Volume 88, Issue 10

Seven separate samples of hydrous ringwoodite with compositions ranging from Fo 100 to Fo 89 and hydrogen contents from 0.2 to 1.1 wt% were synthesized in the 5000 ton multi-anvil press at the Bayerisches Geoinstitut. Synthesis conditions ranged from 18 to 22 GPa and 1400 to 1500 °C. The crystals were characterized by single-crystal X-ray diffraction, electron microprobe, IR and Mössbauer spectroscopy, and by analytical and high-resolution transmission electron microscopy. The crystals are optically isotropic, and the Fe-bearing samples are deep blue in color. Mössbauer spectroscopy and ELNE spectroscopy applied to the Fe-bearing samples indicates about 10% of the iron is in the ferric state. High-resolution TEM examination of one of the Fe-bearing samples indicates that the crystals are homogeneous and free of significant inclusions or exsolution features. Infrared spectra show a broad absorption band extending from about 2500 to 3600 cm -1 with maxima ranging from 3105 for the pure magnesian samples to 3150 cm -1 for the Fo 89 samples. The crystal structures of the seven ringwoodite samples were refined by X-ray single-crystal diffraction. Refinement of cation site occupancies indicates full occupancy of the tetrahedral site for all samples, whereas the occupancy of the octahedral site appears to decrease systematically with H content. The principal hydration mechanism involves octahedral cation vacancies. The IR spectra are consistent with protonation of the short O-O approach on the tetrahedral edge, which would imply partial Mg-Si disorder.

We have determined the equation of state of phase Egg, AlSiO 3 OH, at room temperature up to 40 GPa, using X-ray powder diffraction with synchrotron radiation. We determined the isothermal bulk modulus K 0T = 157 ± 4 GPa with a pressure derivative K' 0T = 6.5 (4) by fitting a third order Birch- Murnaghan equation of state. When K' 0T is fixed at 4, we obtain K 0T = 183 ± 2 GPa. This value can be compared to other hydrous phases existing in the transition zone as well as to non-hydrous phases, such as kyanite, Al 2 SiO 5 . We find that despite the presence of hydrogen, the bulk modulus of phase Egg remains high, unlike other low-pressure hydrous minerals. In addition, we found that phase Egg is more compressible along the b axis, where the O-H bonds are oriented. Our results are in good agreement with previous theoretical calculations, performed on the similar hydrous phase δ-AlOOH, that show that the O-H bond strengthens with pressure, suggesting that the presence of water stored in these phases does not soften the material at pressures corresponding to lower mantle conditions.

High-pressure synchrotron infrared (IR) absorption spectra were collected at ambient temperature for OH-chondrodite and OH-clinohumite up to 38 and 29 GPa, respectively, using argon as the pressure- transmitting medium. The crystal structures of both clinohumite and chondrodite are preserved up to the maximum pressure. However, disordering of the silicate framework appears to become more pronounced at high pressure based on significant broadening of the IR bands with increasing pressure. All three OH bands in both structures shift linearly to higher frequency with pressure up to 18 GPa. Above 18 GPa, the variation of OH frequency with pressure remains linear; however, the slopes for the three OH bands are significantly different as a result of different degrees of hydrogen bonding. The IR results are compared to those from recent Raman studies in which water was used as the pressuretransmitting medium.

The HP behavior of the natural zeolite yugawaralite and of the synthetic zeolite Na-A was studied by in situ synchrotron X-ray powder diffraction, using a non-penetrating P-transmitting medium. The unit-cell parameters of yugawaralite were refined up to the pressure of 10 GPa, at which reductions were found of about 7, 2.4, 7, 1.3, and 15% for a, b, c, β, and V, respectively. Contractions of 6.5 and 18.4% were found for a and V, respectively, for zeolite Na-A in the range 10 -4 to 6.8 GPa. Diffraction patterns collected during decompression show that the effects induced by high pressure on both samples are almost completely reversible. These results are compared with those obtained under similar experimental conditions for other natural zeolites, with the aim of rationalizing the deformation mechanisms of these porous materials and comparing their flexibility under high-pressure and high-temperature conditions.

