Henrik Skogby

Hydrogen in nominally anhydrous upper-mantle minerals concentration levels and implications

Several of the supposedly anhydrous major minerals of the upper mantle have been shown to regularly contain small amounts of hydrogen. The concentrations measured in the most important minerals obtained from mantle xenoliths are, expressed in ppm H 2 O, 100-1300 for clinopyroxene, 60-650 for orthopyroxene, 0- 140 for olivine and 1-200 for garnet. Hydrogen is normally structurally incorporated as hydroxyl ions, and in many cases the hydrogen ions seem to act as charge compensators associated with point defects, such as metal vacancies or substitution by mono- or trivalent cations. The determination of the exact amount of hydrogen stored in these nominally anhydrous upper mantle minerals is a key-step toward quantification of the water content of the mantle, as well as understanding of its internal water cycle. For instance, a concentration of 100 ppm H 2 O homogeneously distributed within the upper mantle above 410 km depth is approximately equivalent to a 100 m water layer at the Earth9s surface. However, the relatively fast kinetics of dehydrogenation with concomitant oxidation of iron within these minerals, implies that hydrogen as well as Fe 3+ concentrations in equilibrium with mantle conditions might be different from those measured from recovered xenolith samples. High-pressure experimental measurements of hydrogen solubility as a function of PH 2 O show a trend similar to the hydrogen contents of natural samples, with hydrogen saturation levels that decrease following the mineral series: diopside > enstatite > olivine > pyrope. Except pyrope, these minerals may incorporate more than 1000 ppm H 2 O. Based on recent data of water solubility, stability and partitioning, we suggest that an entire upper mantle saturated in hydrogen is highly unprobable and that the maximum average amount of hydrogen stored in the nominally anhydrous minerals of the upper mantle is around 600 ppm H 2 O. Despite the important progress achieved during the last years, our knowledge of the concentration of hydrogen stored as point defects in the mantle above 410 km is still too poorly constrained. The importance of nominally anhydrous minerals for the water budget of the upper mantle is now well established but still awaits complete quantification.

Cation ordering and structural variations with temperature in MgAl2O4 spinel: An X-ray single-crystal study

Abstract The equilibrium distribution of Mg and Al between the tetrahedral and octahedral sites of a flux grown, stoichiometric MgAl2O4 spinel was investigated between 600 and 1100 °C by single-crystal X-ray diffraction of quenched samples. The cation distribution for both ordering and disordering runs was obtained by minimizing accurate crystallographic parameters and effective ionic radii. Along with the variation of the degree of inversion from 0.18 to 0.29 between 600 and 1100 °C, both unit cell and oxygen positional parameter decreased linearly. Multiple non-linear least-squares fit of our data with the thermodynamic model of O’Neill and Navrotsky (1983) gave α = 23 ± 2 KJ/mol and β = 13 ± 4 KJ/mol. The influence of both cation inversion and thermal expansion on T-O and MO bond length variation was determined by comparison of our data with previous in situ studies. In the thermal interval investigated, the inversion accounts for change of -0.014 Å for T-O and +0.007 Å for M-O. Mean linear polyhedral thermal expansion coefficients of 6.5 × 10-6 °C-1 and 8.9 × 10-6 °C-1 were calculated for T and M sites, respectively.

The intensity of amphibole OH bands in the infrared absorption spectrum

Polarized IR spectra of single-crystals of amphiboles show that the molar absorptivity of the fundamental vibrational OH band is strongly wavenumber dependent. The intensity increases by a factor of 1.6 from 3674 cm-1 (tremolite endmember; MgMgMg) to 3618 cm-1 (grunerite endmember; FeFeFe). Spectra obtained from Ca and Fe-Mg amphiboles consist of sharp, well-resolved bands on a well-defined background. The high intensity of the OH bands in Ca and Fe-Mg amphiboles makes it sometimes necessary to thin the samples to under 2 μm thickness, whereas alkali amphiboles can be measured at 25–50 μm thickness.

