Band 95, Heft 11-12

Forsterite (Mg 2 SiO 4 ) is a silicate mineral frequently studied in the Earth sciences as it has a simple crystal structure and fast dissolution kinetics (elemental release rates under typical conditions on the order of 10 -7 mol/m 2 /s 1 ). During the dissolution process, spectroscopic techniques are often utilized to augment solution chemical analysis and to provide data for determining reaction mechanisms. Nuclear magnetic resonance (NMR) is able to interrogate the local bonding arrangement and coordination of a particular nuclide to obtain structural information. Although previous NMR studies have focused on the silicon and oxygen environments in forsterite, studies focusing on the two nonequivalent magnesium environments in forsterite are limited to a few single-crystal studies. In this study, we present the results of 25 Mg MAS, MQMAS, and static QCPMG experiments performed on a powdered sample of a pure synthetic forsterite. We also present spectral fits obtained from simulation software packages, which directly provide quadrupolar parameters for 25 Mg nuclei occupying each of the two nonequivalent magnesium sites in the forsterite structure. These results are compared to calculations of the electric field gradient tensor conducted in previous ab initio studies to make definitive assignments correlating each peak to their respective magnesium site in the forsterite structure. Althought previous NMR investigations of forsterite have focused on single-crystal samples, we have focused on powdered forsterite as the increased surface area of powdered samples makes them more amenable to laboratory-scale dissolution studies and, ultimately, the products from chemical weathering may be monitored and quantified.

The crystal structures of biogenic Mn oxides produced by three fungal strains isolated from stream pebbles were determined using chemical analyses, XANES and EXAFS spectroscopy, and powder X-ray diffraction. The fungi-mediated oxidation of aqueous Mn 2+ produces layered Mn oxides analogous to vernadite, a natural nanostructured and turbostratic variety of birnessite. The crystallites have domain dimensions of ~10 nm in the layer plane (equivalent to ~35 MnO 6 octahedra), and ~1.5-2.2 nm perpendicularly (equivalent to ~2-3 layers), on average. The layers have hexagonal symmetry and from 22 to 30% vacant octahedral sites. This proportion likely includes edge sites, given the extremely small lateral size of the layers. The layer charge deficit, resulting from the missing layer Mn 4+ cations, is balanced mainly by interlayer Mn 3+ cations in triple-corner sharing position above and/or below vacant layer octahedra. The high surface area, defective crystal structure, and mixed Mn valence confer to these bio-minerals an extremely high chemical reactivity. They serve in the environment as sorption substrate for trace elements and possess catalytic redox properties.

Mullite (Al 4+2x Si 2-2x O 10-x ) is known to exhibit a very distinct compositional variation in which the lattice constant in the a direction expands linearly, while that in the b direction slightly contracts with an increase of the x-value. In this study, first-principles density functional theory (DFT) simulations were applied to examine the cause. The atomic structure and charge density were examined. We found that the local charge redistribution due to a newly formed vacancy leads to the onset and recurrence of localized atomic relaxation. The charge redistribution and atomic relaxation causes the clockwise and counterclockwise rotations of the neighboring octahedral units. These rotations contribute to expansion in the a-axis and contraction in the b-axis. The mechanism is supported by the previous experimental measurements of the Al-O2 bond lengths projecting in the (001) plane. We conclude that the rotations of the octahedral units are the fundamental mechanism responsible for compositionally induced variations of mullite. Results derived from simulations also provide evidence for a preferred occurrence of oxygen vacancies parallel to the crystallographic b axis. It thus supports earlier findings of a partial ordering in mullite.

Since its first discovery in nature, natrolite has been largely known as a sodium aluminosilicate zeolite, showing very limited preference toward cation exchange. Here we show that fully K-exchanged natrolite can be prepared from natural Na-natrolite under mild aqueous conditions and used to subsequently produce Rb- and Cs-exchanged natrolites. These cation-exchanged natrolites exhibit successive volume expansions by ca. 10, 15.7, and 18.5% for K-, Rb-, and Cs-forms, respectively, compared to the original Na-natrolite. This constitutes the largest, ever-reported volume expansion via cation substitution observed in zeolites and occurs by converting the elliptical channels into progressively circular ones. The observed cation-dependent changes in the channel volume and shape thus show the flexibility limits of the natrolite framework and suggest the possible existence of compositionally altered analogues in suitable environments as well as a novel means to tailor the cation selectivity of this class of small pore zeolites toward various industrial and environmental applications.