In-situ synchrotron X-ray powder diffraction experiments on goethite were performed up to 24.5 GPa using a diamond-anvil cell at BL-18C in the Photon Factory, Japan. The compression behavior of goethite is anisotropic: the a axis, which is close to the direction of the hydrogen bond, is almost twice as compressible as the b and c axes. A second-order Birch-Murnaghan fit (K' = 4) to the unitcell volumes and pressures up to 24 GPa gave a bulk modulus K 0 = 111(2) GPa. The pressure medium apparently affects the compression behavior of goethite, as a sudden strain broadening occurs above 11 GPa, where solidification of the 4:1 methanol:ethanol pressure medium occurs. The crystal structure of goethite was refined by means of the Rietveld method at several pressures below 10 GPa. The main compression occurs in the “vacant channels” in the crystal structure, where the OH ···O bridges are located. The hydrogen-bonded O···O distance shortens with increasing pressure at the rate of about -0.023 Å/GPa and reaches 2.60(1) Å at 9 GPa.

An isosymmetric displacive structural transformation in the hydrous layer silicate dickite [Al 2 Si 2 O 5 (OH) 4 , monoclinic Cc, a = 5.161(3), b = 8.960(6), c = 14.459(10) Å, β = 96.77(1)°], occurring under hydrostatic compression above 2.0 GPa, has been studied using single-crystal X-ray diffraction and diamond-anvil cell techniques. The structure of the high-pressure phase, determined in situ, is monoclinic with space group Cc with unit-cell parameters a = 5.082(3), b = 8.757(6), c = 13.771(9) Å, and β = 89.60(2)° at 4.1 GPa. The positions of all hydrogen atoms at both ambient and high pressure have been determined by a combination of simulated annealing and energy minimization. The mechanism of the transformation, which may be general for other hydrous layered materials, involves a shift of the 1:1 layers with respect to each other by the vector [1/6, 1/6, 0] and is accompanied by the formation of new hydrogen bonds.

The incorporation of hydrogen into the coesite structure was investigated at pressures ranging from 4.0-9.0 GPa and temperatures from 750-1300 °C using Al and B doped SiO 2 starting materials. The spectra show four sharp bands (ν 1 , ν 2a , ν 2b , and ν 3 ) in the energy range of 3450-3580 cm -1 , consistent with the hydrogarnet substitution [Si 4+(T2) + 4O 2- = va T2 + 4OH - ], two weak sharp bands at 3537 and 3500 cm -1 (v 6a and ν 6b ) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm -1 (ν 4 and ν 5 ) attributed to substitution of Si 4+ by Al 3+ + H. More than 80% of the dissolved water is incorporated via the hydrogarnet substitution mechanism. The hydrogen solubility in coesite increases with pressure and temperature. At 7.5 GPa and 1100 °C, 1335 H/10 6 Si is incorporated into the coesite structure. At 8.5 GPa and 1200 °C, the incorporation mechanism changes: in the IR spectra four new sharp bands appear in the energy range of 3380-3460 cm -1 (ν 7 -ν 10 ) and the ν 1 -ν 3 bands disappear. Single crystal X-ray diffraction, Raman spectroscopy, polarized single-crystal and in situ high-pressure FTIR spectroscopy confirm that the new bands are due to OH- in coesite. The polarization and high-pressure behavior of the ν 7 -ν 10 OH bands is quite different from that of the ν 1 -ν 3 bands, indicating that the H incorporation in coesite changes dramatically at these P and T conditions. Quantitative determination of hydrogen solubility in synthetic coesite as a function of pressure, temperature, and chemical impurity allow us to interpret observations in natural coesite. Hydrogen has not previously been detected in natural coesite samples from ultra high-pressure metamorphic rocks. In this study, we report the first FTIR spectrum of a natural OH-bearing coesite. The dominant substitution mechanism in this sample is the hydrogarnet substitution and the calculated hydrogen content is about 900 ζ ± 300 H/10 6 Si. The coesite occurs as an inclusion in diamond together with an OH-bearing omphacite. The shift of the OH-bands of coesite and omphacite to lower energies indicates that the minerals are still under confining pressure.

The pressure-induced structural phase transition in kalicinite, KHCO 3 , has been studied by neutron powder diffraction, and infrared (IR) and Raman spectroscopy at high pressure and room temperature. The neutron diffraction study of deuterated kalicinite (KDCO 3 ) revealed that for the one site for hydrogen (deuterium) found in the low-pressure phase, the O-D···O angle decreases from 176 to 161° and the distance between donor and acceptor O atoms of the O-D···O group decreases from 2.66 to 2.59 Å in the pressure range from 0 to 2.5 GPa. The crystal structure of the high-pressure polymorph was not determined. Infrared spectra were obtained at pressures up to 6.3 GPa using a diamond anvil cell. At ambient pressure, the O-H stretching, O-H···O in-plane bending, and O-H···O out-of-plane bending modes occur at 2620, 1405, and 988 cm -1 , respectively. The frequency of the O-H stretch mode was nearly constant in the pressure range from 0 to 2.8 GPa, while that of O-H···O in-plane bending and out-of-plane modes increased with increasing pressure up to 2.8 GPa and remained constant above the phase transition pressure. The Raman spectra showed a clear phase transition at 2.8 GPa. The three Raman modes observed are assigned to internal vibrational modes of HCO 3 - and this suggests that the surrounding environment did change dramatically at the phase transition. These results suggest that the phase transition in kalicinite is triggered by the distortion of C-O-H bond at high pressure.

