Showing papers in "American Mineralogist in 2000"

The absolute energy positions of conduction and valence bands of selected semiconducting minerals

Abstract: The absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge energies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H2O oxidation potential (relative to absolute vacuum scale), energies for conduction band edges are close to, or lower than, the H2O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sulfides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H2O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.

Measurement of crystal size distributions

Abstract: Studies of crystal size distributions (CSD) can reveal much about how rocks solidify and under what conditions. Data from two-dimensional sections can be readily acquired at many different scales, from electron microscope images, thin sections, slabs, outcrops, and so on, but the conversion to true, three-dimensional values is complex. The widely used Wager method does not have a good theoretical basis and does not give accurate results. A modification of the Saltikov correction method is proposed here that is more accurate and can account for different crystal shapes and fabrics. Population densities determined by this method differ by factors of 0.02 to 100 from those determined by the Wager method. Published CSDs determined using other methods can be recalculated if the crystal shape and fabric parameters can be estimated. The method has been incorporated into a new program, CSDCorrections.

Application of new experimental and garnet Margules data to the garnet-biotite geothermometer

Abstract: Garnet-biotite experiments now exist from Ferry and Spear (1978, FS), Perchuk and Lavrent’eva (1983, PL), and Gessmann et al. (1997, GE). Recent garnet Margules parameters exist from Berman and Aranovich (1996, BA), Ganguly et al. (1996, GA), and Mukhopadhyay et al. (1997, MU). A comparison of ΔGmix plots for garnet binaries shows similarity among models with some significant differences. In all, 20 garnet-biotite geothermometers were retrieved from the various data using a modified form of the approach of Holdaway et al. (1997, HO) such that experimental vs. calculated values of T , in °C, were regressed stepwise to a line with intercept of zero and slope of one, maximizing r2. All tests assumed the GA model for garnet MnMg and the HO Fe3+ correction and Δ W BioTi . The experimental combinations (sets) were: (1) GE + FS with GE values of VIAlBio in FS; (2) PL + FS with GE VIAlBio in FS; (3) PL + FS with GE VIAlBio in FS, increasing W GrMnMg by 5 kJ; (4) PL + FS with no VIAlBio in FS, increasing W GrMnMg by 5 kJ; (5) PL + FS with 0.10 VIAlBio in FS, increasing W GrMnMg by 5 kJ; (6) PL + FS + GE with 0.10 VIAlBio in FS, increasing W GrMnMg by 5 kJ. Models are designated by a number for the experimental set and letters to designate the above garnet model, e.g., 5BA. Experimental sets vary in maximum r2 as follows: (1) 0.892–0.896; (2) 0.943–0.956; (3) 0.943–0.955; (4) 0.980–0.982; (5) 0.980–0.983; and (6) 0.939–0.944. Application to the HO Maine biotite-garnet data gives the following sequence of increasing quality from experimental sets: 1 < 2 < 3 < 6 < 4 ≅ 5. The various garnet models show only small differences in r2 and in quality with the Maine data base. For all 20 models, average T (°C) for the Maine M3 staurolite zone varies between 554(15) and 588(13). The best models appear to be 5BA, 5GA, and 5MU, which allow for 0.10 VIAl in FS biotite. A model based on an average of the three garnet Margules models, 5AV, has some justification and provides a slight improvement. Model 5AV gives (in J) Δ G ex = 40198 – 7.80 T; W BioFeMg = 22998 – 17.40 T ; Δ W Al Bio = 245559 – 280.31 T , and Maine M3 average staurolite zone T = 571(12) °C. The high quality of sets based mainly on PL experiments (2–5 above) results in part from their experimental P of 6 kbar. The results suggest that FS biotite contained only minor VIAl. The wide ranges of derived exchange and biotite Margules parameters, which provide reasonable calibrations, indicate that while accuracy in parameters is important, consistency is even more important. These geothermometers (5BA, 5GA, 5MU, 5AV) are available on PC disk. I recommend geothermometer 5AV.

In-situ Raman spectroscopy of quartz: A pressure sensor for hydrothermal diamond-anvil cell experiments at elevated temperatures

Abstract: Changes in frequency and linewidth of the 206 and 464 cm−1 A1 Raman modes of quartz were determined over temperatures from 23 to 800 °C and simultaneously at pressures ranging between 0.1 MPa and 2.1 GPa using a hydrothermal diamond-anvil cell (HDAC). The frequency shift of the 464 cm−1 peak can be used as a secondary pressure standard for SiO2-saturated systems in HDAC experiments at temperatures up to 560 °C. The frequency of this peak depends quasilinearly on pressure in the studied pressure range. The global slope (∂ν464/∂ P ) T is 9 ± 0.5 cm−1/GPa. A significant variation of this slope with temperature was not observed. Including literature data, the temperature induced frequency shift of the 464 cm−1 mode is described by (ΔνT)464, P = 0.1 MPa (cm−1) = 2.50136·10−11 · T 4 + 1.46454·10−8· T 3 − 1.801·10−5· T 2 − 0.01216· T + 0.29 where −196 ≤ T (°C) ≤ 560. The pressure dependence of the linewidth of the 464 cm−1 line increases with temperature. The frequency shifts and linewidths for the 206 cm−1 mode indicate that this line can be used as an alternative to the ruby fluorescence technique as a pressure sensor to about 5 GPa for experiments at room temperature. Both the frequency and linewidth of this mode show significant cross-derivatives (∂2 v 206/∂ P ∂ T ) and (∂2 FWHM 206/∂ P ∂ T ).

