Band 98, Heft 1

Joint refinement of powder neutron diffraction data of hydrogenated and deuterated brucite over the temperature range of 10-295 K clearly shows significant isotope effects in the structural parameters. Mg(OH) 2 has a 0.31% larger volume than that of Mg(OD) 2 at room temperature, which is mostly due to the c-axis expansion of Mg(OH) 2 as compared to Mg(OD) 2 . The isotope effect in the a-axis has the opposite, but smaller, behavior as compared to the c-axis. These differences are slightly enhanced with reduction of the cell volume upon cooling. The temperature dependence of the isotropic atomic displacement parameters (ADPs) with the single site model show that the ADP of the H atom is approximately larger than that of the D atom by the amount expected from the reduced-mass difference, but this difference is not evident with the split site model. Despite the shorter c-axis of the deuterated form, nearest-neighbor D···D distances are longer than the H···H distances, because the O-H distances are longer than the O-D distances, which necessarily places the H atoms closer together by 0.03 Å or more within the interlayer space. This study showcases an example of when a joint Rietveld refinement is ideally suited, by combining data for the deuterated and hydrogenated forms of brucite. The approach reduces the number of least-squares variables, and reduces the systematic errors. It can be a general method to analyze isotope effects in materials studied by neutron diffraction.

We performed hydration experiments of pure and Nb-, Cr-, and V-doped synthetic dry (H 2 O < 3 ppm) single rutile crystals. They were equilibrated with pure H 2 O in hydrothermal experiments at constant conditions of 600 °C, 400 MPa, and fO2 near the Ni-NiO buffer, run time between ~25 min and 14 days. Slabs cut parallel to (110) of the reacted single crystals (1 to 2 mm3) were analyzed for H + by FTIR. Hydration occurs almost spontaneously and the H 2 O-equivalent is uniformly distributed in the samples, but depends extremely on trace element contents. In pure rutile, the average H 2 O-content is 314 ± 50 ppm, the saturation level at these conditions. Rutile doped with 500 ppm Nb has a lower average H 2 O content of ~235 ppm, rutile with 2000 ppm Cr has ~900 ppm H 2 O, and rutile with 2000 ppm V does not incorporate H 2 O. During stepwise heating at atmospheric pressure of a reacted Nbdoped rutile, H + is quickly released between 450 and 550 °C. UV-VIS spectra of unreacted colorless and reacted deep blue pure rutile show the rutile-characteristic sharp absorption edge in the UV spectra. The reacted rutile has a broad absorption band at 6500 cm -1 wavenumber attributed to intervalence charge transfer transition Ti 3+ +Ti 4+ → Ti 4+ +Ti 3+ . The reduction of Ti 4+ to Ti 3+ is charge balanced by the incorporation of H + . The Nb-doped rutile changed its color from light greenish-blue (untreated) to deep blue. In the untreated Nb rutile, the UV-VIS absorption band at 6500 cm -1 indicates that Nb 5+ is charge balanced by Ti 3+ . In the reacted Nb-rutile the absorption band is more intense, but compared with the pure rutile, H+ incorporation is lower by the equivalent of Ti 3+ reduced in the untreated rutile. Reacted Cr-rutile almost retains its brownish-orange color, but the spectrum shows a prominent Ti 3+ / Ti 4+ IVCT band at ~6400 cm -1 with moderate intensity considering the high-H 2 O contents of ~900 ppm. The high-H + contents are best explained by the reduction of Cr 4+ to Cr 2+ . The UV-VIS spectra of the dark-blue to opaque V-doped rutile show a very strong absorption toward low energies, which is likely caused by reduction of Ti 4+ to Ti 3+ for charge balance of V 5+ . This forms a deep narrow window of transmittance in the range 25 000-20 000 cm -1 , which causes the dark-blue color. To explore the possible use of H-in-rutile as a geohygrometer, geothermobarometer, and oxybarometer, we measured the H + content in a natural rutile crystal from a retrograded eclogite with a zoned trace element (Fe, Nb, and Zr) content. The crystal reveals a slight correlation between the variable H 2 O (~200 to 900 ppm) and its trace element concentrations. The observations indicate that the preservation of H + contents in this natural rutile is a complicated interplay of diffusive reequlibration of fast H + , slower Fe and very slow other trace elements. An interpretation of the H 2 O contents of the natural crystal in terms of f O2 or a H2O is not possible.