A method based on an analysis of the Laplacian of the electron density distribution is used to locate positions of nonbonding electron-pairs and favorable docking sites for hydrogen in high-pressure silicates, including stishovite, akimotoite, wadsleyite, ringwoodite, MgSiO 3 , and CaSiO 3 perovskite. In stishovite, model hydrogen positions (h) are located off the shared O···O edge of the octahedra with the Oh vector oriented perpendicular to [001]. In akimotoite, two model positions for hydrogen are found, one along an edge of the MgO 6 octahedron with an Oh vector aligned close to [001] and the other within the face of the MgO 6 octahedra, with Oh vectors oriented perpendicular to [001]. Geometric and electrostatic considerations suggest that both of these sites would require vacant Mg sites. In wadsleyite, potential sites for protonation exist for all O atoms. Hydrogen bonded to O1 resides in a cage-like site with the Oh dipole oriented parallel to [001]. Two symmetrically equivalent sites for hydrogen exist at O2 in a second cage-like structure with Oh vectors aligned approximately along [100]. The calculations also identify potential sites for hydrogen at O3 and O4, but suggest that these could only be occupied by hydrogen if vacancies exist at adjacent Mg sites. Model hydrogen positions in ringwoodite are predicted along O···O edges of the MgO 6 octahedron, and, if occupied, would require vacancies at adjacent Mg sites. MgSiO 3 perovskite is distinguished by having only one potential site for hydrogen. The Oh vector lies within the (110) plane and geometric and electrostatic considerations suggest that a vacancy is required at the Mg site if this site is protonated. No localized sites for hydrogen are found in CaSiO 3 perovskite. We suggest that non-bonded electron pairs associated with oxide anions involved in octahedral frameworks with tilt angles of less than 180° provide more favorable docking sites for protons than those involved in wider angles. The results from the electron density calculations show very good agreement with available crystallographic and spectroscopic data and demonstrate that the strategy used in this study can be useful in locating protons in Earth materials.

Clinozoisite and topaz are examined to pressures of 36 and 24 GPa, respectively, at 300 K using infrared spectroscopy. Both minerals metastably persist throughout this pressure range at ambient temperature. Both stretching and bending vibrations of the silicate tetrahedra are tracked upon compression and decompression of each phase, as are their hydroxyl-stretching vibrations. The hydroxylstretching vibrations lie at 3650 cm -1 in topaz and 3351 cm -1 in clinozoisite and shift by 0.6 (± 0.3) cm -1 /GPa and -5.1 (± 0.9) cm -1 /GPa, respectively. Therefore, the hydrogen bonding in clinozoisite and topaz responds completely differently to pressure: hydrogen bonding increases in clinozoisite, but decreases in topaz. The clinozoisite hydroxyl-stretching vibration also broadens by almost a factor of six under compression. Both the clinozoisite peak shift and its pressure-induced broadening are simulated using a model of the hydrogen bond potential. The markedly different structural responses of these two minerals to pressure are interpreted on crystal-chemical grounds, and demonstrate that the bonding of hydrogen in possible subduction-related hydrous phases varies dramatically, depending on crystal chemistry.

A haplogranite composition (73 wt% SiO 2 ) has been studied experimentally to investigate the kinetics of crystallization. A series of equilibrium experiments over a range of pressures (5-200 MPa) and temperatures (825-1185 °C) determined the stable phases at water-saturated conditions. The haplogranite system crystallizes plagioclase over a broad range of pressures and temperatures, with crystalline silica and then alkali feldspar becoming stable at progressively lower temperatures and pressures. Plagioclase An contents decrease and bulk crystal fractions increase as experimental conditions tend to lower pressures and temperatures. A second series of experiments investigated the kinetics of crystallization induced by depressurization and associated water exsolution. A nucleation delay of at least 4 hours is estimated for all final pressures (P f ) studied. The amount of crystallization increases as P f decreases and experimental duration and undercooling increase. Nucleation and growth rates are observed to increase with undercooling, although at very high undercoolings, growth rates start to decrease. Comparison with other experimental studies shows the importance of composition and water content to nucleation and growth rates. Increasing water content lowers melt viscosity, aiding crystallization.