Raman spectroscopic characteristics of Mg-Fe-Ca pyroxenes

Abstract: Raman spectra of several compositions of (Mg, Fe, Ca)SiO3 pyroxenes were collected at ambient conditions. More than 10 Raman vibrational modes were observed for these pyroxenes in the wavenumber range between 200 and 1200 cm−1. In general, these pyroxenes are characterized by (1) the Si-O stretching modes above 800 cm−1; (2) the Si-O bending modes between 500 and 760 cm−1; (3) SiO4 rotation and metal-oxygen translation modes below 500 cm−1. For a constant Ca content, frequencies of the Raman modes in the enstatite-ferrosilite (opx) and diopside-hedenbergite (cpx) series generally decrease with an increase in Fe content. This phenomenon is attributed to an increase in both the bonding lengths and the reduced mass as Fe2+ is substituted for Mg. However, two modes at ~900 cm−1 in the enstatite-ferrosilite series increase in frequencies as Fe content increases. A possible explanation is to the shortening in the Si-O-Si bridging bonding bonds when the M2 sites are preferentially occupied by the iron cation. The effect of Fe substituting for Mg on the frequency shift in cpx is less profound than opx because the larger M2 was occupied by calcium and the substitution of iron and magnesium in the M1 site results in a less significant change in the bond length. The major-element composition of the (Mg, Fe, Ca)-pyroxenes, especially the orthopyroxene series, can be semi-quantitatively determined on the basis of the peak positions of their characteristic Raman modes.

Monazite geochronology of UHP and HP metamorphism, deformation, and exhumation, Nordøyane, Western Gneiss Region, Norway

Abstract: U-Th-Pb monazite geochronology is combined with previous structural analysis and quantitative estimates of metamorphic conditions to date the thermotectonic evolution of UHP and HP plates (820 °C, 39 kbar; 780 °C, 18 kbar) metamorphosed during the Late Silurian-Early Devonian collision between Baltica and Laurentia. The Upper Plate contains a microdiamond-bearing, kyanite-garnet-graphite gneiss and associated kyanite eclogites, independently indicating probable diamond-forming conditions. In situ dating of monazite in the microdiamond sample, using the SHRIMP II at the Geological Survey of Canada, yielded ages of 415 ± 6.8 Ma for those included in garnet and 398 ± 6 Ma for those in the matrix. These ages compare to 408.0 ± 5.6 and 397.5 ± 4.4 Ma determined using the electron microprobe at the University of Massachusetts. Both methods also identified complexly zoned detrital cores up to 150 micrometers in diameter with ages of 1100–950 Ma and scattered grains with ages of 900–500 Ma, but no ages of 1680–1650 Ma equivalent to the local Baltica basement were found. Agreement between the two techniques allowed evaluation of monazite age domains (198 analyses) from the microdiamond rock and a kyanite-garnet-sillimanite mylonite produced from it, using a combination of high-resolution element imaging and trace-element analysis of U, Th, Pb, and Y. This comparison yielded three mean ages of 407.0 ± 2.1 Ma, 394.8 ± 2.3 Ma, and 374.6 ± 2.7 Ma. Combining this geochronology with previous P - T estimates, we propose that the UHP unit reached its maximum depth of 125 km, at a maximum age of 407 Ma when monazite was included in garnet, and experienced 65 km of exhumation at an average rate of 10.9 mm/year during top-southeast thrusting that brought it into contact with the HP unit. Following these events, both units were exhumed together at an average rate of 3.8 mm/year until reaching a depth of 37 km at 395 Ma, where these rocks experienced extensive re-equilibration, and top-west and left-lateral shearing. After 395 Ma, these units continued to be exhumed at an average rate of 0.8 to 1.4 mm/year until 375 Ma, the time of last equilibration of asymmetric monazite porphyroclasts in the mylonite. The exhumation histories of these units record a change in mechanism from syncollisional exhumation through late- to post-orogenic collapse that was a consequence of plate reorganization.

Equation of state of magnetite and its high-pressure modification: Thermodynamics of the Fe-O system at high pressure

Abstract: Fe 3 O 4 has been studied by high-pressure diffraction to 43 GPa. No major changes in the spinel-type structure of magnetite is observed below 21.8 GPa. At higher pressure a sluggish transition to a high-pressure modification, h-Fe 3 O 4 , is observed. The X-ray diffraction pattern of the high-pressure modification is consistent with the orthorhombic unit cell (CaMn 2 O 4 -type structure, space group Pbcm ) recently proposed for h-Fe 3 O 4 by Fei et al. (1999), however, it is also consistent with a more symmetric CaTi 2 O 4 -type structure (space group Bbmm ). Bulk modulus values for magnetite, K T0 = 217 (2) GPa, and h-Fe 3 O 4 , K T0 = 202 (7) GPa, are calculated from the pressure-volume data using a third-order Birch-Murnaghan equation of state. A thermodynamic analysis of the Fe-O system at high pressure is presented. The proposed equation of state of h-Fe 3 O 4 gives an increased stability of wustite relatively to a two-phase mixture of iron and h-Fe 3 O 4 compared to earlier equations of state and removes an inconsistency in the thermodynamic description of the Fe-O system at high pressure.

Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH.