Under hydrothermal conditions, diffusion-driven solid-state reactions can compete with fluid-mediated reaction mechanisms. We have obtained an insight into the complex textures resulting from this competition by studying experimentally the transformation of Au-Ag-telluride sylvanite to Au-Ag alloy under hydrothermal conditions, and exploring the effects of temperature (160-220 °C), pH (2-10), and redox conditions on the sample textures and the reaction kinetics. Sylvanite transformed to Au-Ag alloy over all hydrothermal conditions investigated, but not under dry conditions. The replacement was pseudomorphic, as the Au-Ag alloy preserved the external dimensions of the sylvanite grains. The resulting Au-Ag alloy was porous, consisting of worm-like aggregates with diameters ranging from 200 nm to 1 μm. In addition to Au-Ag alloy, a range of other phases were observed as intermediate products, including petzite (Ag 3 AuTe 2 ), hessite (Ag 2 Te), and two compositions of calaverite: an Ag-rich-Te-depleted composition, (Au 0.78 Ag 0.22 )Te 1.74 , and a normal calaverite, (Au 0.93 Ag 0.07 )Te 2 . The transformation of sylvanite to Au-Ag alloy follows a complex reaction path, with competing reactions proceeding either via interface-coupled dissolution and reprecipitation (ICDR) mechanism or via solid-state exsolution. Initially, sylvanite was replaced by an Au-Ag alloy following an ICDR mechanism, with sylvanite dissolution being the rate-limiting step relative to Au-Ag alloy precipitation. Tellurium was lost to the bulk solution as tellurite or telluride complexes, depending on the redox conditions. Once the concentration of Te in solution reached a critical state, the reaction switched and sylvanite dissolution was coupled to the precipitation of an Ag-rich-Te-depleted calaverite. This Agrich- Te-depleted calaverite decomposes via exsolution to calaverite and phase X (Ag 3+x Au 1-x Te 2 with 0.1 < x < 0.55), which in turn breaks down to a mixture of low petzite and low hessite below 120 °C via exsolution. As the reaction continues, the calaverite and phase X are all transformed to Au-Ag alloy via ICDR. In the ICDR reactions the Au-Ag alloy precipitated locally near the telluride dissolution site. Such local Au-Ag alloy precipitation is facilitated by fast heterogeneous nucleation onto the sylvanite, calaverite, and petzite surfaces. The dissolution of sylvanite and of the intermediate telluride species, and the overall reaction, are oxidation reactions. The diffusion of oxygen through the porous Au-Ag alloy layer plays an important role in sustaining the reaction. A similar combination of dissolution-reprecipitation and solid-state processes may be responsible for the formation of some of the Au and Au-Ag telluride assemblages observed in Nature. These processes may also play a role in the formation of mineral assemblages in Cu-Fe sulfide systems, where the solid-state mobility of Cu + ions is relatively high at moderate temperatures. The interplay of different reaction mechanisms results in complex textures, which could easily be misinterpreted in terms of complex geological evolution. At 220 °C, solid-state replacement of sylvanite by Au-Ag alloy is slow (months), but under hydrothermal conditions sylvanite grains ~100 μm in size can be fully replaced in as little as 96 h, providing a possible alternative to roasting as a pre-treatment of telluride-rich gold ores.