Assessment of the glaucophane substitution, [M4] Ca + [VI] Mg ↔ [M4] Na + [VI] Al, along the Ga and F analogue of the tremolite-glaucophane join, ⃞Ca 2 Mg 5 Si 8 O 22 F 2- ⃞Na 2 Mg 3 Ga 2 Si 8 O 22 F 2 (= F-Ga-glauc), was conducted at 900 °C and 1.5 GPa in 10 mol% increments. Several chemical exchanges deviating from the glaucophane substitution were identified, namely the “edenite” substitution [A] ⃞ + [IV] Si ↔ [A] Na + [IV] Ga toward F-Ga-nyböite, the [A] ⃞ + [M4] Na + [VI] Ga + [IV] Si ↔ [A] Na + [M4] Mg + [VI] Mg + [IV] Ga exchange toward F-Ga-Na-magnesiokatophorite, and the [A] ⃞ + [VI] Ga ↔ [A] Na + [VI] Mg exchange toward F-Ga-eckermannite. Minor displacements from the join due to the [M4] Na + [VI] Ga ↔ [M4] Mg + [VI] Mg exchange toward “cummingtonite” and the richterite exchange [A] ⃞ + [M4] Ca ↔ [A] Na + [M4] Na were also observed. Amphibole syntheses with low F-Ga-glauc contents (≤ 60 mol%) were primarily influenced by the “edenite” exchange, while those with high F-Ga-glauc contents (≈ 70 mol%) were dominated by the “glaucophane” substitution. Powder X-ray diffraction Rietveld refinements were obtained without restrictions for the amphiboles formed in the range 0-40 mol% FGa- glauc. Refinements for samples in the ranges of 50-70 and 100 mol% F-Ga-glauc were possible only after applying some restrictions to the A- and M4-site occupancies because of the appearance of a layered silicate and an unidentified Ga-rich phase that complicated the refinement. The successful refinements indicated that amphibole exhibits [VI] Ga ordering at the M2 site and [IV] Ga ordering at T1. The refinements also show a transition in A-site occupancy from A2 to Am with increasing FGa- glauc content. The inability to synthesize stoichiometric glaucophane and the types of chemical deviations that are observed in this analogue system are virtually identical to what has been observed in previous studies. Extrapolation of the results from this and previous studies suggests that ideal glaucophane would be obtained at approximately 3.85 GPa and 585 °C.

We present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from Sri Lanka in the temperature range 350 to 650 °C and with different solutions (2 M AlCl 3 , 2 M CaCl 2 , pure H 2 O, and a multi-cation solution). Under these conditions, sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems. The reaction zones are characterized by a marked increase in the cathodoluminescence intensity, a decrease of the back-scattered electron emission, and an increased degree of structural order, as revealed by micro-Raman and infrared spectroscopy. Sensitive high-resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations (e.g., Ca 2+ , Ba 2+ , Mg 2+ , Al 3+ ) from the solution and lost variable amounts of Zr, Si, Hf, the REE, U, Th as well as radiogenic Pb. A comparison between “dry” and “hydrothermal” annealing trends shows that the kinetics of structural recovery, including recrystallization of the amorphous phase in metamict zircon, is strongly enhanced under hydrothermal conditions. This finding suggests that water “catalyzes” structural recovery processes in metamict zircon. We found that the structure of the reacted areas does not resemble that of crystalline zircon, i.e., is still characterized by a temperature-dependent degree of disorder, which would not be expected if the reaction is controlled by a coupled dissolution and re-precipitation process. Instead, the alteration process can be described best by a diffusion-reaction-recrystallization model. In this model, it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts. We found that the rate and the extent of solid-state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements. This interpretation is indicated by the observation that trace elements, including U and Th, were preferentially lost during the reaction with a fluid at low temperatures, where recrystallization of the amorphous material was slow or not activated at all. The observed chemical alteration patterns are believed to reflect a competition between the kinetics of long-range diffusion and ion exchange and the kinetics of the short-range diffusion necessary for the recrystallization process.