Crystal chemistry of the magnetite-ulvöspinel series

Abstract Spinel single crystals of 19 compositions along the magnetite-ulvöspinel join were synthesized by use of a flux-growth method. To obtain quantitative site populations, the crystals were analyzed by single-crystal X-ray diffraction, electron-microprobe techniques, and Mössbauer spectroscopy. All results were processed by using an optimization model. The unit-cell parameter, oxygen fractional coordinate, and tetrahedral bond length increase with increasing ulvöspinel component, whereas the octahedral bond length decreases marginally. These changes result in sigmoidal crystal-chemical relationships consistent with cation substitutions in fully occupied sites. As a first approximation, the Akimoto model T(Fe3+1-XFe2+x)M(Fe2+Fe3+1-XTiX)O4 describes the cation substitutions. Deviations from this model can be explained by an electron exchange reaction TFe2+ + MFe3+ = TFe3+ + MFe2+, which causes MFe2+ ≠ 1 and TFe2+/Ti ≠ 1. The resultant S-shaped trends may be related to a directional change in the electron exchange reaction at Ti ≈ 0.7 apfu. In general, variations in structural parameters over the whole compositional range can be split into two contributions: (1) a linear variation due to the TFe3+ + MFe3+ = TFe2+ + MTi4+ chemical substitution and (2) non-linear variations caused by the internal electron exchange reaction. In accordance with bond-valence theory, strained bonds ascribable to steric effects characterize the structure of magnetite-ulvöspinel crystals. To relax the bonds and thereby minimize the internal strain under retained spinel space group symmetry, the electron exchange reaction occurs.

Hydrogen incorporation in enstatite

A series of synthetic enstatites containing up to 1000 ppm (wt) Al were analysed with FTIR-spectroscopy. Pure enstatite exhibits only two essential O-H absorption bands at 3070 and 3362 cm −1 . As Al is incorporated in the crystal the absorption band at 3362 cm-1 is continuously split into two other peaks. In addition several new bands above 3400 cm −1 progressively evolve with Al incorporation. The intensities of these peaks correlate well with the Al-content of the sample. The observations are consistent with a model, where hydrogen in pure, Al-free enstatite is linked to the oxygen atoms O(3A) and O(3B), which are located at the SiO 4 -tetrahedron faces pointing towards the cavity between two M2-positions along the crystallographic b-axis. In Al-bearing samples additional water is mainly incorporated between the \(oxygen\ atoms\ O(2A)\ {\mbox{--}}\ O(1\ A)\ and\ O(2B)\ {\mbox{--}}\ O(1B)\) .

Structural relaxation around Cr3+ and the red-green color change in the spinel (sensu stricto)-magnesiochromite (MgAl2O4-MgCr2O4) and gahnite-zincochromite (ZnAl2O4-ZnCr2O4) solid-solution series

Abstract Optical absorption spectra of flux-grown single crystals in the spinel s.s.-magnesiochromite and gahnite-zincochromite solid solutions were recorded with the aim of exploring variations in local Cr-O bond distance as a function of composition. With increasing Cr contents, the crystals vary in color from pale red to intensely red to dark greenish. These variations are reflected in the optical spectra by the position and intensity of the two spin-allowed electronic d-d transitions in six-coordinated Cr3+ at ~18 000 (ν1) and 25 000 cm-1 (ν2). From the shift of the ν1 band position, a decrease in crystal field splitting, 10Dq, for six-coordinated Cr3+ with increasing Cr contents is evident in both solid-solution series. Based on published Cr-O bond distances for the CrO6 polyhedra in magnesiochromite and zincochromite of 1.995 and 1.991 Å, respectively, and applying the ligand field relationships, local Cr-O bond distances in gahnite and spinel with Cr contents at trace levels are determined to be 1.974(2) and 1.960(3) Å, respectively. These local Cr-O distances result in relaxation parameters (ε) equal to 0.69(2) and 0.60(3) for Cr-O bonds in the Mg(Al1-xCrx)2O4 and Zn(Al1-xCrx)2O4 series, respectively. However, the presently obtained Racah B values indicate increasing Cr-O bond covalency with increasing Cr3+ contents. This suggests that color changes and accompanying 10Dq variations may be due to variations in Cr-O bond covalency along the two solid-solution series, without or with very minor local Cr-O bond distance variation. Consequently, the ε values obtained from the present optical absorption spectra should be regarded as minimum values.