Synchrotron X-ray powder diffraction and infrared (IR) spectroscopy studies on natural brucite were conducted up to 31 GPa using diamond-anvil cell (DAC) techniques at beamlines X17C and U2A of National Synchrotron Light Source (NSLS). The lattice parameters and unit-cell volumes were refined in P3̄m1 space group throughout the experimental pressure range. The anisotropy of lattice compression decreases with pressure due to a more compressible c axis and the compression becomes nearly isotropic in the pressure range of 10-25 GPa. The unit-cell volumes are fitted to the third-order Birch-Murnaghan equation of state, yielding K 0 = 39.4(1.3) GPa, K 0 ′ = 8.4(0.4) for the bulk modulus and its pressure-derivative, respectively. No phase transition or amorphization was resolved from the X-ray diffraction data up to 29 GPa, however, starting from ~4 GPa, a new infrared vibration band (~3638 cm -1 ) 60 cm -1 below the OH stretching A 2u band of brucite was found to coexist with the A 2u band and its intensity continuously increases with pressure. The new OH stretching band has a more pronounced redshift as a function of pressure (-4.7 cm -1 /GPa) than the A 2u band (-0.7 cm -1 / GPa). Comparison with first-principles calculations suggests that a structural change involving the disordered H sublattice is capable of reconciling the observations from X-ray diffraction and infrared spectroscopy studies.

Feldspar crystals typically show a range of exsolution and polysynthetic twinning textures that can present problems for their full characterization, but at the same time give important information about their genesis. We present an integrated procedure for the micro-texture analysis, twin law identification plus crystal structure refinement of all components in a feldspar intergrowth. This procedure was applied to perthitic intergrowths in feldspars from two different pegmatites in the Larvik plutonic complex in the southern part of the Oslo region, Norway. It revealed that the two starting high-temperature (HT) feldspars had similar global chemical compositions but underwent significantly different cooling histories, with cooling times probably differing by over an order of magnitude. Powder X-ray diffraction with Rietveld refinement was used for a preliminary identification of the mineral components and concluding quantitative phase analysis. Electron microprobe analysis was used to bracket the chemical compositions of the constituents. Electron backscatter diffraction was used to reveal the texture of the samples, twin laws and spatial distribution and crystallographic orientation of the crystal domains. Single-grain X-ray diffraction recorded by an area detector was applied for a simultaneous integration of reflection intensities for all crystallographic domains with different orientations and severe diffraction overlaps. The crystal structures were refined using the program JANA2006 that allows a simultaneous calculation for structurally different components. Combined results of various methods helped improve accuracy and resolve ambiguities that arise from the application of a single technique. The approach is widely applicable to the study of mineral intergrowths and bridges an existing gap in the routinely accessible data on the structural characteristics of rock constituents.

Microstructural and geochronological analysis of shocked zircon has greatly advanced understanding the formation and evolution of impact structures. However, fundamental aspects of shock-produced planar microstructures in zircon remain poorly known, such as their deformation mechanisms, crystallographic orientations, and how planar microstructures visible at the grain scale by scanning electron microscopy correlate to microstructures visible at sub-micrometer scales by transmission electron microscopy and electron backscatter diffraction (EBSD). To unify observations of planar microstructures in zircon made at different scales into a consistent framework, we integrate the results of: (1) three-dimensional crystallographic modeling of planar microstructure orientations, with (2) 360° external prism backscattered electron imaging at the grain scale, and (3) polished section cathodoluminescence and EBSD analysis at the sub-micrometer scale for a suite of detrital shocked zircons eroded from the Vredefort Dome in South Africa. Our combined approach resulted in the documentation of seven planar microstructure orientations that can be correlated from grain to sub-micrometer scales of observation: (010), (100), (112), (11̄2), (1̄12), (1̄1̄2), and (011). All orientations of planar microstructures exhibit minor variations in style, however all are considered to be fractures; no amorphous ZrSiO 4 lamellae were identified. We therefore favor the usage of “planar fracture” (PF) over “planar deformation feature” (PDF) for describing the observed planar microstructures in zircon based broadly on the nomenclature developed for shocked quartz. Some {112} PFs visible at the grain scale contain impact microtwins detectable by EBSD, and are the first report of polysynthetic twinning in zircon. The microtwins consist of parallel sets of thin lamellae of zircon oriented 65° about <110> and occur in multiple crosscutting {112} orientations within single grains. Curviplanar fractures and injected melt are additional impact-related microstructures associated with PF formation. Crosscutting relations of shock microstructures reveal the following chronology: (1) Early development of c-axis parallel PFs in (010) and (100) orientations; (2) the development of up to four {112} PFs, including some with microtwins; (3) the development of curviplanar fractures and the injection of impact derived melt; (4) the development of (011) PFs associated with compressional deformation; and (5) grain-scale non-discrete crystal plastic deformation. Experimental constraints for the onset of PFs, together with the absence of reidite, suggest formation conditions from 20 to 40 GPa for all of the planar microstructures described here.