Crystal growth experiments were conducted using potassium alum and calcite crystals in aqueous solution under both non-stirred and stirred conditions to elucidate the mechanism for size-dependent (proportionate) and size-independent (constant) crystal growth. Growth by these two laws can be distinguished from each other because the relative size difference among crystals is maintained during proportionate growth, leading to a constant crystal size variance (β 2 ) for a crystal size distribution (CSD) as the mean size increases. The absolute size difference among crystals is maintained during constant growth, resulting in a decrease in size variance. Results of these experiments show that for centimeter-sized alum crystals, proportionate growth occurs in stirred systems, whereas constant growth occurs in non-stirred systems. Accordingly, the mechanism for proportionate growth is hypothesized to be related to the supply of reactants to the crystal surface by advection, whereas constant growth is related to supply by diffusion. Paradoxically, micrometer-sized calcite crystals showed proportionate growth both in stirred and in non-stirred systems. Such growth presumably results from the effects of convection and Brownian motion, which promote an advective environment and hence proportionate growth for minute crystals in non-stirred systems, thereby indicating the importance of solution velocity relative to crystal size. Calcite crystals grown in gels, where fluid motion was minimized, showed evidence for constant, diffusion-controlled growth. Additional investigations of CSDs of naturally occurring crystals indicate that proportionate growth is by far the most common growth law, thereby suggesting that advection, rather than diffusion, is the dominant process for supplying reactants to crystal surfaces.

We have extended a recently developed Monte Carlo technique which includes explicit exchange as well as movement of ions to systems involving heterovalent exchange. These Monte Carlo computer simulations, based on analytical inter-atomic potentials, are capable of providing detailed quantitative information concerning the thermodynamics of ordering of spinel (MgAl 2 O 4 ), gahnite (ZnAl 2 O 4 ), hercynite (FeAl 2 O 4 ), NiAl 2 O 4 , and magnesioferrite (MgFe 2 O 4 ) over a range of pressures and temperatures. At all temperatures and pressures ionic relaxation, lattice vibrations, and pressure are explicitly taken into account. Each compound has a larger expansion coefficient and smaller bulk modulus in the normal than in the inverse spinel structure. We predict only a small variation of order parameter with pressure, and that this will be more pronounced for inverse than normal spinels. We examine, briefly, the consequences of our results for the kinetics of cation ordering in these solids.

Structural modifications induced by the I1̅-I2/c displacive transition in Ca 0.2 Sr 0.8 Al 2 Si 2 O 8 feldspar (An 20 SrF 80 ) have been investigated in situ by single-crystal X-ray diffraction at 20, 200, 400, 500, and 620 °C. Crystals were synthesized from the melt, cooled slowly to 1300 °C, and then quenched in air. At room temperature (a = 8.361, b = 12.973, c = 14.259 Å, α = 90.79, β = 115.55, γ = 90.62°, V = 1394.9 Å 3 ; space group: I1̅; Q od = 0.88), the polyhedra of the non-tetrahedral cation have different configurations at the Ca/Sr(0) and Ca/Sr(z) sites. In monoclinic Sr feldspar, the Sr-O B and Sr-O D distances are regular, but in triclinic An 20 SrF 80 feldspar the O B (m0) atom in the Ca/Sr(0)-polyhedron and the O D (mz) atom in the Ca/Sr(z)-polyhedron are displaced. The topochemical symmetry of the framework is essentially monoclinic and the average dimensions of the pseudo-symmetrical tetrahedra do not change within the error limits. With increasing temperature, the distances between the Ca/Sr cations and the pseudo-related O atom pairs converge on the values adopted at the I1̅-I2/c transition. At the transition point, the M-polyhedron assumes a regular coordination, similar to that observed in monoclinic Sr feldspar at room temperature. The results obtained indicate T tr = 520 ± 10 °C, in agreement with the transition temperature obtained from the changes of cell dimensions. The variation of cos 2 α* with temperature is consistent with the solution to Landau 2-4-6 potential with T c = 506 ± 7 °C.

