and 3640/ 3592 cm-1 are attributed to OH dipoles oriented parallel to [100]. An OH band doublet at 3570/3535 cm-1 shows both, a strong absorption parallel to [100] and a strong component parallel to [001]. On the basis of the pleochroic scheme and under the assumption of vacancies on Si- and M-sites it is proposed that O1 is partially replaced by OH defects pointing to the vacant Si-site. O3 is donator oxygen of OH dipoles lying near the O3-O1 tetrahedral edge or roughly pointing to a vacant M2-site. Also O2 can act as donator oxygen of an OH group oriented along the O2-O3 edge of a vacant M1 octahedron. The splitting of the bands is explained by the presence of Fe2+ in cation sites surrounding the OH defects.

The effect of As-Sb substitution in the Raman spectra of tetrahedrite-tennantite and pyrargyrite-proustite solid solutions

Abstract Lawsonite single crystals were investigated by polarized FTIR spectroscopy at wave-numbers between 8000 and 1000 cm-1 and temperatures between 82 and 325 K. This temperature range contains three lawsonite phases-Cmcm > 273 K, 273 K > Pmcn > 150 K, P21cn < 150 K-which are characterized by different rotations of hydroxyl groups and H2O molecules. Unlike previous studies of H2O in minerals, which assumed weakly bonded, symmetric H2O molecules, the highly asymmetric H2O molecule in lawsonite required a modified approach that uses the single, uncoupled O-H stretching frequencies and orientations of the individual OH groups in the H2O molecule. The formation of a strong hydrogen-bond system with decreasing temperature is characterized by a shift of O-H stretching bands from 2968 and 3252 cm-1 at 325 K to 2817 and 3175 cm-1 at 82 K. These frequencies are in good agreement with the corresponding hydrogen-bond lengths (H···O = 1.66 and 1.74 Å, O-H···O = 2.60 and 2.66 A) at low temperatures. The orientations of the O-H vectors determined from polarized IR measurements also confirm the H-atom positions refined from previous X-ray structure determinations at low temperatures. However, the disagreement between spectroscopically determined distances (and orientations) and those from X-ray refinements at ambient conditions indicates that the room-temperature Cmcm structure of lawsonite contains dynamically disordered hydroxyl groups and H2O molecules. The smooth changes of stretching and bending frequencies across the phase boundaries at 273 and 150 K also suggest that the lawsonite phase transitions are of a dynamic order-disorder type rather than a displacive type. Deuteration experiments on differently oriented, single-crystal lawsonite slabs at 350°C and 1.2-2.5 kbar showed that lawsonite has a preferred H-diffusion direction parallel to [001]. This is consistent with the crystal structure showing channels parallel to [001], which are solely occupied by H atoms. The spectra of isotopically diluted samples, which are almost identical to those of natural lawsonite, indicate that band-coupling effects are generally weak. The FTIR powder spectra of the lawsonite-type mineral hennomartinite, SrMn2[Si2O7](OH)2·H2O, are similar to the lawsonite Z spectra and confirm the existence of both strong and weak hydrogen bonds in its structure.

The hydrogen bond system in natrochalcite-type compounds — an FTIR spectroscopic study of the H3O2 unit

Many known occurrences of the zeolites erionite and offretite have been characterized by electron probe microanalysis, X-ray powder diffraction, and optical microscopy. For the first time, a substantial amount of experimentally consistent and homogeneous chemical and crystallographic data have been evaluated for these natural zeolites. Systematic analysis of the data, performed by statistical multivariate analysis, leads to the following conclusions: (1) the two zeolites have well-defined compositional fields in the chemical space describing the extraframework cation content, best illustrated in a Mg-Ca(1Na)K(1Sr1Ba) diagram; (2) no discrimination is possible on the basis of the framework Si/Al ratio because of the extensive compositional overlap between the two species, however the Si-Al content in the framework tetrahedra is the major control on the unit-cell volume dimensions, particularly in erionite; (3) the crystal chemistry of the Mg cations is a major factor in controlling the crystallization of the mineral species; (4) cation compositions at the boundary of the recognized compositional fields might be due to chemical averaging of two-phase intergrowths, although these mixed-phase occurrences are much less common than previously thought; (5) the sign of optical elongation is not a distinctive character of the two phases, it is related to the Si/Al ratio in the framework tetrahedra of each zeolite type and cannot be used for identification purposes; (6) the zeolite mineral species epitaxially overgrown on levyne in all cases is identified as erionite; in a few cases offretite was found to be overgrown on chabazite; (7) erionite samples epitaxially overgrown on levyne are substantially more Al-rich and Mg-poor than the erionite samples associated with other zeolites.

Crystal chemistry and optics of bazzite from Furkabasistunnel (Switzerland)

Oriented single-crystals of stibnite, bismuthinite, kermesite, stephanite and bournonite were investigated by polarized Raman spectroscopy. The obtained spectra were compared to those of the sulfosalt minerals tetrahedrite and pyrargyrite. Whereas the latter show isolated SbS 3 groups with ideal trigonal symmetry, the former show distorted XS 3 (X = Sb,Bi) groups with lower symmetry. Moreover, in stibnite and isostructural bismuthinite the pyramidal groups are interconnected to infinite ribbons, and even to sheets in kermesite. The internal vibrations, i.e. the stretching and bending modes, of the SbS 3 groups occur at ~340–180 cm −1 , those of the BiS 3 groups in bismuthinite at ~280–150 cm −1 . The higher mass and the longer bond distances of the BiS 3 groups readily explain the lower wavenumbers of the latter. However, even for the SbS 3 groups a negative correlation between bond distances and vibrational wavenumbers is observed. Moreover, in those minerals with a wide range of pyramidal bond distances, i.e. stibnite, kermesite, bismuthinite, a more extended range of vibrational modes is observed. A clear distinction between Raman spectra of separated and interconnected SbS 3 groups is not observed.