ALBERT

Description: We report high-pressure Raman and Brillouin spectroscopy results measured in the (010) plane of a natural antigorite single crystal. We find that structural changes at 〉6 GPa lead to (1) an intensity crossover between Raman modes of the Si-O-Si bending vibrations, (2) changes of the compression behavior of Raman modes related to the SiO 4 tetrahedra, (3) changes of the pressure derivative of the Raman shifts associated with OH stretching vibrations, (4) the emergence of a new Raman band in the OH spectral region, (5) a softening of the elastic constants c 33 and c 11 , and (6) a directional change of the slowest compressional wave velocity in the a-c plane. In addition to the structural insights at high-pressure, the unique characteristics of our single-crystal sample allows for first direct measurements of the acoustic velocity anisotropy in a plane perpendicular to the basal a - b plane. Comparison to previously published data indicates that the elastic anisotropy of antigorite strongly depends on the FeO and/or Al 2 O 3 content. In contrast, it seems not to be affected by increasing temperature as inferred from an additional high-temperature experiment performed in our study. These constraints are important for the interpretation of seismic anisotropy observations in subduction zone environments.

Description: Majoritic garnet, characterized by an excess of silicon (〉3 Si per formula unit), is considered one of the major phases of the Earth’s transition zone from 410–660 km depth. Quantifying the H 2 O content of nominally anhydrous mantle minerals is necessary to evaluate their water storage capacity from experiments and modeling the Earth’s deep water cycle. We present mineral-specific infrared absorption coefficients for the purpose of quantifying the amount of water incorporated into majorite as hydroxyl point defects. A suite of majoritic garnet samples with varying proportions of Si, Fe, Al, Cr, and H 2 O was synthesized at conditions of 18–19 GPa and 1500–1800 °C. Single-crystals were characterized using X-ray diffraction, electron microprobe analysis, secondary ion mass spectrometry (SIMS), IR, Raman, and Mössbauer spectroscopy. We utilize SIMS and Raman spectroscopy in combination with IR spectroscopy to provide IR absorption coefficients for water in majoritic garnets with the general mineral formula (Mg,Fe) 3 (Si,Mg,Fe,Al,Cr) 2 [SiO 4 ] 3 . The IR absorption coefficient for majoritic garnet in the OH stretching region is frequency dependent and ranges from 10 470 ± 3100 Lmol –1 cm –2 to 23 400 ± 2300 Lmol –1 cm –2 .

Description: Olenitic tourmaline with high amounts of tetrahedral B (up to 2.53 [4] B pfu) has been synthesized in a piston-cylinder press at 4.0 GPa, 700 °C, and a run duration of 9 days. Crystals are large enough (up to 30 x 150 μm) to allow for reliable and spatially resolved quantification of B by electron microprobe analysis (EMPA), single-crystal X-ray diffraction, and polarized single-crystal Raman spectroscopy. Tourmalines with radial acicular habit are zoned in [4] B-concentration [core: 2.53(25) [4] B pfu; rim: 1.43(15) [4] B pfu], whereas columnar crystals are chemically homogeneous [1.18(15) [4] B pfu]. An amount of 1.4(1) [4] B pfu was found in the columnar tourmaline by single-crystal structure refinement (SREF) ( R = 1.94%). The EMPA identify [T] Si –1 [V,W] O –1 [T] B 1 [V,W] (OH) 1 as the main and [X] –1 [T] Si –1 [X] Na 1 [T] B 1 as minor exchange vectors for [4] B-incorporation, which is supported by the SREF. Due to the restricted and well-defined variations in chemistry, Raman bands in the OH-stretching region (3000–3800 cm –1 ) are unambiguously assigned to a specific cation arrangement. We found the sum of the relative integrated intensity ( I rel ) of two low-frequency bands at 3284–3301 cm –1 (1) and 3367–3390 cm –1 (2) to positively correlate with the [4] B concentrations: [4] B [pfu] = 0.03(1) x [ I rel (1) + I rel (2)]. Hence, those bands correspond to configurations with mixed Si/B occupancy at the T site. Our semi-quantitative correlation also holds for well-characterized natural [4] B-bearing tourmaline from the Koralpe, Austria. This work shows the potential for Raman spectroscopy as a non-destructive method for the chemical classification of (precious) natural tourmaline, and as a tool to rapidly characterize chemical zonation of tourmalines in thin section.