Mixed-habit diamonds have experienced periods of growth where they were bounded by two surface forms at the same time. Such diamonds are relatively rare and therefore under-investigated. Under certain physical and chemical conditions, smooth octahedral faces grow concurrently with rough, hummocky cuboid faces. However, the specific conditions that cause this type of growth are unknown. Here we present a large array of spectroscopic data in an attempt to investigate the impurity and carbon-isotope characteristics, as well as growth conditions, of 13 large (>6 mm diameter) plates cut from mixed-habit diamonds. The diamonds all generally have high nitrogen concentrations (>1400 ppm), with the octahedral sectors enriched by 127-143% compared to their contemporary cuboid sectors. Levels of nitrogen aggregation are generally low (2-23% IaB) with no significant difference between sectors. IR-active hydrogen features are predominantly found in the cuboid sectors with only very small bands in the octahedral sectors. Platelet characteristics are variable; only one sample shows a large B′ band intensity in the octahedral sector, with no platelets occurring in the cuboid sector. Other samples either show a small B′ band in both sectors, or just in the cuboid sector, or none at all. These data support a model that shows the concentration-adjusted aggregation rate of nitrogen to be the same in both sectors, whereas the subsequent platelet development is reduced in the cuboid sectors. This is because the interstitial carbon atoms have interacted with disk-crack-like defects only found in cuboid sectors, which in turn reduces their chances of aggregating to form platelets. These disk-crack-like defects are also thought to be the most likely site for the IR-active hydrogen features and they maybe intrinsic to cuboid growth in mixed-habit diamonds. When they are graphitized, as they are in all of the diamonds in this study, this may reflect a heating event prior to volcanic exhumation. Spectroscopic analysis of the green cathodoluminescence exhibited by all of the diamonds shows nickel centers to be present in only the cuboid sectors. Carbon isotope data, obtained by secondary ion mass spectrometry, show very little variation in seven of the diamonds. The total range of 217 analyses is -7.94 to -9.61 (±0.15)‰, and the largest variation in a single stone is 0.98‰. No fractionation in carbon isotopes is seen between octahedral and cuboid sectors at the same growth horizon. These data suggest that the source fluid chemistry, as well as pressure, temperature, and oxygen fugacity were very stable over time, allowing such large volumes of mixed-habit growth to occur. The high concentration of impurities, namely nitrogen and hydrogen, is probably the critical factor required to cause mixed-habit growth. The impurity and isotopic data fall in line with previous modeling based on diamond growth from reduced carbonates with the loss of a 13 C-enriched CO 2 component.

We present the first thermal infrared reflectance spectral characterization of moganite and mixtures of moganite with microcrystalline quartz. We find that for relatively high (>50%) abundances of moganite, the absolute reflectance for samples is significantly reduced. Using microscopic-Raman (~1 μm/pixel) measurements, we estimate the moganite content for various samples. We then compare Raman-derived moganite abundances with microscopic infrared reflectance (25 μm/pixel) spectra to determine the effects of increasing moganite abundance on thermal infrared spectra. We find that moganite is broadly spectrally similar to quartz with major reflectance maxima located between ~1030 and 1280 cm -1 and ~400 and 600 cm -1 ; but there are characteristic differences in the peak shapes, peak center positions, and especially the relative peak reflectance magnitudes. For regions with high (>50%) moganite content, the relative magntitudes of the reflectance maxima at 1157 and 1095 cm -1 (R 1095 / R 1157 band ratio) can be used to estimate the moganite content. This work demonstrates the utility of thermal infrared microspectroscopy in isolating phases that are intimately mixed in a sample, and has applications in planetary science, where constituent phases of quartz-rich sedimentary rocks can be identified using remote or in situ thermal infrared spectroscopy.