Abstract: The structural transformation of high pH Na-rich buserite (NaBu) to H-exchanged hexagonal birnessite (HBi) at low pH was studied by simulation of experimental X-ray diffraction patterns. Four HBi samples were prepared by equilibration of NaBu at constant pH in the range pH 5-2. The samples differ from each other by the presence of one (at pH 2 and 3) or two (at pH 4 and 5) phases, and by the structural heterogeneity of these phases which decreases with decreasing pH. The sample obtained at pH 5 is a 4:1 physical mixture of a 1H phase ( a = 4.940 A, b = a /√3 = 2.852 A, c = 7.235 A, β = 90°, γ = 90°) and of a 1M phase ( a = 4.940 A, b = a /√3 = 2.852 A, c = 7.235 A, β = 119.2°, γ = 90°) in which successive layers are shifted with respect to each other by + a /3 along the a axis as in chalcophanite. Both the 1H and 1M phases contain very few well-defined stacking faults at pH 5. At pH 4, the sample is a 8:5 physical mixture of a 1H phase containing 15% of monoclinic layer pairs and of a 1M phase containing 40% of orthogonal layer pairs. Any further decrease of the pH leads to the formation of a single defective 1H phase. This 1H phase contains 20% and 5% of monoclinic layer pairs at pH 3 and 2, respectively. Independent of pH, all phases contain 0.833 Mnlayer cations, 0.167 vacant layer sites, and 0.167 interlayer Mn cations located either above or below layer vacancies per octahedron. A structural formula is established at each pH. The origin of the observed phase and structural heterogeneities has been analyzed. 1H and 1M phases are assumed to inherit their specific structural and crystal chemical features from the two distinct NaBu modifications. NaBu type I, with a high proportion of Mn4+layer cations, is thought to be responsible for the monoclinic layer stacking because this configuration allows Mn cations from adjacent layers to be as far as possible from each other, thus minimizing the electrostatic repulsion between these high charge cations. In contrast, NaBu type II has a high interlayer charge induced by Mn3+layer for Mn4+layer substitutions. Consequently, the 1H phase has a high amount of interlayer protons and achieves compensation of the unfavorable overlap of layer and interlayer Mn cations, in projection on the ab -plane, by the presence of strong hydrogen bondings between layers. The higher proportion of defined stacking faults in both 1H and 1M phases at pH 4 compared to pH 5 can be attributed to the increase in reaction rate with decreasing pH. At lower pH (3 and 2) the formation of strong hydrogen bonds between adjacent layers controls the layer stacking mode and leads to the formation of a unique 1H phase. The proportion of well-defined stacking faults in this phase decreases from pH 3 to 2.

Surface area and porosity of primary silicate minerals

Abstract: Surface area is important in quantifying mineral-water reaction rates. Specific surface area (SSA) was measured to investigate controls on this parameter for several primary silicate minerals (PSM) used to estimate rates of weathering. The SSA measured by gas adsorption for a given particle size of relatively impurity-free, laboratory-ground samples generally increases in the order: quartz ≈ olivine ≈ albite 4000 cm2/g) and values measured with N2 were observed to be up to 50% larger than values measured with Kr. For laboratory-ground Amelia albite and San Carlos olivine, SSA can be calculated using log (SSA , cm2/g) = b + m log ( d ), where d = grain diameter (μm), b = 5.2 ± 0.2 and m = −1.0 ± 0.1. A similar equation was previously published for laboratory-ground quartz. Some other samples showed SSA higher than predicted by these equations. In some cases, high SSA is attributed to significant second phase particulate content, but for other laboratory-ground samples, high SSA increased with observed hysteresis in the adsorption-desorption isotherms. Such hysteresis is consistent with the presence of pores with diameters in the range 2 to 50 nm (mesopores). In particular, porosity that contributes to BET-measured SSA is inferred for examples of laboratory-ground diopside, hornblende, and all compositions of plagioclase except albite, plus naturally weathered quartz, plagioclase, and potassium feldspar. Previous workers documented similar porosity in laboratory-ground potassium feldspar. Surface area measured by gas adsorption may not be appropriate for extrapolation of interface-limited rates of dissolution of many silicates if internal surface is present and if it does not dissolve equivalently to external surface. In addition, the large errors associated in measuring SSA of coarse and/or impurity-containing silicates suggest that surface area-normalized kinetics in both field and laboratory systems will be difficult to estimate precisely. Quantification of the porosity in laboratory-ground and naturally weathered samples may help to alleviate some of the discrepancy between laboratory- and field-based estimates of weathering rate.

Structure type and bulk modulus of Fe3S, a new iron-sulfur compound

Abstract: We performed a series of experiments in the system Fe-FeS at a pressure of 21 GPa and temperatures between 950 and 1400 °C, and we found two new iron-excess iron-sulfur compounds, Fe3S and Fe2S, formed at subsolidus temperatures. Powder X-ray diffraction data revealed that Fe3S has a tetragonal cell, isostructural with Fe3P (space group I 4). The tetragonal unit-cell dimensions for Fe3S are a = 9.144(2) A and c = 4.509(2) A, with a zero-pressure density of 7.033 g/cm3. Static compression experiments on Fe3S were carried out in a diamond-anvil cell, using synchrotron X-ray diffraction technique. A least-squares fit to the experimental data at room temperature yielded bulk modulus K = 170 ± 8 GPa with a corresponding pressure derivative K ′ = 2.6 ± 0.5 or K = 150 ± 2 GPa with fixed K ′ = 4.