Description: The 3.65 Å phase [MgSi(OH) 6 ] is likely to be formed by decomposition of the hydrous 10 Å phase [Mg 3 Si 4 O 10 (OH) 2 ·H 2 O] at pressures of 9–10 GPa. In this study, we use a combination of X-ray diffraction and first-principles simulations to constrain the equation of state and elasticity of the 3.65 Å phase. We find that the equation of state results for the 3.65 Å phase, from X-ray diffraction data are well represented by a third-order Birch-Murnaghan formulation, with K 0 = 83.0 (±1.0) GPa, K 0 ' = 4.9 (±0.1), and V 0 = 194.52 (±0.02) Å 3 . Based on the first-principles simulations, the full single-crystal elastic constant tensor with monoclinic symmetry shows significant anisotropy with the compressional c 11 = 156.2 GPa, c 22 = 169.4 GPa, c 33 = 189.3 GPa, the shear components c 44 = 55.9 GPa, c 55 = 58.5 GPa, c 66 = 74.8 GPa, and c 46 = 1.6 GPa; the off-diagonal components c 12 = 38.0 GPa, c 13 = 26.5 GPa, c 23 = 22.9 GPa, c 15 = 1.5 GPa, c 25 = 1.5 GPa, and c 35 = –1.9 GPa at zero pressure. At depths corresponding to 270–330 km, the seismological X-discontinuity has been observed in certain regions. We find that the formation of 3.65 Å from layered hydrous magnesium silicates (LHMS) such as 10 Å angstrom phases occurs at around 9 GPa, i.e., coinciding with the seismic X-discontinuity. The LHMS phases have significant seismic anisotropy. Based on the full elastic constant tensor, although among the dense hydrous magnesium silicate (DHMS) phases, the 3.65 Å phase reveals considerably larger elastic anisotropy, it is significantly smaller than the LHMS phases. This change in seismic anisotropy in hydrous phases might be one of the plausible explanations for the seismic X-discontinuity.

Description: Guyanaite, β-CrOOH, is a structural analogue of the high-pressure oxyhydroxide phases -AlOOH and -FeOOH. Here, it was synthesized in piston-cylinder and multi-anvil press experiments at 4–13.5 GPa and 900–1100 °C. The deuterated phase β-CrOOD was synthesized at 4 GPa and 600 °C. The samples were characterized at ambient conditions by X-ray diffraction, diffuse optical reflectance spectroscopy and infrared absorption (IR) spectroscopy. In addition, the structural evolution of β-CrOOH and β-CrOOD with increasing pressure up to about 20 GPa was studied by in situ IR spectroscopy in a diamond anvil cell (DAC). This investigation was complemented by first-principles calculations in the framework of the density-functional theory (DFT). A pronounced geometric isotope effect and very short O-H. . .O bond lengths of 2.497(3) Å for β-CrOOH and 2.541(3) Å for β-CrOOD are observed at ambient pressure. In the IR spectra, no bands show up above 2000 cm –1 , which indicates strong hydrogen bonding. The evolution of OH- and OD-related vibrational bands with pressure studied by IR spectroscopy shows a discontinuity at about 5 GPa. The DFT calculations suggest that this change in compression mechanism is related to a second-order phase transition from the low-pressure phase with asymmetric hydrogen positions (space group P 2 1 nm or Pnnm ) to a high-pressure phase with space group Pnnm that is characterized by symmetric hydrogen bonds with two identical OH bond lengths of 1.20(1) Å. Using density functional perturbation theory, the most prominent high-frequency modes observed in the IR spectra are assigned to O-H-O bending vibrations. The transition pressure for hydrogen bond symmetrisation in β-CrOOH is considerably lower than in other hydrous phases of recent interest, such as -AlOOH or phase D.