We have studied mineralogical characteristics of Fe-Si oxide precipitates from hydrothermal fields of the Valu Fa Ridge, Lau Basin, especially the role that the neutrophilic Fe-oxidizing bacteria played in their formation, using various analytical techniques (XRD, SEM, EPMA, TG/DTA, and FTIR). According to this examination, the Fe-Si oxide formation can be divided into two stages. At the initial stage, the Fe-oxidizing bacteria bound and oxidized Fe 2+ into Fe 3+ to fix CO 2 , triggering precipitation of Fe-oxyhydroxide (ferrihydrite) and construction of a loose network of Fe-rich filaments. Subsequently, the decreased porosity of the network resulting from the gradual growth of the filaments led to a decline in the mixing between seawater and the hydrothermal fluids. Then the conductive cooling of the network resulted in saturation of the dissolved Si with respect to amorphous silica. As a result, significant precipitation of opal-A occurred through inorganic polymerization. However, part of the silica was immobilized by bonding to Fe-OH functional groups and yielded unpolymerized silica, which is characterized by Fe-O-Si bond. Owing to the incorporation of Si into the ferrihydrite structure and its adsorption on the ferrihydrite surface, the modern hydrothermal Fe-Si oxides are thermally stable. DSC measurements indicate the full segregation of cristobalite from hematite at about 800 °C in an O 2 atmosphere. These observations indicate that primary alternating Si- and Fe-rich layers may be absent in the Archean ocean and that alternating bands in BIFs represent a diagenetic process; our work thus provides a potential clue that can be used to unravel the precipitation and diagenetic mechanisms of Precambrian banded iron formations (BIF).

Hyperspectral cathodoluminescence (CL) mapping, combined with electron probe microanalysis (EPMA) and Fourier transform infrared spectroscopy, was used for the reconstruction of crystallization conditions of quartz from porphyry environments. Quartz eyes from the two porphyry deposits Rio Blanco (Chile) and Climax (U.S.A.) were studied. Three peaks are found to be responsible for the total CL emission: 1.93, 2.05, and 2.72 eV. The first two peaks are assigned to O-M (with M being an alkali ion) and oxygen vacancies, respectively. The 2.72 eV peak shows a linear correlation with the Ti concentration determined by EPMA point measurements. In addition, a negative correlation between the 1.93 eV emission and the Al concentration was observed. Quartz grains often form clusters in which adjacent grains show identical CL patterns, indicating that they crystallized attached to each other and were not disturbed later. Quartz cores display sector zoning and enrichment in Li, OH, and sometimes Al, which points to rapid crystallization from an extremely evolved melt. Quartz rims show high Ti, and low Li and OH contents, indicating crystallization from a less evolved melt either at higher temperatures or at higher titanium activities. The Al and Ti distribution patterns are frequently not correlated and both show uneven distribution indicating fast growth from inhomogeneous melts. Only Ti displays sharp transitions and fine oscillatory zoning, which can be explained by the higher mobility of Al in the quartz lattice. The quartz eyes crystallized after magma emplacement under non-equilibrium conditions. It is likely that the crystallization occurred from the melt enriched in Al, Li, and OH and probably other metals and/or volatiles on the brink of fluid exsolution. Subsequent fluid exsolution brought about disequilibrium to the system, resulting in dissolution of quartz and redistribution of elements between the melt and the fluid. The OH, Li, and other alkali metals and volatiles partitioned into the fluid, whereas Ti and Al remained in the melt. Resorption of quartz caused by the fluid exsolution continued until equilibrium was reached again, after which crystallization of quartz rims began from the water-, alkali-, and volatile-poor melt with higher Ti activity. Further accumulation of Al and Ti in the residual melt led to crystallization of extremely Al- and Ti-rich quartz.

Cerium L 3 absorption edge (L 3 -edge) X-ray absorption near edge structure (XANES) spectra were obtained from ~7 × 5 μm areas on green titanite and brown titanite (both with total Ce ~ 0.6 wt%) using the X-ray microprobe at the Pacific Northwest Consortium-X-ray Science Division (PNC-XSD) Insertion Device (ID) line of the Advanced Photon Source (APS). Using a wavelength-dispersive X-ray (WDX) fluorescence detector with a bent LiF (220) crystal monochromator (E/ΔE ~ 1000), we have overcome the challenge of having to measure trace amounts of Ce in a Ti-rich sample of which the energy of the fluorescence X-rays from Ce L 3 -edge and Ti K-edge excitation cannot be resolved with solid-state detectors. We show that both Ce 3+ and Ce 4+ are present in our titanite samples by examining the Ce L 3 -edge XANES spectra. Our results show that to correctly determine trace element substitution mechanisms in titanite (and other minerals), it is necessary to determine multivalent element concentrations, including Ce 3+ /Ce 4+ . We present a new approach for predicting and evaluating multivalent trace element substitution in titanite and other minerals.