Oxidation-reduction mechanism of iron in dioctahedral smectites: I. Crystal chemistry of oxidized reference nontronites

Abstract: The crystal chemistry of Fe in four nontronites (Garfield, Panamint Valley, SWa-1, and NG-1) was investigated by chemical analysis, X-ray goniometry, X-ray absorption pre-edge spectroscopy, powder and polarized extended X-ray absorption fine structure (EXAFS, P-EXAFS) spectroscopy, and X-ray diffraction. The four reference nontronites have Fe/(Fe + Al + Mg) ratios ranging from 0.58 to 0.78, and are therefore representative of the different chemical compositions of dioctahedral ferruginous smectites. Pre-edge and powder EXAFS spectroscopy indicate that NG-1 contains 14 to 20% of tetrahedrally coordinated Fe 3+ , whereas the other three samples have no detectable IV Fe 3+ . The partitioning of VI Fe 3+ between cis (M2) and trans (M1) sites within the octahedral sheet was determined from the simulation of X-ray diffraction patterns for turbostratic nontronite crystallites by varying the site occupancy of Fe. Based on this analysis, the four nontronite samples are shown to be trans-vacant within the detection limit of 5% of total iron. The in-plane and out-of-plane local structure around Fe atoms was probed by angular P-EXAFS measurements performed on highly oriented, self-supporting films of each nontronite. The degree of parallel orientation of the clay layers in these films was determined by texture goniometry, in which the half width at half maxi- mum of the deviation of the c* axis of individual crystallites from the film plane normal, was found to be 9.9° for Garfield and 19° for SWa-1. These narrow distributions of orientation allowed us to treat the self-supporting films as single crystals during the quantitative analysis of polarized EXAFS spectra. The results from P-EXAFS, and from infrared spectroscopy (Madejova et al. 1994), were used to build a two-dimensional model for the distribution of Fe, and (Al,Mg) in sample SWa-l. In this nontronite, Fe, Al, and Mg atoms are statistically distributed within the octahedral sheet, but they exhibit some tendency toward local ordering. Fe-Fe and (Al, Mg)-(Al,Mg) pairs are preferen- tially aligned along the (010) direction and Fe-(Al,Mg) pairs along the (31 - 0), and (3 - 1 0) directions. This distribution is compatible with the existence of small Fe domains separated by (Al,Mg), and empty octahedra, which segregation may account for the lack of magnetic ordering observed for this sample at low temperature (5 K) (Lear and Stucki 1990).

Synchrotron XPS evidence for Fe2+-S and Fe3+-S surface species on pyrite fracture-surfaces, and their 3D electronic states

Abstract: A X-ray photoelectron Fe 2p 3/2 spectrum of a pristine pyrite fracture surface was collected using synchrotron radiation with the source tuned to 800 eV. Comparison of this highly surface sensitive Fe 2p spectrum with Fe 2p spectra collected by conventional means (1487 eV Al K α source) reveals that the high binding energy tail of the pyrite Fe 2p 3/2 line results primarily from Fe surface state contributions. The three major contributions to the spectrum are interpreted to be: (1) Fe 2+ resident on bulk sites; (2) Fe 2+ resident on surfaces, edges and corners; (3) Fe 3+ surface states produced during fracture by an auto-redox reaction involving Fe and S. The intense main peak is ascribed to the bulk state, whereas the high binding energy tail of the spectrum is composed primarily of Fe 2+ and Fe 3+ surface state contributions. Fe 2+ on bulk sites is octahedrally coordinated (O h symmetry). All valence electrons of Fe on bulk sites are paired (diamagnetic) and a singlet photopeak at 707 eV is consequently produced. Fracture produces Fe 2+ surface states with lower coordination than bulk sites. Fe 2+ located at surfaces, edges and corners experiences modified Ligand Field Stabilization Energies (LFSE) which results in stabilization of the d z 2 orbital and destabilization of the d xy orbital. Promotion of a d xy electron to the d z 2 orbital makes surface Fe 2+ surface states paramagnetic resulting in multiplet splitting of their associated photopeaks. The Fe 3+ surface state is necessarily paramagnetic and its photoemissions are consequently multiply split. Analysis of photopeak structures and binding energy splittings of Fe 2+ and Fe 3+ surface states demonstrates that they are located at the appropriate binding energies, and span the appropriate energy range, to satisfactorily explain the high binding energy tail on of the Fe 2p 3/2 spectrum.

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 M-O 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 A for T-O and +0.007 A 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.

Effect of alkalis, phosphorus, and water on the surface tension of haplogranite melt

Abstract: The sessile drop method has been used for measurements of the surface tension of haplogranite (HPG) melts containing an excess of alkalis and phosphorous (HPG8, HPG8 + 5 wt% Li 2 O, 5 wt% Na 2 O, 20 wt% Na 2 O, 5 wt% K 2 O, 5 wt% Rb 2 O, 5 wt% Cs 2 O, 10 wt% P 2 O 5 ) and of Armenian rhyolite in the temperature interval, 650–1665 °C, and at 1 bar pressure. Sessile drops were placed on graphite substrates in a Pyrox tube furnace purged with Ar. Drop shape was monitored with a videocamera and stored in a videorecorder. The surface tension was calculated by measuring the two principal radii of curvature of the drop shape in vertical cross section. The precision of the method was checked against the surface tension of water. The surface tension of HPG and rhyolite melt is ~280–300 ± 5 mN/m in the temperature interval 1200–1400 °C. Temperature dependence of the surface tension of haplogranite melts and rhyolite is weak and positive (dσ/d T = 0.06 to 0.09 mN/m/°C). Addition of 5 wt% of alkali oxides (except Li 2 O) results in a decrease of the surface tension of haplogranite melts. The HPG melts with 10 wt% P 2 O 5 have 30% higher surface tension than haplogranite melts with excess alkalis, and a negative temperature derivative (dσ/d T = −0.1 mN/m/°C). The HPG melts with 20 wt% Na 2 O and 5 wt% Li 2 O exhibit a decrease in surface tension with temperature (dσ/d T = −0.02 and −0.10 mN/m/°C, respectively). The surface tension of HPG8 melt saturated with water at 1–4 kbar was measured on sessile drops quenched at high pressure in an internally heated gas vessel at temperatures of 800–1200 °C. Water pressure significantly decreases the surface tension of melt from 270 mN/m at 1 bar (1000 °C) to 65 mN/m at 4 kbar. At 1 bar in “dry” conditions, dσ/d T = +0.056 mN/m/°C and at 3 kbar of water pressure, dσ/d T = +0.075 mN/m/°C. The decrease in the surface tension of HPG melt at a water pressure of several kbars is from −10 to −30 mN/m/wt% H 2 O. The increase of water content to more than 10 wt% in granite melts may not result in any significant decrease in the surface tension, which may be explained by formation of a surface sublayer having physical properties very distinct from those of the bulk.