Mineralogical characterization of the precipitates developed in passive systems treating minepolluted waters is an essential tool to fully understand and control the removal processes taking place in these systems. In 2008, after five years of operation, a section of the permeable reactive barrier (PRB) at Shilbottle, Northumberland, was subjected to a low intrusive/non-destructive solid sampling. These solid samples were mineralogically characterized by XRD, ESEM-EDS, and sequential extractions. In addition to the solid sampling, 44 water samples obtained in the PRB from January 2004 to August 2009 were used to study the mineral stability of some selected phases in these waters. It was observed that the main iron phases in the PRB were those associated with mineral phases typically developed in non-reducing environments (schwertmannite and goethite), while the presence of a significant amount of pyrite was also observed. The low residence time of the water within the PRB (from 10 to 40 h) appears to be the reason for the absence of a more reducing and less acidic environment in the reactive substrate. An increase of residence time in the PRB, by increasing reactive mixture porosity and resizing the PRB, changes in the reactive material employed (smaller limestone grain size and inclusion of zerovalent iron) and changes in the PRB design (isolating top layer and forced homogeneous flow upward through all the reactive material) are proposed for future reconditioning of the system.

X-ray diffraction has been widely used in analyzing Ca-Mg carbonates. Compositions of biogenic and inorganic (Ca,Mg)CO 3 crystals are often calculated by comparing their d 104 values with published empirical curves. However, previous studies suggested that these curves do not apply to very high-Mg calcite and disordered dolomite. Based on synthesized high-Mg calcite and disordered dolomite, a new empirical curve between values of magnesian calcite d 104 and MgCO 3 content in the calcite-disordered dolomite solid-solution series is constructed. This new curve is consistent with the significant cell parameter changes accompanying the Mg-Ca cation disorder in dolomite, and it can help the characterization of the MgCO 3 content of both natural and synthetic magnesian calcite and disordered dolomite, especially for the mineral mixtures that are not suitable for other analysis methods.

Selected phlogopite flakes from Mt. Vulture in southern Italy were studied using a combination of single-crystal techniques: electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS), single-crystal X-ray diffraction (SCXRD), and micro-X-ray absorption near-edge spectroscopy (XANES). The latter technique was employed to analyze the structure of the Fe-K absorption edge over the region from 7080-8100 eV and to determine Fe 3+ /∑Fe at a micrometer scale, albeit with large error bars due to known effects of orientation on pre-edge energy. The annite component, Fe/(Mg-i-Fe), of the samples studied ranged from 0.16 to 0.31, the Ti content from 0.11 to 0.27 atoms per formula unit (apfii) and the Ba content from 0.03 to 0.09 apfu. SIMS analysis showed H 2 O (wt%) = 1.81-3.30, F (wt%) = 0.44-1.29, and Li 2 O (wt%) = 0.001-0.027. The intra single-crystal chemical variability for major/minor elements (Mg, Fe, Al, Ba, Ti, and K) was found particularly significant for samples VUT191_11 and PG5_1, less significant for the other samples of the set. SIMS data relative to crystals VUT187_24, VUT191_10, VUT191_11, and VUT187_28 showed a noteworthy variation in the concentrations of some light elements (H, Li, and F) with coefficient of variation CV (as 1σ%) up to -18% for H 2 O. The analyzed micas belong to the 1M polytype. Structure refinements using anisotropic displacement parameters were performed in space group Clim and converged at 3.08 <R < 3.63,3.32 <RW < 3.98%. Micro-XANES results yielded Fe 3+ /£Fe from 51-93%. Previous Mossbauer data from powdered samples suggested Fe 3+ /∑Fe values ranging from 49-87%. However, the Fe 3+ content determined by both techniques is sometimes remarkably different, in part because of the large errors (±10-15%) presently associated with the micro-XANES technique and m part because the Fe 3+ content of a single crystal may significantly depart from the average value obtamed from routine Mossbauer analysis. The combination of EMPA, SIMS, and micro-XANES resulted in the characterization of the samples at a comparable spatial scale. By means of in-situ data and the results of crystallographic investigations, the occurrence of different relative amounts of M3*-oxy [ VI M 2+ + (OH) - ↔ VI M 3+ + O 2- + ½H2↑], Ti-oxy substitutions [ VI M 2+ + 2(OH) - ↔ VI Ti 4+ + 2O 2- + H 2 ↑], and Ti-vacancy (⃞) substitution (2 VI M 2+ ↔ + VI Ti 4+ + VI ⃞) was ascertained for the studied samples.