Our ability to calculate the depths and temperatures at which magmas partially crystallize can prove crucial as petrologists test hypotheses of magma transport and evolution. Yet whereas numerous magma transport arguments involve hydrous and SiO 2 -rich volcanic products, current clinopyroxene-liquid thermobarometers have been calibrated only from basaltic liquid compositions. To remedy this deficiency, new thermobarometers have been calibrated using new experiments that include hydrated (water-undersaturated) and SiO 2 -rich liquids ranging to 71.3 wt% SiO 2 . As with prior models, the new calibrations are based on jadeite crystallization and jadeite-diopside + hedenbergite exchange equilibria: Here, T is in Kelvins and P is in kbar. Jd cpx is the mole fraction of jadeite in clinopyroxene, where pyroxene cations are calculated on the basis of 6 O atoms, and Jd is the lesser of Na or VI Al; remaining Al is used to form CaTs. DiHd cpx is the mole fraction of diopside + hedenbergite in clinopyroxene, calculated as the fraction of Ca remaining after forming CaTs (= VI Al - Jd), CaTiAl 2 O 6 [= ( IV Al - CaTs)/2], and CaCr 2 SiO 6 (=Cr/2). Terms such as Al liq refer to the cation fraction of AlO 1.5 in the liquid, Fmliq is the sum FeO liq + MgO liq , and Mg'liq is the cation fraction ratio MgO liq /(MgO liq + FeO liq ). Errors are similar to earlier models that utilize basalt compositions only. For the barometer, R 2 = 0.97 and the standard error of estimate (SEE) is 1.7 kbar for the regression data; for the thermometer R 2 = 0.96 and the SEE is 33 K. The models are applied to Neogene lavas from the Tibetan Plateau, and Neogene-Quternary lavas from the eastern Snake River Plain (SRP), Idaho. Crystallization depths for Tibet cluster at the middle/lower crust boundary. Magma-crust density relationships suggest that the middle crust may act as a level of neutral buoyancy. In the SRP, however, magmas appear to bypass several possible density traps. We suggest that fracture properties, such as dike size and aspect ratio, control magma transport in the SRP.

Using synchrotron X-ray fluorescence (XRF) microanalysis, including XRF tomography, and Xray absorption near-edge structure (XANES) analyses, the distribution, and nature of incorporation of Fe and Ni impurities in as-grown diamond crystals, synthesized under high-pressure and hightemperature (HPHT) conditions, have been characterized. We find significantly different behavior for Fe and Ni as impurities in diamond. Nickel is dispersed and sector-zoned, with concentrations in {111} growth sectors at least 3 times those in {100} sectors, whereas Fe exists in the form of microaggregates or clusters with no observable sector correlation. Fe K-edge XANES shows that Fe is oxidized in diamond and has a valence of 2+. Comparison of XANES spectra from numerous standard compounds indicates that Fe is very likely bonded with oxygen as FeO.

This work describes the sequence of transformations and their reaction kinetics during the oxidation of olivine in air at high temperature. A natural olivine sample from the layered series of the Ivrea-Verbano igneous complex (Western Alps, Italy) was heated in the temperature range 25-1300 °C in air and investigated by in situ, real time powder X-ray diffraction (PXRD). The evolution of the peaks (measured integrated intensities) was followed in non-isothermal conditions using variable heating rates (b = 20, 22, 25, 27, and 30 °C/min). The total time of the experiments ranged from about 256 (b = 30 °C/min) to 277 (b = 20 °C/min) min including the time for the data collections. An additional isothermal run was performed at 800 °C. The analysis of the kinetic data was attempted with the use of different equations including the classical Avrami theory for solid-state reactions. The kinetic results were confirmed by independent experimental data from electron microscopy (SEM, TEM). In the transformation sequence, hematite appears at about 600 °C producing amorphous segregations. of silica that later recombine with forsterite to form pyroxene Hematite is stable up to 1130 °C where it is transformed into magnetite. The rate limiting step for the formation of hematite is a twodimensional diffusion with constant or decelerating nucleation rate and apparent activation energy of 15 kcal/mol. The concentration of Fe 3+ in Fe-rich regions favors the heterogeneous nucleation of hematite, which may take place on existing defects or at the grain boundaries with impurity phases such as serpentine. At 1130 °C, magnetite is formed at the expenses of hematite, with a contracting volume interface-controlled reaction in two or three dimensions with an apparent activation energy of 30-31 kcal/mol. The hematite to magnetite transformation is direct, without a metastable amorphous intermediate. It is described by the “shrinking core model,” with the formation of a magnetite outer layer at the surface of the hematite particles that proceeds toward the core of the reacting hematite by diffusion of the oxygen throughout the newly formed magnetite layer. Its rate is limited by the advancement of the reaction front. The rate-limiting step for the formation of pyroxene is two-dimensional diffusion with decelerating nucleation rate with an apparent activation energy of 29 kcal/mol.