Structure of synthetic 2-line ferrihydrite by electron nanodiffraction

Abstract: Comparison of experimental single-crystal electron diffraction patterns of synthetic two-line ferrihydrite (2LFh) with simulated single-crystal electron-diffraction patterns indicates that a synthetic 2LFh sample contains highly disordered material and nanocrystals with structures based on hexagonal (ABAB) and cubic (ABC) stacking of close-packed layers of O2− and OH− ions. An apparently continuous variation in ordering exists between the highly disordered material and each of the crystalline structures, suggesting that both nanocrystalline structures represent local extremes of three-dimensional ordering. Experimental diffraction patterns were obtained using electron nanodiffraction, a technique in which the finely focused beam from a field-emission gun in an electron microscope can be used to produce diffraction patterns from areas <1 nm across. Nanodiffraction patterns from the highly disordered material have diffuse streaks rather than distinct reflections, and are consistent with a two-dimensional structure that consists of close-packed anionic layers with essentially complete stacking disorder and nearly random distribution of Fe atoms. The structure with cubic stacking is similar to maghemite and has ~25% of the Fe in tetrahedral sites. The structure with hexagonal stacking consists of double chains of face-sharing Fe octahedra; each octahedron shares one face, two edges, and three corners with adjacent octahedra. Previous results from transmission electron microscopy, powder X-ray and electron diffraction, and synchrotron-based techniques reflect the overall high degree of structural disorder rather than the characteristics of the maghemite-like and double-chain structures.

Oxidation-reduction mechanism of iron in dioctahedral smectites: II. Crystal chemistry of reduced Garfield nontronite

Abstract: The crystallochemical structure of reduced Garfield nontronite was studied by X-ray absorption pre-edge and infrared (IR) spectroscopy, powder X-ray diffraction, polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy, and texture goniometry. Untreated and highly reduced (>99% of total Fe as Fe 2+ ) nontronite samples were analyzed to determine the coordination number and the crystallographic site occupation of Fe 2+ , changes in in-plane and out-of-plane layer structure and mid-range order between Fe centers, and to monitor the changes in structural and adsorbed OH/H 2 O groups in the structure of reduced nontronite. Contrary to earlier models predicting the formation of fivefold coordinated Fe in the structure of nontronites upon reduction, these new results revealed that Fe maintains sixfold coordination after complete reduction. In-plane P-EXAFS evidence indicates that some of the Fe atoms occupy trans-sites in the reduced state, forming small trioctahedral domains within the structure of reduced nontronite. Migration of Fe from cis- to trans sites during the reduction process was corroborated by simulations of X-ray diffraction patterns which revealed that about 28% of Fe 2+ cations exist in trans sites of the reduced nontronite, rather than fully cis occupied, as in oxidized nontronite. Out-of-plane P-EXAFS results indicated that the reduction of Fe suppressed basal oxygen corrugation typical of dioctahedral smectites, and resulted in a flat basal surface which is characteristic of trioctahedral layer silicates. IR spectra of reduced nontronite revealed that the dioctahedral nature of the nontronite was lost and a band near 3623 cm −1 formed, which is thought to be associated with trioctahedral [Fe 2+ ] 3 OH stretching vibrations. On the basis of these results, a structural model for the reduction mechanism of Fe 3+ to Fe 2+ in Garfield nontronite is proposed that satisfies all structural data currently available. The migration of reduced Fe ions from cis-octahedra to adjacent trans-octahedra is accompanied by a dehydroxylation reaction due to the protonation of OH groups initially coordinated to Fe. This structural modification results in the formation of trioctahedral Fe 2+ clusters separated by clusters of vacancies in which the oxygen ligands residing at the boundary between trioctahedral and vacancy domains are greatly coordination undersaturated. The charge of these O atoms is compensated by the incorporation of protons, and by the displacement of Fe 2+ atoms from their ideal octahedral position toward the edges of trioctahedral clusters, thus accounting for the incoherency of the Fe-Fe1 and Fe-Fe2 distances. From these results, the ideal structural formula of reduced Garfield nontronite is Na 1.30 [Si 7.22 Al 0.78 ] [Fe 2+ 3.65 Al 0.32 Mg 0.04 ]O 17.93 (OH) 5 in which the increased layer charge due to reduction of Fe 3+ to Fe 2+ is satisfied by the incorporation of protons and interlayer Na.

Supersilicic clinopyroxene and silica exsolution in UHPM eclogite and pelitic gneiss from the Kokchetav massif, Kazakhstan