The 10 Å phase is a high-pressure hydrous magnesium silicate whose composition appears to depend on synthesis conditions. We have measured the compressibility to 10.5 GPa and thermal expansivity to 400 °C of samples of 10 Å phase synthesized in long experiments (400 and 169 h, respectively) designed to maximize compositional equilibrium. The structure was refined using a metrically trigonal unit cell. Compression is highly anisotropic, especially over the first 2 GPa of compression, indicating weak bonding across the interlayer. There is an inflection in the compression curve of c at 8 GPa, suggesting a change in compression mechanism or the onset of non-hydrostaticity in the pressure medium. Fitting the compression data collected below 8 GPa to a Murnaghan equation-of-state gives V 0 = 734.8(7) Å 3 , K 0 = 25(1) GPa, K′ = 18(1). Thermal expansion is also strongly anisotropic: coefficients for data up to 200 °C are α a = 0.15(5) × 10-5 K -1 , α c = 3.1(2) × 10 -5 K -1 , α V = 3.4(2) × 10 -5 K -1 . Above 200 °C, the expansivity of c decreased, and all parameters showed a contraction after the experiment, suggesting partial dehydration at high temperatures. Comparison of our compressibility data with those of previous studies suggests that 10 Å phase synthesized in short experiments does not retain all of its interlayer H 2 O during quenching and decompression. In contrast, samples annealed for many hours at high pressure and temperature are stabilized by small amounts of hydrogarnet-type substitution and consequent hydrogen bond strengthening.

Interfacial tension between immiscible liquids is an important thermodynamic parameter of silicate melt unmixing and a property that determines the kinetics of phase separation. In this study, we present experimental measurements of interfacial tension between immiscible Fe-rich and silica-rich melts in the system K 2 O-FeO-Fe 2 O 3 -Al 2 O 3 -SiO 2 . We have also measured densities and surface tensions of the individual immiscible liquid phases. The measurements were carried out in air at 1500-1550 °C by the maximum detachment force method employing vertical cylinder geometry and using a gravimetric balance system. We have chosen the most oxidized and contrasting liquid compositions containing 73 and 17 wt% SiO 2 and 14 and 80 wt% FeOt, respectively, that have been shown to coexist in air at and above 1465 °C. Interfacial tension between the synthetic immiscible liquids decreases with increasing temperature from 16.4 ± 3.1 mN/m at 1500 °C to 7.8 ± 1.1 mN/m at 1550 °C. Interfacial tension between natural, less compositionally contrasting ferrobasaltic and rhyolitic melts should be even lower by a factor of 2 or 3. Very low interfacial tension implies easy nucleation of immiscible liquid droplets and very slow coarsening of resulting silicate emulsions.

As shown by in situ infrared spectroscopy and analysis of quenched samples, phyllosilicates (muscovite, sericite, pyrophyllite, and talc) under dehydroxylation conditions lack the characteristic bands near 1600 cm -1 (bending) and 5200 cm -1 (combination) of H 2 O, and they contain virtually no H 2 O but an abundance of OH. This observation appears to be at variance with the formal description of dehydroxylation in bulk samples as 2(OH) → H 2 O + O, whereas it is suggested that hydrogen diffuses in the form of (OH)- or/and H + in dehydroxylation. The upper limit of H 2 O in the dehydroxlated bulk is likely to be at the parts per million level in phyllosilicates that contain structural OH ions equivalent to 4-5 wt% H 2 O. The observations suggest that H 2 O molecules are probably formed near the surface of the sample.