Thermal infrared emission and reflectance spectra (250-1400 cm-1; ~7-40 mm) of experimentally shocked albite- and anorthite-rich rocks (17-56 GPa) demonstrate that plagioclase feldspars exhibit characteristic degradations in spectral features with increasing pressure. New measurements of albite (Ab 98 ) presented here display major spectral absorptions between 1000-1250 cm -1 (8-10 mm) (due to Si-O antisymmetric stretch motions of the silica tetrahedra) and weaker absorptions between 350-700 cm -1 (14-29 mm) (due to Si-O-Si octahedral bending vibrations). Many of these features persist to higher pressures compared to similar features in measurements of shocked anorthite, consistent with previous thermal infrared absorption studies of shocked feldspars. A transparency feature at 855 cm -1 (11.7 μm) observed in powdered albite spectra also degrades with increasing pressure, similar to the 830 cm-1 (12.0 μm) transparency feature in spectra of powders of shocked anorthite. Linear deconvolution models demonstrate that combinations of common mineral and glass spectra can replicate the spectra of shocked anorthite relatively well until shock pressures of 20-25 GPa, above which model errors increase substantially, coincident with the onset of diaplectic glass formation. Albite deconvolutions exhibit higher errors overall but do not change significantly with pressure, likely because certain clay minerals selected by the model exhibit absorption features similar to those in highly shocked albite. The implication for deconvolution of thermal infrared spectra of planetary surfaces (or laboratory spectra of samples) is that the use of highly shocked anorthite spectra in end-member libraries could be helpful in identifying highly shocked calcic plagioclase feldspars.

High spatial resolution (10-30 μm), in situ oxygen isotopic analyses by secondary ion mass spectrometry (SIMS), coupled with high-resolution transmission electron microscopy (HRTEM), were used to show that uraninite from the Oklo-Okélobondo natural fission reactors that occur in near surface environments, have low δ 18 O values and nanotextures that are consistent with interaction with ground water. These low δ 18 O values (-14.4 to -8.5‰) suggest that the minerals exchanged with meteoric groundwater. In contrast, reactor zones that occur at depth have largely retained their original O isotopic composition (-10.2 to -5.6‰) and uraninites are well-crystallized and essentially defect-free. These observations clearly demonstrate that by combining both HRTEM and in situ O isotopic analyses by SIMS, it is possible to characterize the nano-scale porosity and postdepositional alteration of U-bearing phases.

The Rizhao garnet clinopyroxenite occurs as small mantle slices faulted into subducted continental crust in the Sulu ultrahigh-pressure (UHP) terrane, China. Two representative garnet clinopyroxenites with exsolution microstructures were studied: (1) a massive ilmenite-rich specimen, composed of coarse-grained clinopyroxene containing abundant exsolution rods of grossularrich garnet (>20 vol%) + ilmenite (~3-4 vol%) in a fine-grained matrix of clinopyroxene + garnet + ilmenite; and (2) megacrystic garnet-bearing sample, characterized by garnet containing inclusions of clinopyroxene and ilmenite; the clinopyroxene inclusions show exsolution rods and blebs with variable amounts of garnet (1-15 vol%) and minor ilmenite. Both exsolved and matrix garnets have similar grossular-rich compositions. The aggregate composition of clinopyroxene + garnet + ilmenite intergrowth is similar to the whole-rock composition. We propose that the parental phase of exsolved garnet + ilmenite lamellae and clinopyroxene host was either a homogeneous clinopyroxene (hypothesis A) or a majoritic garnet (hypothesis B) that experienced three discrete evolution stages. If the parental phase was clinopyroxene (ABO 3 ), with an Si deficiency in the B site, the Rizhao garnet clinopyroxenite could have an initial assemblage clinopyroxene ± Grt ± Ilm formed at nearsolidus conditions (≤ 1400 °C, at 5 GPa) in the upper mantle. The second stage is defined by garnet exsolution from the primary clinopyroxene involving decreasing temperature and/or increasing pressure, related to continental subduction. Coexisting clinopyroxene host and garnet exsolution (Grt exs ) recrystallized at temperatures of ~900 °C at an assumed minimum P of 5 GPa. The third stage is represented by recrystallization of exsolution phases to form the fine-grained matrix of Cpx + Grt + Ilm at 700 °C and ≥ 3 GPa during the early exhumation of the UHP terrane. The speculative hypothesis B implies that the proposed majoritic garnet was from the mantle transition zone (<450 km depth); this suggestion remains to be tested.