Abstract: Abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the Kokchetav massif, Kazakhstan. These rocks are diamond-grade, ultrahigh-pressure (UHP) metamorphic rocks that recrystallized at P > 6 GPa and T > 1000 °C. Zircon is an excellent container, which effectively protects peak UHP metamorphic phases from retrogression. Therefore, to ascertain the pre-exsolution composition of the clinopyroxene, we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond-bearing biotite gneiss. Clinopyroxene in zircon has an excess of Fe 3+ + VI Al over IV Al + Na + K, and calculated cation totals significantly less than 4.0 per six O atoms. The stoichiometry of these pyroxenes can be reconciled if the Ca-Eskola end-member (Ca 0.5 □ 0.5 AlSi 2 O 6 ) is considered. The zircon-hosted clinopyroxene in the eclogite contains up to 9.6 mol% of the Ca-Eskola component, and in the biotite gneiss up to 18.2 mol%, whereas the matrix clinopyroxene contains much less (1.3 mol%, on average). Recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6.8 mol% Ca-Eskola component, which is consistent with the composition of the clinopyroxene inclusions in zircon. We conclude that the Ca-Eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction: \[2\ Ca_{0.5}{\square}_{0.5}AlSi_{2}O_{6}\ {\rightarrow}\ CaAl_{2}SiO_{6}\ +\ 3\ SiO_{2}\] resulting in exsolution of the quartz rods in the matrix clinopyroxene. Subducted crustal and supracrustal rocks form the Ca-Eskola clinopyroxene at high pressures and temperatures. The vacancy-containing clinopyroxene may have an important bearing on the physico-chemical properties of the subducted slab at upper mantle depth.

Kyanite eclogite thermobarometry and evidence for thrusting of UHP over HP metamorphic rocks, Nordøyane, Western Gneiss Region, Norway

Abstract: Quantitative estimates of metamorphic conditions are combined with previously reported structural analysis to develop the thermotectonic evolution of two separate lithotectonic units metamorphosed during the Late Silurian-Early Devonian collision between Baltica and Laurentia. A structurally higher plate, regionally correlated with the Blaho Nappe, contains kyanite eclogites associated with microdiamond-bearing kyanite-garnet-graphite gneiss on the north coast of Fjortoft and correlated with kyanite eclogites at Nogva, Flemsoya. The kyanite eclogites, containing the assemblage kyanite-garnet-omphacite-coesite (now polycrystalline quartz pseudomorphs) ± zoisite ± phengite, yield conditions of 820 °C and 34–39 kbar at Fjortoft and of 820 °C and 30–36 kbar at Nogva, best characterized by two recently recalibrated geothermobarometers. The conditions at Fjortoft overlap the diamond-graphite phase boundary and represent the first quantitative petrologic determination of UHP diamond-forming conditions in crustal rocks outside the Dabie Mountains, China and the Kokchetav Massif, Kazahkstan. In a structurally lower plate, eclogitized mylonite with the assemblage kyanite-garnet-omphacite-quartz-oligoclase, produced from the mid-Proterozoic Haram Gabbro that has intruded diorite country rock, yields 780 °C and 18 kbar. This result agrees with other normal HP estimates by Mork (1985) from partially to completely eclogitized gabbro in the same unit on Flemsoya. We propose that the UHP plate reached a maximum depth of 125 km and then experienced 65 km of exhumation during top-southeast thrusting that brought it into contact with the HP plate. Following this, both plates were exhumed together until reaching a depth of 37 km where they experienced extensive amphibolite-facies re-equilibration and top-west or left-lateral shearing. Temporal details of these histories were determined by monazite U-Th-Pb geochronology described in a companion paper.

Accuracy of equation-of-state formulations

Abstract: The accuracy of equation-of-state formulations is compared for theoretical total energies or experimental pressure-volume measurements for H{sub 2}, Ne, Pt, and Ta. This spans the entire range of compression found for minerals and volatiles in the Earth. The Vinet equation is found to be most accurate. The origin of the behavior of different equation-of-state formulations is discussed. It is shown that subtle phase transitions can be detected by examining the residuals from an equation-of-state fit. A change in the electronic structure of Ta is found at high pressures using this procedure, and a possible new transition in H{sub 2}.

Determination of water contents of granite melt inclusions by confocal laser Raman microprobe spectroscopy

Abstract: A new method of determining the water content of melt inclusions using confocal laser Raman microprobe spectroscopy is described. The water content of melt inclusions can be determined in the concentration range of 0 to 20 wt% with a high spatial resolution (~2 μm). Because the method works in reflection, minimal sample preparation is necessary. The method is fast, has good accuracy and precision (±0.25 wt%), and has the potential to become a useful, high resolution spectroscopic tool for melt inclusion studies.

Eclogite-facies relics and inferred ultrahigh-pressure metamorphism in the North Dabie Complex, central-eastern China

Abstract: Mineral assemblages and microstructures of a newly identified (retrogressed) eclogite (sensu lato) in the North Dabie Complex (NDC), central-eastern China, indicate early very high-pressure metamorphic relics preserved in a dominant amphibolite-facies host, where no eclogite (sensu stricto) has been positively identified before. The investigated eclogitic rock shows distinct multistage recrystallization, with granulite- and amphibolite-facies assemblages overprinting eclogite-facies relics. The minimum temperature for the eclogite-facies metamorphism is estimated to be ~800−820 °C. A spectacular microstructure of oriented quartz needles (~2–20 μm wide, ~5–200 μm long) in matrix Ca-Na clinopyroxene implies the prior existence of a non-stoichiometric “supersilicic” omphacite stabilized at ultrahigh-pressure (UHP, ≥25 kbar) conditions, although no coesite or coesite pseudomorphs have been found in the samples. The absence of coesite may be due to the lack of free silica at UHP conditions or the consumption of silica during retrograde reactions. The inferred UHP conditions metamorphism is further supported by Sm-Nd ages that are equivalent to Triassic metamorphic ages from UHP eclogites in the southeastern Dabie Mountains. This finding expands the UHP terrane northward about 50 km; spatial distribution of subduction/collision-related UHP rocks includes parts of the NDC.