Seven natural analcime samples with atomic Si/Al ratios from 1.97 to 2.63 were investigated to explore the effects of intermediate range structure and Al for Si substitution up to the fourth nearest neighbor coordination shell on the 29 Si NMR chemical shifts in the framework aluminosilicates. With increasing bulk Si/Al ratio, the 29 Si chemical shifts of all Si(nAl) resonances become more negative (more shielded), consistent with previously reported trends for faujasite and LTA zeolite (Newsam 1985). For our analcimes, the total observed changes in chemical shift for the Si(3Al), Si(2Al), and Si(1Al) sites are ~0.5, 0.6, and 1.1 ppm, respectively, demonstrating that the effect of Si/Al ratio is more significant for the Si sites with a smaller number of next-nearest neighbor Al atoms. The mean value of the change in chemical shift per added Al on fourth nearest neighbor sites is ~2.8 ppm [2.3 ppm if Si(3Al) is excluded]. This value is similar to the results of recent QM/MM calculations and is somewhat larger than those previously reported for faujasite and LTA framework zeolite (~1.4 and 1.3 ppm). This difference correlates with the overall denser structure of analcime, including smaller cages and shorter Si-fourth neighbor distances. Combining these results with the known changes in 29 Si chemical shifts for framework silicates due to changes in the first coordination shell, tetrahedral polymerization and second neighbor Al for Si substitution for tetrahedrally coordinated Si, we present an empirical relation between the changes in 29 Si chemical shift and interatomic distance between Si and nearby atoms.

The Mössbauer spectra of an albite glass and melt doped with 57 Fe (bulk composition NaFe 0.04 AlSi 3 O 8 ) were measured in situ to 1100 °C. A specially designed furnace was used where a disk of the sample was enclosed in a capsule of graphite. The furnace was purged with argon to avoid oxidation of the graphite, and shields of boron nitride (BN) were used to protect both the γ-ray source and the detector from thermal radiation. Room-temperature spectra show contributions from paramagnetic Fe 2+ , paramagnetic Fe 3+ , and magnetic Fe 3+ . The signal of Fe 2+ disappears around 400 °C, and the signal of Fe 3+ disappears at about 1100 °C. This behavior cannot be due to changes in the oxidation state of iron because in situ visible and near infrared spectra of the same sample show that Fe 2+ persists to at least 1000 °C and the quenched samples do not show any change in oxidation state. Rather, the disappearance of Fe 2+ and Fe 3+ in the spectra is likely due to the structural relaxation time around these cations becoming comparable to the Mössbauer timescale (lifetime of excited state ≈ 10 -8 s). We show that the temperature at which the Fe 3+ contribution disappears corresponds to the expected glass transition temperature of a “ferrialbite” NaFeSi 3 O 8 melt for a timescale of 10 -8 s. Our data therefore confirm that viscous flow is due to the rearrangement of the chemical bonds between oxygen anions and network-forming cations. Furthermore, our data show that Mössbauer spectroscopy can be used to study the speciation and relaxation time around iron in silicate melts even at temperatures above the macroscopic glass transition temperature as defined from viscosimetry and calorimetry.

We present in situ measurements of the unit-cell volume of a natural terrestrial ilmenite (Jagersfontein mine, South Africa) and a synthetic reduced ilmenite (FeTiO 3 ) at simultaneous high pressure and high temperature up to 16 GPa and 1273 K. Unit-cell volumes were determined using energy-dispersive synchrotron X-ray diffraction in a multi-anvil press. Mössbauer analyses show that the synthetic sample contained insignificant amounts of Fe 3+ both before and after the experiment. Results were fit to Birch- Murnaghan thermal equations of state, which reproduce the experimental data to within 0.5 and 0.7 GPa for the synthetic and natural samples, respectively. At ambient conditions, the unit-cell volume of the natural sample [V 0 = 314.75 ± 0.23 (1σ) Å 3 ] is significantly smaller than that of the synthetic sample [V 0 = 319.12 ± 0.26 Å 3 ]. The difference can be attributed to the presence of impurities and Fe 3+ in the natural sample. The 1 bar isothermal bulk moduli KT0 for the reduced ilmenite is slightly larger than for the natural ilmenite (181 ± 7 and 165 ± 6 GPa, respectively), with pressure derivatives K 0 ′ = 3 ± 1. Our results, combined with literature data, suggest that the unit-cell volume of reduced ilmenite is significantly larger than that of oxidized ilmenite, whereas their thermoelastic parameters are similar. Our data provide more appropriate input parameters for thermo-chemical models of lunar interior evolution, in which reduced ilmenite plays a critical role.