Short-range order in tourmaline: a vibrational spectroscopic approach to elbaite

Polarized Fourier-transform infrared and Raman spectra were acquired on an elbaite sample previously characterized by electron- and ion microprobe analysis, X-ray diffraction and structure refinement. Spectra from the two vibrational spectroscopy techniques reveal a close similarity in the OH-stretching region, with three main absorption bands strongly polarized in the c-axis direction. By means of bond-valence theory arguments, the observed OH bands are interpreted and assigned to specific local cation arrangements around the O1 (≡W) and O3 (≡V) anion sites. In combination with the relatively simple composition of the studied sample, bond-valence constraints are used to identify stable anion-cation arrangements, which permit the occurrence of short-range ordering to be assessed. Evidence for nearly complete short-range order at the O1 site, with the stable arrangements Y(LiAlAl)0.6–W(OH)0.6 and Y(LiLiAl)0.4–W(F)0.4, are presented. These two local arrangements can be further expanded to obtain the larger ordered clusters [W(OH)–Y(LiAl2)–V(OH)3–Z(Al)6]0.6 and [W(F)–Y(Li2Al)–V(OH)3–Z(Al)6]0.4.

Compositional dependence of cation distribution in some synthetic (Mg,Zn)(Al,Fe3+)2O4 spinels

The compositional dependence of the intracrystalline cation distribution was investigated in four synthetic spinels belonging to binary solid solutions. Spinel single crystals were flux-grown in the (Mg,Zn)(Al,Fe 3+ ) 2 O 4 system, and cation distribution was determined by means of single-crystal X-ray structural refinement, electron-microprobe analysis and Mossbauer spectroscopy. Experimental data were processed and a detailed topochemical model was obtained for each sample: IV (Mg 0.76 Al 0.24 ) VI (Al 1.76 Mg 0.24 )O 4 ; IV (Zn 0.65 Mg 0.22 Al 0.13 ) VI (Al 1.87 Mg 0.13 )O 4 ; IV (Mg 0.73 Al 0.18 Fe 3+ 0.09 ) VI (Al 1.62 Mg 0.27 Fe 3+ 0.11 )O 4 and IV (Mg 0.62 Zn 0.15 Al 0.15 Fe 3+ 0.08 ) VI (Al 1.61 Mg 0.23 Fe 3+ 0.16 )O 4 . Zinc was found to occupy only the tetrahedrally coordinated site and its presence strongly influenced intersite cation partitioning. In Zn-bearing crystals both intracrystalline exchanges Mg ⟷ Al and Mg ⟷ Fe 3+ turned out to be limited. Zinc in octahedral coordination, which is rare in natural spinels, was shown to be favoured by high Fe 3+ contents and high equilibration temperatures. The cation distributions determined experimentally were found to be in close agreement with those calculated using the general thermodynamic model for spinel solid solutions by O9Neill & Navrotsky (1984).

Tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3OH, a new mineral species of the tourmaline supergroup from Grotta d’Oggi, San Pietro in Campo, island of Elba, Italy