Solubilities of noble metals in Fe-containing silicate melts as derived from experiments in Fe-free systems

Abstract: The solubilities of noble metals (NM: Ir, Pd, Au, Pt, and Ru) in FeO-free silicate melts are known from recent experimental work. In this paper, calculations are presented that relate solubilities of NM in FeO-free melts to those in FeO-containing melts. The main difference between these two systems is the formation of Fe-NM alloys in FeO-containing melts. At f O 2 conditions of the QFM buffer and 1200 °C, binary alloys of Fe with Au, Ru, Ir, Pd, or Pt containing 1, 2, 10, 17, and 25 at% Fe, respectively, are in thermodynamic equilibrium with a silicate melt with 10 mol% FeO. Thus alloy formation leads to a significant reduction in the solubility of Pt and to a lesser reduction in the solubility of Pd. The effects for Ir and Ru are small, and for Au almost negligible. The reduction in solubilities depends on temperature, oxygen fugacity, and FeO content of the silicate melt. Formation of FePt-alloys would lead to a preferred depletion of Pt in partial melts from the Earth’s mantle, which, however, is not observed. One explanation is that mantle melting occurs under very oxidizing conditions (QFM+2). The calculations presented here should be considered a first step toward gaining a better understanding of the behavior of NM during igneous processes.

Fluoride sites in aluminosilicate glasses: High-resolution 19F NMR results

Abstract: We present here high-resolution, 19 F NMR spectra, collected with sample spinning rates to 25 kHz, for sodium and calcium silicate and aluminosilicate glasses. Several distinct fluoride ion sites are well resolved and can be assigned to various coordination environments based on clear similarities to crystalline model compounds. In aluminosilicates, fluoride with one Al and several Na or Ca neighbors predominate, but silicon-fluoride groups may also be significant. Small concentrations of the latter can also be detected in sodium silicate glasses, suggesting a possible role in reduction of viscosity. Fluoride sites with no Si neighbors are, however, predominant in Al-free sodium silicates and probably in Ca silicates as well.

Hydrogen in diopside: Diffusion profiles

Abstract: The kinetics of diffusion for hydrogen in diopside single crystals from Jaipur, India, were determined by performing room pressure dehydration experiments at temperatures from 700–850 °C and an oxygen fugacity of 10 −14 bar. The hydrogen diffusivities were determined for the [100], [010], and [001]* directions either from concentration profiles for hydroxyl in samples after annealing or from bulk hydroxyl concentrations as a function of anneal time for sequential dehydration experiments. The rate of diffusion is anisotropic, with fastest transport along the [100] and [001]* axes and slowest along the [010] axis. Fits of the data to an Arrhenius law yield activation energies and pre-exponential terms of 181 ± 38 kJ/mol and 10 −2.1±1.9 m 2 /s for diffusion parallel to [100], and 153 ± 32 kJ/mol and 10 −3.4±1.6 m 2 /s for diffusion parallel to [001]*. For diffusion parallel to [010], the data were measured over an insufficient temperature range to calculate the activation energy for diffusion. However, these diffusivities were approximately an order of magnitude slower than those for diffusion parallel to [100] or [001]*. The measured rates and anisotropy for self-diffusion of hydrogen in diopside are consistent with those determined from hydrogen-deuterium exchange in Russian diopside (Hercule and Ingrin 1999). The hydrogen diffusivities are also similar in magnitude to those for olivine (Mackwell and Kohlstedt 1990) and are large enough that the hydrogen content of millimeter-size diopside grains with compositions near Jaipur diopside will adjust to changing environmental conditions in time scales of hours at temperatures as low as 800 °C. As xenoliths ascending from the mantle remain at high temperatures (i.e., >1000 °C) but experience a rapid decrease in pressure, diopside grains may dehydrate during ascent. Thus, low water contents for diopside crystals from xenoliths cannot be taken as indicative of low water contents in the mantle.

Patterns of mineral occurrence in metamorphic rocks

Abstract: Patterns in the occurrence of minerals in metamorphic rocks suggest additional opportunities for investigating chemical and physical processes during metamorphism. Three such patterns are reviewed. First, trace minerals in metamorphic rocks commonly occur with regular distributions indicating their participation in prograde reactions that can be mapped as isograds. Examples include the distribution of allanite and monazite in pelitic rocks and of zircon and baddeleyite in siliceous dolomites. Recognition of these isograds points to the potential for developing a chronology of specific chemical reactions during metamorphism and for defining the P - T conditions of those reactions. Second, the mineralogical products of retrograde metamorphism in many cases occur in distinctive associations that are consistent with partial mineral-fluid equilibrium. Examples include the distribution of retrograde calcite, quartz, and tremolite in siliceous limestones and of retrograde tremolite, dolomite, brucite, and serpentine in siliceous dolomites from contact aureoles. Among other things, application of partial equilibrium to retrograde metamorphic rocks leads to constraints on the amount and direction of fluid flow in contact aureoles as they cool. Third, pseudomorphs are typically absent from prograde metamorphic rocks but are common in retrograde metamorphic rocks. The distribution may be explained by the effect of “force of crystallization.” The pattern of occurrence of pseudomorphs thus suggests novel phenomena during metamorphism that develop from an interplay between chemical and mechanical processes.

Fivefold-coordinated aluminum in tectosilicate glasses observed by triple quantum MAS NMR

Abstract: Eight glasses with molar Mg/2Al ≈ 1 in the system MgO-Al2O3-SiO2 have been studied by magic angle spinning (MAS) NMR spectroscopy. Using triple quantum (3Q) NMR techniques we find evidence for significant concentrations of Al coordinated to five O atoms in all glasses, the proportion increasing with decreasing Mg/Al and decreasing silica content. In glasses with Mg/2Al = 1, up to 6% of the Al is estimated to be coordinated to five rather than four O atoms. Calculations of the polymerization state of these liquids made assuming that all aluminum is in tetrahedral coordination charge balanced by magnesium are thus seriously in error. Such errors may be of even greater importance at the high temperatures and pressures relevant to the Earth and materials sciences.