Metamict zircon crystals have been thermally annealed between 500 and 1800 K and analyzed using infrared and optical spectroscopy in the spectral region of 1400-7000 cm -1 . Recrystallization and dehydroxylation via complex proton/OH diffusion, redistribution, and incorporations of additional hydrogen-related species within the crystal structure of zircon occur at temperatures above 700 K in partially metamict zircon and above 1200 K in heavily amorphized material. Thermally induced changes in O-H stretching spectra are different between E || c and E ⊥ c in weakly metamict zircon. The O-H stretching band near 3342 cm -1 (with E ⊥ c) in an untreated sample shifts to 3277 cm -1 at 1200 K, where the frequency of O-H stretching bands with E || c increases. Conversions of hydrogen-related species were observed and extra OH bands were found at temperatures between 1200 and 1600 K. A dramatic change of OH spectra was recorded between 1600 and 1800 K in partially metamict crystals, resulting in additional absorption features (near 3098 and 2998 cm -1 along E ⊥ c). U 4+ and U 5+ related spectra are also affected by high-temperature annealing. For highly metamict zircon, the U 4+ band near 4830 cm -1 shows an increase in intensity above 1200 K. Additional IR bands at 2146 and 2344 cm -1 appear in the spectra of metamict zircon annealed at high temperatures. Their frequencies are consistent with stretching vibrations of CO and CO 2 .

The present paper reports a crystal-chemical study of minerals belonging to the proustite-pyrargyrite group by evaluating 32 natural samples of proustite and 27 samples of pyrargyrite from different localities that have variable chemical compositions. The unit-cell parameters were modeled as a function of the Sb contents. The a parameter is strongly influenced by the As ↔ Sb substitution, whereas the influence on the c parameter is very minor (nearly constant trend). The following equations were obtained from the linear fitting of the data: a pred = 10.8433(3) + 0.2019(4)Sb (apfu) c pred = 8.7189(6) + 0.0059(9)Sb (apfu) V pred = 887.77(7) + 34.0(1)Sb (apfu). The crystal structure of the members of the proustite-pyrargyrite solid solution consists of two sets of spiral chains parallel to the c-axis. Each chain contains alternating Ag and S atoms, with each chain being connected by As and Sb atoms, which are the apices of flat pyramidal AsS 3 /SbS 3 groups. Each S atom is part of a different Ag-S chain. The compositional data support the concept that proustite-pyrargyrite solid solutions re-equilibrate and exsolve to near end-member upon cooling. Examples of intermediate compositions are rare in nature and must have quenched above the solvus.

Biotic and abiotic oxidation experiments were performed on ground pyrite grains. MicroRaman (μRaman) spectroscopy was used to characterize and statistically analyze the reaction products formed at the pyrite surface. Reaction compounds consist of iron and sulfur-oxy species for all experiments including various amounts of sulfates, iron oxi-hydroxides, polysulfides, and elemental sulfur as well as scarce amounts of thiosulfate. These compounds are distributed as micrometric to submicrometric bumps. μRaman spectroscopy allowed for clear recognition of four phases of iron oxi-hydroxides and, without any correlation, four molecular structures of sulfate groups. Peaks associated with the oxidation products were assigned and are discussed according to hydration and to most probable chemical bonds with pyrite surfaces. Sulfates formed in solution (Td local symmetry) are distinguished from anhydrous sulfates (C 3v to 0 local symmetries) formed directly at the surface of pyrite grains. Proposed structures are related to the surrounding chemical properties (e.g., local acidity) and to the heterogeneous electronic properties of pyrite grains. The distribution and combination of oxidation compounds at the surface of pyrite grains provide clues that distinguish bio-oxidized from chemically oxidized pyrite surfaces. Bio-oxidized surfaces are characterized by the exclusive presence of C 3v sulfates that reflect active electronic circulations within the pyrite lattice. In contrast, air-oxidized pyrite surfaces exhibit a high proportion of 0 symmetry sulfates adsorbed on hematite bumps indicating a strong passivation of the mineral surface.

Various manganese valence quantification methods using manganese L 2,3 and oxygen K electron energy loss near-edge spectra (ELNES) were applied to determine the relative portions of individual valence state in a mixed (Mn 3+ , Mn 4+ ) valence system. Multiple linear least-squares (MLLS) fitting of Mn L 2,3 ELNES using reference spectra and Gaussian peak fitting of Mn L 3 edge are newly developed and the feasibility of these methods was tested on a set of cryptomelane minerals with different valence states. The selection of appropriate standards is crucial to the success of the MLLS method. The O K-edge structures for manganese oxides can provide valuable guidance in the selection of appropriate reference spectra for quantitative determination of Mn valence state. Gaussian peak fitting, however, failed to determine the Mn valence for (Mn 3+ , Mn 4+ ) minerals due to the small separation between the primary L 3 peaks from Mn4+ and Mn3+ valence. As to the methods based on calibration curves, the energy difference between Mn L 3 and oxygen K, i.e., ΔE (L 3 -O K) vs. valence, is the most valence-sensitive method in the range of Mn 3+ and Mn 4+ and yields good agreement with the actual values.