Abstract Tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3OH, is a long-expected new mineral of the tourmaline supergroup. It occurs in an aplitic dike of a LCT-type pegmatite body from Grotta d’Oggi, San Pietro in Campo, island of Elba, Italy, in association with quartz, K-feldspar, plagioclase, elbaite, and schorl. Crystals are greenish yellow with a vitreous luster, a white streak, and show no fluorescence. Tsilaisite has a Mohs hardness of approximately 7; it is brittle with a sub-conchoidal fracture, and has a calculated density of 3.133 g/cm3. In plane-polarized light, tsilaisite is pleochroic, O = pale greenish yellow, E =very ale greenish yellow; it is uniaxial negative, ω = 1.645(5), Ɛ = 1.625(5). Tsilaisite is rhombohedral, space group R3m, a = 15.9461(5), c = 7.1380(3) Å, V = 1571.9(1) Å3, Z = 3. The strongest eight ‑raydiffraction lines in the powder pattern [d in Å(I)(hkl)] are: 3.974(100)(220), 2.942(94)(122), 2.570(79) (051), 2.034(49)(152), 4.205(41)(211), 6.329(22)(101), 2.377(21)(003), and 1.592(21)(550). Analysis by a combination of electron microprobe, secondary ion mass spectrometry, and optical absorption spectroscopy gives SiO2 = 36.10(3), TiO2 = 0.32(4), Al2O3 = 37.10(5), MnO = 9.60(10), CaO = 0.09(4), Na2O = 2.11(7), K2O = 0.03(1), F = 0.79(3), B2O3 = 10.2(6), Li2O = 0.8(1), H2O = 3.1(2), sum 99.95 wt%. The unit formula is X(Na0.67⃞0.30Ca0.02K0.01)Y(Mn2+1.34Al1.14Li0.54Ti0.04)ZAl6T(Si5.94Al0.06)B2.91O27V(OH)3W(OH0.39F0.41O0.20). The structure, refined also taking into account the positional disorder of the O1 and O2 anions, converged to statistical indices R1 for all reflections of about 2%. The resulting site populations indicate that the Z site is occupied by Al and that the Y site is dominated by Mn2+. Aluminum is incorporated at Y through two types of substitutions: YAl+WO2- → YMn2++WOH, which has the result of replacing OH at the W site by O2-, and Y(Al+Li)+WF → 2YMn2++WOH, which relates fluor-elbaite to the tsilaisite component. Infrared absorption spectra measured in the principal OH-stretching region were interpreted on the basis of local arrangements consistent with the short-range bond-valence requirements. A compositional trend from fluor-elbaite to tsilaisite is observed in samples from Elba Island. The occurrence of tsilaisite is very rare in nature, as a consequence of both the requirement of extraordinary petrogenetic conditions and limited structural stability.

Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Abstract Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with quartz, Cr-V-bearing tremolite and muscovite-celadonitechromphyllite- roscoelite, diopside-kosmochlor-natalyite, Cr-bearing goldmanite, escolaite-karelianite, dravite-oxy-vanadium-dravite, V-bearing titanite and rutile, ilmenite, oxyvanite-berdesinskiite, shreyerite, plagioclase, scapolite, zircon, pyrite, and an unnamed oxide of V, Cr, Ti, U, and Nb. Crystals are emerald green, transparent with a vitreous luster, pale green streak, and conchoidal fracture. Vanadio-oxy-chromium-dravite has a Mohs hardness of approximately 7½, and a calculated density of 3.3 g/cm3. In plane-polarized light, vanadio-oxy-chromium-dravite is pleochroic (O = dark green, E = pale green) and uniaxial negative: ω = 1.767(5), ε = 1.710(5). Vanadio-oxy-chromium-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1260(2), c = 7.3759(1) Å, V = 1661.11(4) Å3, Z = 3. Crystal chemistry analysis resulted in the empirical structural formula: X(Na0.89K0.06□0.05) Y(V3+2.77Mg0.17Fe3+0.06) Z(Cr3+ 1.85Al1.59V3+ 0.78Mg1.78) T[(Si5.95Al0.05)O18] B(BO3)3V(OH2.91O0.09) W(O0.86F0.14). The crystal structure of vanadio-oxy-chromium-dravite was refined to a statistical index R1 of 1.16% using 2543 unique reflections collected with MoKa X-radiation. Ideally, vanadio-oxy-chromiumdravite is related to oxy-chromium-dravite and oxy-vanadium-dravite by the homovalent substitution V3+ ↔ Cr3+. Tourmaline with chemical compositions classified as vanadio-oxy-chromium-dravite can be either Cr3+-dominant or V3+-dominant as a result of the compositional boundaries along the solid solution between Cr3+ and V3+ that are determined at Y+Z(V5Cr2), corresponding to NaY(V3)Z(V2Cr2Mg2) Si6O18(BO3)3(OH)3O, and Y+Z(V1.5Cr5.5), corresponding to NaY(V1.5Cr1.5)Z(Cr4Mg2)Si6O18(BO3)3(OH)3O.