The MD simulation of the equation of state of MgO: Application as a pressure calibration standard at high temperature and high pressure

Abstract: Molecular dynamics (MD) simulation is used to calculate the elastic constants and their temperature and pressure derivatives, and the T-P-V equation of state of MgO. The interionic potential is taken to be the sum of pairwise additive Coulomb, van der Waals attraction, and repulsive interactions. In addition, to account for the observed large Cauchy violation of the elastic constants of MgO, the breathing shell model (BSM) is introduced in MD simulation, in which the repulsive radii of O ions are allowed to deform isotropically under the effects of other ions in the crystal. Quantum correction to the MD pressure is made using the Wigner-Kirkwood expansion of the free energy. Required energy parameters, including oxygen breathing parameters, were derived empirically to reproduce the observed molar volume and elastic constants of MgO, and their measured temperature and pressure derivatives as accurately as possible. The MD simulation with BSM is found to be very successful in reproducing accurately the measured molar volumes and individual elastic constants of MgO over a wide temperature and pressure range. The errors in the simulated molar volumes are within 0.3% over the temperature range between 300 and 3000 K at 0 GPa, and within 0.1% over the pressure range from 0 up to 50 GPa at 300 K. The simulated bulk modulus is found to be correct to within 0.7% between 300 and 1800 K at 0 GPa. Here we present the MD simulated T-P-V equation of state of MgO as an accurate internal pressure calibration standard at high temperatures and high pressures.

Molecular modeling of the structure and dynamics of the interlayer and surface species of mixed-metal layered hydroxides: Chloride and water in hydrocalumite (Friedel's salt)

Abstract: The dynamical behavior of Cl− and H2O molecules in the interlayer and on the (001) surface of the Ca-aluminate hydrate hydrocalumite (Friedel’s salt) over a range of temperatures from −100 to 300 ° C was studied using isothermal-isobaric molecular dynamics computer simulations. This phase is currently the best available model compound for other, typically more disordered, mixed-metal layered hydroxides. The computed crystallographic parameters and density are in good agreement with available X-ray diffraction data and the force field developed for these simulations preserves the structure and density to within less than 2% of their measured values. In contrast to the highly ordered arrangement of the interlayer water molecules interpreted from the X-ray data, the simulations reveal significant dynamic disorder in water orientations. At all simulated temperatures, the interlayer water molecules undergo rapid librations (hindered hopping rotations) around an axis essentially perpendicular to the layers. This results in breaking and reformation of hydrogen bonds with the neighboring Cl− anions and in a time-averaged nearly uniaxial symmetry at Cl−, in good agreement with recent 35Cl NMR measurements. Power spectra of translational, librational, and vibrational motions of interlayer and surface Cl− and H2O were calculated as Fourier transforms of the atomic velocity autocorrelation functions and compared with the corresponding spectra and dynamics for a bulk aqueous solution. The ordered interlayer space has significant effects on the motions. Strong electrostatic attraction between interlayer water molecules and Ca atoms in the principal layer makes the Ca· · · OH2 bond direction the preferred axis for interlayer water librations. The calculated diffusion coefficient of Cl− as an outer-sphere surface complex is almost three times that of inner-sphere Cl−, but is still about an order of magnitude less than that of Cl− in bulk aqueous solution at the same temperature.

The dissolution of hectorite: In-situ, real-time observations using atomic force microscopy

Abstract: The dissolution of individual hectorite (a trioctahedral smectite) particles has been observed at molecular scales in acidic aqueous solution with atomic force microscopy (AFM). A new sample preparation technique was used to attach nanometer-sized hectorite particles to a mica substrate. The reactive surface area of individual hectorite particles was identified and its change during a dissolution experiment was quantified. The dissolution of hectorite under pH 2 conditions takes place exclusively at the edge surfaces. In contrast, the basal surface is completely unreactive within the investigated time scale of several hours. The short edges of the hectorite laths were found to react somewhat more quickly than the long edges. The edge surface area represents 1.5–3.3% of the total surface area. The total surface area has been determined from the actual particle dimensions derived from AFM data to be 730 m 2 /g. The dissolution rate normalized to the reactive edge surface area (ESA) has been determined to be 7.3 × 10 −9 mol hectorite/(m 2 -s), which represents a total surface area (TSA) normalized dissolution rate of 1.9 × 10 −10 mol hectorite/(m 2 -s). The ESA/TSA ratio increases by about 15% within 1 h exposure to a pH 2 aqueous solution at 22 °C.

A shell model for the simulation of rhombohedral carbonate minerals and their point defects

Abstract: The electronic polarization of oxygen ions has been explicitly incorporated in a shell model to better simulate the structure of calcite and related rhombohedral carbonate minerals. Pair-potentials for Ca{sup 2+} ions and C and O comprising the carbonate molecular ion were simultaneously fitted to experimental lattice, elastic, dielectric, and vibrational data for calcite, and the structure and elastic properties of aragonite. The resulting potential parameters for the CO{sub 3}{sup 2{minus}} group were then transferred to models for the structures and bulk moduli of the carbonate minerals incorporating Mn, Fe, Mg, Ni, Zn, Co, Cd, and thus a fully consistent set of interaction parameters for calculating the properties of the carbonate minerals was obtained. Defect energies for doping the divalent cations into the calcite structure, and for calcium and carbonate ion vacancies were calculated. In addition, various disorder types for dolomite, including anti-site defects, stacking defects, stacking defects, and the energy related to increasing the Ca/Mg ration in the dolomite structure were simulated. The theoretical enthalpy for domomite ordering (34.4 kJ/mol) compares very well with experimental measurements.