Description: A temperature-dependent in situ micro-FTIR and micro-Raman spectroscopic investigation was performed on powdered (FTIR, Raman) and single-crystal (Raman) lizardite-1 T samples between room temperature and 819 °C. Between room temperature and 665 °C, the OH stretching bands shift to lower wavenumbers, demonstrating a weak expansion of the O3-H3...O2 interlayer distance. Band deconvolution of FTIR and Raman spectra at room temperature show differences in the number of bands in the OH stretching region with respect to group theory: four (FTIR) and six (Raman) OH stretching bands, respectively. This number reduces to four (Raman) at non-ambient temperatures either caused by LO-TO splitting, the presence of non-structural OH species or the presence of different lizardite polytypes and/or serpentine polymorphs as well as defects. During dehydroxylation, the evolution of the integrated intensity of the OH bands suggests a transport of hydrogen and oxygen as individual ions/molecule or OH – . A significant change in Raman spectra occurs between 639 and 665 °C with most lizardite peaks disappearing contemporaneously with the appearance of forsterite-related features and new, non-forsterite bands at 183, 350, and 670 cm –1 . A further band appears at 1000 cm –1 at 690 °C. The long stability of Si-O-related bands indicates a delayed decomposition of the tetrahedral sheet with respect to the dehydroxylation of the octahedral sheet. Moreover, evidence for a small change in the ditrigonal distortion angle during heating is given. In general, all appearing non-forsterite-related frequencies are similar to Raman data of talc and this indicates the presence of a talc-like intermediate.

Description: We report the synthesis of h-magnetite, ideally h-Fe 3 O 4 with considerable amounts of substitutional cations (Cr, Mg, Al, Si) and quenchable to ambient conditions. Two types of experiments were performed at 18 GPa and 1800 °C in a multi-anvil press. In one, we used an oxide mixture with a majoritic stoichiometry Mg 1.8 Fe 1.2 (Al 1.4 Cr 0.2 Si 0.2 Mg 0.2 )Si 3 O 12 , with Si and Mg in excess as starting material (MA-367, MA-380). In the second type of experiment (MA-376), we started from an oxide mixture on the composition of the Fe-oxide phase obtained in MA-367. The Fe-oxide phases of both experiments were investigated by electron microprobe and transmission electron microscopy including electron diffraction tomography. Our investigations show that the Fe-oxide phases crystallize in the structure-type of h-magnetite. However, electron diffraction data show that keeping the cell setting from literature, this phase crystallizes in space group Amam and not in space group Bbmm as previously proposed. In the experiment MA-367, the Fe-oxide phase are mutually intergrown with majorite, the major phase of the run products. The formula for h-magnetite in this run was calculated as Fe1 (Fe 2+ 0.75 Mg 0.26 ) Fe2 (Fe 3+ 0.70 Cr 0.15 Al 0.11 Si 0.04 ) 2 O 4 . In the experiment on the bulk composition of the Fe-oxide, the main phase was h-magnetite with composition Fe1 (Fe 2+ 1.02 ) Fe2 (Fe 3+ 0.65 Cr 0.19 Al 0.13 Si 0.03 ) 2 O 4 and traces of nearly pure end-member wadsleyite and stishovite. Our results indicate that the substitution of 20 to 30% of Fe (0.7 to 0.9 atoms per formula unit) by smaller cations favored the preservation of the high-pressure form to ambient conditions. We prove that the h-magnetite-type oxide is also stable in chemical systems more complex than Fe-O. Based on our results, which were obtained at 18 GPa and 1800 °C in a system (MA-367) that is closely related to Fe-enriched oceanic lithospheric material, we suggest that a Fe 3 O 4 -rich phase may be present in environments connected to deeply subducted slabs and possibly associated with deep carbonatitic melting. Our observations show that Cr strongly partitions in the oxide phase such that the coexisting silicates are depleted in Cr compared to Fe 3 O 4 -free assemblages. This may significantly affect the chemical signature of melts produced in the deep mantle.