The epitaxial overgrowth of brushite (CaHPO 4 ·2H 2 O) by the interaction of phosphate-bearing, slightly acidic, aqueous solutions with gypsum (CaSO 4 ·2H 2 O) was investigated in situ using atomic force microscopy (AFM). Brushite growth nuclei were not observed to form on the {010} gypsum cleavage surface, but instead formed in areas of high dissolution, laterally attached to gypsum [101] step edges. During the brushite overgrowth the structural relationships between brushite (Aa) and gypsum (A2/a) result in several phenomena, including the development of induced twofold twining, habit polarity, and topographic effects due to coalescence of like-oriented crystals. The observed brushite growth is markedly anisotropic, with the growth rate along the main periodic bond chains (PBCs) in the brushite structure increasing in the order [101] > [101] > [010], leading to tabular forms elongated on [101]. Such a growth habit may result from the stabilization of the polar [101] direction of brushite due to changes in hydration of calcium ions induced by the presence of sulfate in solution, which is consistent with the stabilization of the gypsum [101] steps during dissolution in the presence of HPO 2- 4 ions. The coupling between growth and dissolution was found to result in growth rate fluctuations controlled by the changes in the solution composition.

The high-pressure behavior of protoamphibole (space group Pnmn) was studied by in situ singlecrystal X-ray diffraction on a sample of protomangano-ferro-anthophyllite with formula (Mn 1.39 Fe 0.59 ) (Fe 3.98 Mg 1.02 )Si 8 O 22 (OH) 2 , from Yokone-Yama, Awano Town, Tochigi Prefecture, Japan. Unit-cell parameters were collected at various pressures up to 9 GPa, and structural refinements were obtained from data collected at several pressures up to 7 GPa. Fitting the P-V data to a third-order Birch- Murnaghan equation of state (EoS) gives the following parameters: K T0 = 64(1) GPa, K′ = 7.0(4), and V 0 = 926.4(4) Å 3 . Axial moduli are: K 0a = 30.7(8) GPa, K′ a = 10.8(5), and a 0 = 9.430(2) Å; K 0b = 109(4) GPa, K′ b = 2.7(8), and b 0 = 18.364(4) Å; K 0c = 94(5), GPa, K′ c = 4(1), and c 0 = 5.354(2) Å. The corresponding axial compressibilities (10 -3 GPa -1 ) are β a = 10.9(3), β b = 3.1(1), and β c = 3.5(2), and indicate that the HP behavior of protomangano-ferro-anthophyllite is highly anisotropic, the highest compressibility being along [100]. No discontinuous behavior or polymorphic transitions were observed in the pressure range studied. Structural refinements show that M1, M2, and M3 polyhedra have similar compressibilities, owing to their similar composition. M4 (72% Mn, 28% Fe) is a highly distorted site and is slightly softer than the other octahedra. The major movements in the tetrahedral ribbon concern kinking of the double chain, bending along the [100] direction through the empty A site, and tetrahedral rotation, necessary to maintain coherence with the octahedral layer. The kinking angle of O5-O6-O5, which in air is 179.1(2)°, decreases to 174(2)° at 6.9 GPa. The T1-O7-T1 angle changes from 143.8(3)° to 134(5)° at 6.9 GPa, and the tetrahedral rotation α increases from 0.2(2)° to 4(2)°.

Hydrous wadsleyite (β-Mg 2 SiO 4 ) with 2.8 wt% water content has been synthesized at 15 GPa and 1250 °C in a multi-anvil press. The unit-cell parameters are: a = 5.6686(8), b = 11.569(1), c = 8.2449(9) Å, β = 90.14(1)°, and V = 540.7(1) Å 3 , and the space group is I2/m. The structure was refined in space groups Imma and I2/m. The room-pressure structure differs from that of anhydrous wadsleyite principally in the increased cation distances around O1, the non-silicate oxygen. The compression of a single crystal of this wadsleyite was measured up to 61.3(7) GPa at room temperature in a diamond anvil cell with neon as pressure medium by X-ray diffraction at Sector 13 at the Advanced Photon Source, Argonne National Laboratory. The experimental pressure range was far beyond the wadsleyite-ringwoodite phase-transition pressure at 525 km depth (17.5 GPa), while a third-order Birch-Murnaghan equation of state (EoS) [V 0 = 542.7(8) Å 3 , KT0 = 137(5) GPa, K′ = 4.6(3)] still fits the data well. In comparison, the second-order fit gives V 0 = 542.7(8) Å 3 , KT = 147(2) GPa. The relation between isothermal bulk modulus of hydrous wadsleyite K T0 and water content CH 2 O is: K T0 = 171(1)-12(1) CH 2 O (up to 2.8 wt% water). The axial-compressibility β c is larger than both β a and β b , consistent with previous studies and analogous to the largest coefficient of thermal expansion along the c-axis.