With recent findings of substantial amounts of B in rock-forming minerals, it has become evident a need exists for microprobe techniques allowing quantitative B determinations in minerals. A microprobe method based on the nuclear reaction 11 B(p,α)2α has the potential to become a powerful tool for analyses of minerals with B contents spanning a wide concentration range. In this paper, we present nuclear microprobe analyses of B in a set of silicate minerals containing stoichiometric concentrations of B in the range 1.9-8.8 wt%. Our results strongly indicate that nuclear reaction analyses of silicates utilizing the 11 B(p,α)2α nuclear reaction are virtually matrix-insensitive, and saturation effects are negligible within the studied range of B concentrations. This finding suggests that it is adequate to use a single B standard for calibration purposes for analyses covering wide ranges of B concentrations in silicates. In addition, we demonstrate how B-concentration maps of high contrast and resolution may be produced by scanning the proton beam over the samples.

Electron-diffraction patterns of clinopyroxene with an intermediate composition between kosmochlor and diopside show the presence of satellite reflections in diffuse streaks running perpendicular to (111) planes of the C2/c structure. The position of these satellite reflections indicates the existence of an incommensurate (INC) phase. Based on the characteristics of the diffraction patterns, it is suggested that the INC phase consists of a regular arrangement of kosmochlor-rich and diopside-rich slabs with a periodicity 2.6 to 6.0 times the (111) spacing in the C2/c structure. This is a new ordering scheme in clinopyroxenes.

Sound velocities and single-crystal elastic moduli of hydrous γ-Mg 2 SiO 4 (ringwoodite) containing 2.3 wt% H 2 O have been measured by Brillouin spectroscopy at ambient conditions. The aggregate elastic moduli (VRH averages) are K S = 165.8(5) GPa and m = 107.4(3) GPa for the adiabatic bulk modulus and shear modulus, respectively. Although the aggregate elastic moduli and acoustic velocities of hydrous ringwoodite are smaller than those of anhydrous ringwoodite by about 10% and 3.6% respectively, our results indicate that water has a smaller effect on the elastic moduli and velocities of ringwoodite than implied by previous studies. Our new results are consistent with systematic relationships between changes in the elastic properties and density that accompany the hydration of mineral phases.

Melanophlogite is a naturally occurring clathrasil possessing a framework of linked silicate tetrahedra surrounding small, isolated cages, which can host small molecules. The energetics of a guest-free natural sample was determined by oxide-melt solution calorimetry. Melanophlogite is energetically metastable with respect to α-quartz by 9.5 ± 0.5 kJ/mol, a value similar to that for amorphous silica and for synthetic small-pore zeolitic silicas (Petrovic et al. 1993, Piccione et al. 2001). Thus, its occurrence in nature, for example in environments where it can occlude volcanic gases, is reasonable on energetic grounds. Molecular modeling of the internal pore volume of melanophlogite confirms that this enthalpy follows the trend previously established for a variety of silica zeolites, which defines an internal surface energy of 0.093 ± 0.010 J/m2, similar to that of the external surface energy of amorphous silica. Thus melanophlogite, despite its unique topology and isolated cages, behaves energetically as predicted from the enthalpies of more-open zeolitic frameworks.

The isovolume locus for a diamond-mineral inclusion pair occurs where the relative volumes of the two minerals respond identically to changes in pressure-temperature (P-T) conditions. Thirty potential inclusion minerals have been assessed with a simple linear model of this system. The remnant pressure on the inclusion (diamond at the Earth’s surface) can be estimated by extrapolating the conditions of diamond formation to 0 °C using the slope of the isovolume locus. Remnant pressures can be positive (isovolume locus lower than formation pressure) or negative (locus higher than formation pressure), the latter indicating the inclusion has decompressed completely. When placed in order of increasing isovolume slope, this mineral list defines a pressure preservation index (PPI). Published work confirms that the model is quantitative up to diamond formation pressures of 50 Kb and qualitative beyond. Ten minerals are identified as key PPI indicators-in decreasing order of PPI they are: sanidine, coesite, dolomite, (sphene, garnet, diopside, zircon), magnesiowüstite, spinel, Mg 0.9 Fe 0.1 SiO 3 perovskite. For most diamond formation conditions, inclusions of the first three minerals will retain high remnant pressures, whereas the last three will decompress completely. The central four minerals in brackets will have a highly variable response because their isovolume loci pass through diamond formation conditions. Preliminary calculations suggest that fluid inclusions (H 2 O, CO 2 ) fall at or near the top of the list. For inclusion-bearing diamonds at the Earth’s surface, the model predicts: (1) remnant pressures; (2) confining pressure on inclusion during delivery; (3) the stability of high-pressure polymorphs; (4) retention vs. resetting of radiometric ages; (5) those minerals suited for determining the conditions of formation based on measured remnant pressure; (6) those minerals capable of stabilizing a microdiamond inclusion; and (7) that supercritical fluids preserve inclusions of microdiamond in most minerals.