Description: Due to the similar ionic radius of K + and NH 4 + , K-silicates can incorporate a significant amount of NH 4 . As tourmaline is able to accommodate K in its crystal structure at high and ultrahigh pressure, we test if this also holds true for NH 4 . Piston-cylinder experiments in the system (NH 4 ) 2 O-MgO-SiO 2 -Al 2 O 3 -B 2 O 3 -H 2 O at 4.0 GPa, 700 °C, with B 2 O 3 and NH 4 OH in excess produce an assemblage of tourmaline, phengite, and coesite. The tourmaline crystals are up to 10 x 40 μm in size. EMP analyses indicate that the tourmalines contain 0.22 (±0.03) wt% (NH 4 ) 2 O and are solid solutions mainly along the magnesio-foitite and "NH 4 -dravite" join with the average structural formula X [(NH 4 ) 0.08(1) 0.92(1) ] Y [Mg 2.28(8) Al 0.72(8) ] Z [Al 5.93(6) Si 0.07(6) ] T [Si 6.00(5) O 18 ](BO 3 ) 3 (OH) 4 . NH 4 incorporation is confirmed by characteristic 〈N-H〉 stretching and bending modes in the IR-spectra of single crystals on synthetic tourmaline. Further evidence is the increased unit-cell parameters of the tourmaline [ a = 15.9214(9) Å, c = 7.1423(5) Å, V = 1567.9(2) Å 3 ] relative to pure magnesio-foitite. Incorporation of NH 4 in natural tourmaline was tested in a tourmaline-bearing mica schists from a high- P /low- T (〉1.2 GPa/550 °C) metasedimentary unit of the Erzgebirge, Germany, rich in NH 4 . The NH 4 -concentrations in the three main NH 4 -bearing phases are: biotite (~1400 ppm) 〉 phengite (~700 ppm) 〉 tourmaline (~500 ppm). This indicates that tourmaline can act as important carrier of nitrogen between the crust and the deep Earth, which has important implications for a better understanding of the large-scale light element cycle.

Description: We performed hydration experiments of pure and Nb-, Cr-, and V-doped synthetic dry (H 2 O 〈 3 ppm) single rutile crystals. They were equilibrated with pure H 2 O in hydrothermal experiments at constant conditions of 600 °C, 400 MPa, and f O 2 near the Ni-NiO buffer, run time between ~25 min and 14 days. Slabs cut parallel to (110) of the reacted single crystals (1 to 2 mm 3 ) were analyzed for H + by FTIR. Hydration occurs almost spontaneously and the H 2 O-equivalent is uniformly distributed in the samples, but depends extremely on trace element contents. In pure rutile, the average H 2 O-content is 314 ± 50 ppm, the saturation level at these conditions. Rutile doped with 500 ppm Nb has a lower average H 2 O content of ~235 ppm, rutile with 2000 ppm Cr has ~900 ppm H 2 O, and rutile with 2000 ppm V does not incorporate H 2 O. During stepwise heating at atmospheric pressure of a reacted Nb-doped rutile, H + is quickly released between 450 and 550 °C. UV-VIS spectra of unreacted colorless and reacted deep blue pure rutile show the rutile-characteristic sharp absorption edge in the UV spectra. The reacted rutile has a broad absorption band at 6500 cm –1 wavenumber attributed to intervalence charge transfer transition Ti 3+ +Ti 4+ -〉 Ti 4+ +Ti 3+ . The reduction of Ti 4+ to Ti 3+ is charge balanced by the incorporation of H + . The Nb-doped rutile changed its color from light greenish-blue (untreated) to deep blue. In the untreated Nb rutile, the UV-VIS absorption band at 6500 cm –1 indicates that Nb 5+ is charge balanced by Ti 3+ . In the reacted Nb-rutile the absorption band is more intense, but compared with the pure rutile, H + incorporation is lower by the equivalent of Ti 3+ reduced in the untreated rutile. Reacted Cr-rutile almost retains its brownish-orange color, but the spectrum shows a prominent Ti 3+ /Ti 4+ IVCT band at ~6400 cm –1 with moderate intensity considering the high-H 2 O contents of ~900 ppm. The high-H + contents are best explained by the reduction of Cr 4+ to Cr 2+ . The UV-VIS spectra of the dark-blue to opaque V-doped rutile show a very strong absorption toward low energies, which is likely caused by reduction of Ti 4+ to Ti 3+ for charge balance of V 5+ . This forms a deep narrow window of transmittance in the range 25 000–20 000 cm –1 , which causes the dark-blue color. To explore the possible use of H-in-rutile as a geohygrometer, geothermobarometer, and oxybarometer, we measured the H + content in a natural rutile crystal from a retrograded eclogite with a zoned trace element (Fe, Nb, and Zr) content. The crystal reveals a slight correlation between the variable H 2 O (~200 to 900 ppm) and its trace element concentrations. The observations indicate that the preservation of H + contents in this natural rutile is a complicated interplay of diffusive reequlibration of fast H + , slower Fe and very slow other trace elements. An interpretation of the H 2 O contents of the natural crystal in terms of f O 2 or a H 2 O is not possible.