The dehydration-rehydration process in gmelinite-Na |Na 7.27 K 0.28 Ca 0.15 (H 2 O) 21.85 |[Al 7.71 Si 17.24 O 48 ]- GME, a natural zeolite that can be described as a parallel stacking of double six rings of tetrahedra in the AABB sequence, was studied by X-ray diffraction data. Its space group is P6 3 /mmc with a = 13.764(1) and c = 10.078(1) Å cell parameters. Single-crystal data collections were performed at room conditions and at increasing temperatures, in a hot nitrogen stream, up to the fragmentation of the crystals, which occurs at a temperature as low as 100 °C, and afterward the crystal was cooled down to room conditions. X-ray powder diffraction data showed that gmelinite-Na transforms into a new structure with an AFI-type topology at about 300 °C. At room conditions, extraframework cations are located in two symmetrically independent positions, both of which are coordinated to either framework O atoms or water molecules. When the mineral is heated to 90 °C, about 40% of H 2 O is lost, and one cation site splits over two positions, which are three-coordinated to the framework O atoms. The dehydration process is completely reversible over a period of hours. X-ray single-crystal data has highlighted that gmelinite-Na when quenched at 100 K displays remarkable modifications in its extraframework content, resulting in a strong disorder in its extraframework ions. As in the case of heating, the mineral restores its structural features when brought back to room temperature.

Alkali feldspars in syenite from the Cerro Balmaceda pluton in the Patagonian Andes, Chile, show various petrographic microtextures formed during the magmatic to high- and low-temperature hydrothermal stages in which cathodoluminescence (CL) shows a wide range of blue, violet, and pink to red colors with variable brightness. Their CL spectra exhibit two emission bands: one at 405-420 nm in the blue region and the other at 700-760 nm in the red-infrared (IR) region. Asymmetrically shaped spectral peaks in energy units suggest overlapping of each individual emission, which corresponds to various luminescence centers. Blue emission bands were separated into two spectral peaks fitted by Gaussian curves centered at 3.055-3.076 and 2.815-2.845 eV. A positive correlation is found between emission intensities at 3.055-3.076 eV and TiO 2 contents, suggesting the activation of a Ti 4+ impurity as an emission center. The intensities at 2.815-2.845 eV, where clear and featureless feldspar (CF; not affected by hydrothermal metasomatism) is shown under optical microscopy, which have intensities appreciably higher than those showing patched microperthite (PMP), formed during low-temperature hydrothermal reactions, correlate reciprocally with the intensities of red-IR emission caused by a Fe 3+ impurity center. The peak at 2.815-2.845 eV can be attributed to oxygen defects associated with Al-O-Al and Al-O-Ti bridges. Most of the areas show CL emissions at 700-760 nm in the red-IR region, in which intensities increase with an increase in Fe 2 O 3 contents as impurities. The Fe 3+ ion acts as an activator for the red-IR emission. The Ab-rich and Or-rich phases of PMP have emission components at 1.644 eV (754 nm) and 1.727 eV (717 nm), respectively. The red-IR emission from CF consists of emission components at 1.677 eV (739 nm) and 1.557 eV (796 nm), according to an Fe 3+ impurity center in the Or-rich phase and in the Ab-rich phase as cryptoperthite, respectively. Both components are centered at a wavelength longer than the emission band of Ab-rich and Or-rich phases of PMP, suggesting a change in configurational state around the Fe 3+ ion from the T2 to the T1 site by low-temperature hydrothermal metasomatic reactions. Accordingly, the peak positions of the red-IR emission are controlled by the ordering state of Fe 3+ ion into the T1 site, the existence of multiphase perthite and chemical composition.

