George R. Rossman

Water in Earth's Mantle: The Role of Nominally Anhydrous Minerals

Most minerals of Earths upper mantle contain small amounts of hydrogen, structurally bound as hydroxyl (OH). The OH concentration in each mineral species is variable, in some cases reflecting the geological environment of mineral formation. Of the major mantle minerals, pyroxenes are the most hydrous, typically containing ∼200 to 500 parts per million H2O by weight, and probably dominate the water budget and hydrogen geochemistry of mantle rocks that do not contain a hydrous phase. Garnets and olivines commonly contain ∼1 to 50 parts per million. Nominally anhydrous minerals constitute a significant reservoir for mantle hydrogen, possibly accommodating all water in the depleted mantle and providing a possible mechanism to recycle water from Earths surface into the deep mantle.

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.

Desert Varnish: The Importance of Clay Minerals

Desert varnish has been characterized by infrared spectroscopy, x-ray diffraction, and electron microscopy. It is a distinct morphological entity having an abrupt boundary with the underlying rock. Clay minerals comprise more than 70 percent of the varnish. Iron and manganese oxides constitute the bulk of the remainder and are dispersed throughout the clay layer.

The natural occurrence of hydroxide in olivine

Polarized infrared (IR) spectra of olivine single crystals from 17 different localities show a tremendous variability in both mode and abundance of hydroxide (OH) incorporation. Kimberlitic olivines contain the most total OH at an estimated concentration level of 976 H/106Si, whereas olivines from basalts contain the least at 3 H/106Si. Olivines of metamorphic and hydrothermal origin have widely varying concentration levels intermediate between those of basalts and kimberlites. Over 30 distinct OH absorption bands have been identified. Most of these bands are not unique to individual localities but may be found in samples from several different localities. Pleochroism is consistent among localities, but relative band intensities vary. No evidence is found for molecular H2 in olivine.Hydrous minerals have been identified in olivine by their characteristic OH absorption bands. Serpentine is commonly found and is clearly distinguishable from intrinsic OH. Talc is present in one sample. Prominent OH bands at 3572 and 3525 cm−1 are attributed to humite group minerals.San Carlos, Arizona, olivines annealed in the presence of H2O develop absorption bands which are found in natural samples, however the OH absorption spectra of these annealed olivines are not identical to those of any single natural crystal. Sharp-band OH abundances in annealed samples are an order of magnitude lower than the maximum measured in natural specimens. The mechanical properties determined from these annealed olivines may not be directly applicable to mantle olivine because both the OH sites and concentrations are different.

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

Hydrogen incorporation in olivine from 2–12 GPa

\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 .

Studies of OH in nominally anhydrous minerals

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.

The manganese- and iron-oxide mineralogy of desert varnish☆

The magnitude and direction of equilibrium iron-isotope (^(54)Fe–^(56)Fe) fractionations among simple iron-bearing complexes and α-Fe metal are calculated using a combination of force-field modeling and existing infrared, Raman, and inelastic neutron scattering measurements of vibrational frequencies. Fractionations of up to several per mil are predicted between complexes in which iron is bonded to different ligands (i.e. 4 per mil for [Fe(H_(2)O)_6]^3+ vs. [FeCl_4]^− at 25°C). Similar fractionations are predicted between the different oxidation states of iron. The heavy iron isotopes will be concentrated in complexes with high-frequency metal-ligand stretching vibrations, which means that ^(56)Fe/^(54)Fe will be higher in complexes with strongly bonding ligands such as CN− and H2O relative to complexes with weakly bonding ligands like Cl^− and Br^−. 56Fe/54Fe will also usually be higher in Fe(III) compounds than in Fe(II)-bearing species; the Fe(II) and Fe(III) hexacyano complexes are exceptions to this rule of thumb. Heavy iron isotopes will be concentrated in sites of 4-fold coordination relative to 6-fold coordination. Model results for a ferrous hexacyanide complex, [Fe(CN)_6]^4−, are in agreement with predictions based on Mossbauer spectra (Polyakov, 1997), suggesting that both approaches give reasonable estimates of iron-isotope partitioning behavior.