Description: Ringwoodite [(Mg,Fe) 2 SiO 4 ] is the high-pressure polymorph of olivine stable in the upper mantle between ~525 to 660 km. Information on its temperature-dependent water content and Fe-oxidation state bears important implications on the hydrogen cycle and oxidation state of the Earth’s interior. We conducted several multi-anvil experiments to synthesize iron-bearing (0.11 ≤ x Fe ≤ 0.24) hydrous ringwoodite under oxidizing and reducing conditions. The experiments were performed at 1200 °C and pressures between 16.5 and 18.3 GPa. The incorporation of hydrogen and iron in ringwoodite was studied using Fourier transform infrared (FTIR), Mössbauer (MB), ultraviolet-visible (UV-VIS), and electron energy loss (EEL) spectroscopy. For MB spectroscopy, ringwoodite enriched in 57 Fe was synthesized. The IR spectra of ringwoodite show a broad OH band around 3150 cm –1 and two shoulders on the high-energy side: one intense at 3680 cm –1 and one weak at around 3420 cm –1 . The water content of the samples was determined using FTIR spectroscopy to have a maximum value of 1.9(3) wt% H 2 O. UV-VIS spectra display a broad band around 12 700 cm –1 and a shoulder at 9900 cm –1 representing the spin-allowed dd-transitions of VI Fe 2+ . The weaker band around 18 200 cm –1 is a distinct feature of Fe 2+ -Fe 3+ intervalence charge transfer indicating the presence of Fe 3+ in the samples. EEL spectra yield Fe 3+ fractions ranging from 6(3)% at reducing conditions to 12(3)% at oxidizing conditions. We performed heating experiments up to 600 °C in combination with in situ FTIR spectroscopy to evaluate the temperature-dependent behavior of ringwoodite, especially with respect to hydrogen incorporation. We observed a color change of ringwoodite from blue to green to brown. The heat-treated samples displayed hydrogen loss, an irreversible rearrangement of part of the hydrogen atoms (FTIR), as well as oxidation of Fe 2+ to Fe 3+ evidenced by the appearance of the spin-forbidden dd-transition band for Fe 3+ and the ligand-metal (O 2– -Fe 3+ ) transition band in the optical spectra. An increased Fe 3+ fraction was also revealed by EEL and MB spectroscopy (up to 16% Fe 3+ /Fe). Analyses of MB data revealed the possibility of tetrahedral Fe 3+ in the annealed ringwoodite. These results lead to a reinterpretation of the broad OH band, which is a combination of several bands, mainly [V Mg (OH) 2 ] x ), a weaker high-energy band at 3680 cm –1 ([V Si (OH) 4 ] x ) and a shoulder at 3420 cm –1 ([(Mg/Fe) Si (OH) 2 ] x ).