Granulite-facies quartzites from the Adirondack Highlands (Grenville Province, New York) contain accessory zircon with ubiquitous metamorphic overgrowths. Detrital zircon cores are resorbed, preserve various internal zoning styles and inclusions, and have ages of 1.3 to 2.7 Ga. These ages constrain the timing of deposition of the protolith sandstone and suggest that the Adirondack Highlands were proximal or part of Laurentia during deposition. Metamorphic overgrowths formed in the quartzites during the Shawinigan orogeny (1.20-1.14 Ga). The average volume of overgrowths in eight samples ranges from 62-87%, with a positive correlation between zircon growth and feldspar content (melt productivity during metamorphism). Melt volumes and Zr solubility were too small to allow the overgrowths to have formed in one dissolution-precipitation event. Crystal-size distributions suggest zircon coarsening by the dissolution of small crystals and Zr transfer via a partial melt, and thus zircon overgrowths date anatexis. These results have implications for provenance studies, as dissolution of small zircon grains and growth of new zircon could bias age spectra of metasedimentary rocks.

The effect of dissolved silica on the PVT properties of H 2 O and structure of silica-saturated aqueous fluids in equilibrium with quartz in the SiO 2 -H 2 O system has been determined in situ with the materials at temperature (up to 800 °C) and pressure (up to 1350 MPa) in a constant-volume hydrothermal diamond anvil cell. Pressure was measured with the Raman shift of 13C synthetic diamond and was also calculated from the PVT properties of pure H 2 O. The two sets of pressures thus obtained differ by ≤50 MPa at T < 500 °C. At higher temperatures (and pressures), the pressure difference increases and reaches about 350 MPa at 800 °C. The structure of the silica-saturated aqueous fluid was probed with microRaman and microFTIR methods. Coexisting molecular H 2 O and OH-groups, bonded to Si 4+ , exist above ~400 °C and ~0.4 GPa with their abundance-ratio, OH/H 2 O°, positively correlated with temperature. Hydrogen bonding diminishes with temperature and cannot be detected in the silica-saturated aqueous fluid above ~400 °C and ~0.4 GPa. This behavior resembles that of pure H₂O under similar temperature and pressure conditions. Speciation of dissolved silica in the fluid at 400 °C/760 MPa, and above, comprises Q 0 and Q 1 species, whereas at lower temperature and pressure only Q 0 species were detected. The Q 1 /Q 0 abundance ratio is positively correlated with temperature (and silica content). An excess volume of mixing, V excess H₂O , derived from a comparison of EOS of pure H 2 O and those of silicate-saturated H 2 O, is related to the Q¹ / Q⁰ abundance ratio in silicate-saturated fluid, XQ 1 /X Q⁰ . The V excess H₂O = M H₂O (1/d m-1 H₂O /0.88), V excess H₂O ~ 0 when X Q¹ /X Q⁰ approaches 0.

The study by Thy, Tegner, and Lesher (Thy et al. 2009b) is based on whole-rock compositions in batch-melting experiments at one atmosphere that have a very large temperature variation for a very small plagioclase compositional variation. Their steep T-X slopes of up to 14 °C/An% contrast with the overall regression for the intrusion of <4 °C/An%. This paradox can be evaluated by using separate tests for equilibrium in temperature and composition. The issue of temperature is clouded by the presence of low-temperature components in the beginning of melting experiments. Compositions of plagioclase and liquid are too close to each other compared to expectations from other experiments at 1 atm and at pressure. A ruling hypothesis is perceived here to the effect that every T-X pair represents a valid equilibrium between a liquidus composition and a solidus composition. Therefore, by inversion, each solidus composition defines a valid temperature. Its corollary is that all such pairs receive equal weight and can therefore be legitimately regressed together. The Thy et al. (2009b) regression is passed through the center of gravity of the data rather than through the extrapolated maximum liquidus points. If the ruling hypothesis is falsified, these approaches lose their validity; upon testing, it is falsified. In temperature, testing leads to consequences known to be false. In composition, a test for reproducibility in results also fails, thereby falsifying the principle of stable equilibrium. The falsifications are repeated in at least three ways. The study results cannot furnish a valid thermal path for the fractional crystallization of Skaergaard magma. A selection of the highest-temperature results comes closer to a realistic result based on comparison with earlier experiments that melted out the trapped liquid component of the Skaergaard cumulates. New experimental approaches at the liquidus are needed to help define temperatures and fractionation progress in the Skaergaard intrusion.