The influence of fluorine on cationic and anionic ordering in the mica mineral phlogopite has been investigated using 29 Si, 1 H, and 19 F MAS as well as { 1 H}/{ 19 F} → 29Si CPMAS and CP-depolarization NMR spectroscopies. It can be shown that the mere presence of fluorine achieves a tremendous loss of capability to incorporate aluminum into the phlogopite structure. Fluorine is usually located in Mg-rich octahedral and Si-rich tetrahedral clusters of the phlogopite structure while hydroxyl groups are located in Al-rich octahedral and tetrahedral clusters as derived from { 1 H}/{ 19 F} → 29 Si CPMAS NMR spectroscopies. The ordering effect in these two basic structural clusters can also be proven by a smaller 29 Si linewidth in the { 19 F} → 29 Si CPMAS NMR experiments compared to the usual 29 Si MAS NMR experiment showing a stronger ordering of Si environments near the two different anion types fluorine and hydroxyl. Intensities of the { 1 H}/{ 19 F} → 29 Si CPMAS NMR signals as function of the contact-time show a deviation from the classical I-S model and can be attributed to the I-I*-S model. Time constants like the proton/fluorine spin diffusion time (T df ), the spin-spin relaxation time (T 2 ), the λ parameter (λ), and the proton/fluorine spin-lattice time in the rotating frame (T 1ρ ) were extracted to give information about the local structure.

The formation of fossil oil within clay minerals i.e., mineral-catalyzed decarboxylation, is a mechanism awaiting a thorough chemical explanation. To contribute to such an explanation, the study presented here investigates this mechanism at the level of first-principles, electronic structure computations, employing density functional theory (DFT plus Hubbard-U), planewaves, pseudopotentials, and periodic cells of two types of ferruginous clay minerals, specifically two types of nontronite [Fe 2 (Si,Al) 4 O 10 (OH) 2 ]. The formation of the fossil oil is modeled as a decarboxylation pathway, converting the fatty acid propionic acid, C 2 H 5 COOH to an alkane, C 2 H 6 and the intermediate stages along this conversion pathway are represented by five configurations of interlayer species within the clay minerals. In this study, we test both the effect of the presence of iron on the theoretical stages of decarboxylation, together with the effect of two different density functionals: with and without strong correlations of the d-orbital electrons of iron. We have found that inclusion of the d-orbital electron correlations in the guise of a Hubbard parameter results in the introduction of three new intermediate configurations (one of which is potentially a new transition state), alters the location of the occupied Fermi level orbitals, and changes the band gaps of the clay mineral/interlayer species composites, all of which serves to inform the chemical interpretation of mineral-catalyzed decarboxylation.

In this study, a thermodynamic model for silica and aluminum in high ionic strength solutions at elevated temperatures up to 100 °C is constructed. Pitzer equations are utilized for the thermodynamic model construction. This model is valid up to ionic strengths of ~24 molal (m) in NaOH solutions with silicate concentrations up to ~1.5 m. The speciation of silica (including monomers and polymers) and aluminum at elevated temperatures is taken into account. Also, the equilibrium constants for silicic acid and its polymer species (H 4 SiO 4 , H 5 Si 2 O 7 - , H 4 Si 2 O 7 2- , and H 5 Si 3 O 7 3- ) at elevated temperatures up to 100 °C, are obtained based on theoretical calculations. Using this thermodynamic model, thermodynamic properties, including equilibrium constants, and respective reaction enthalpies are obtained for sodium silicates, zeolite A, and the amorphous form of zeolite A, based on solubility experiments at elevated temperatures. The equilibrium constants for zeolite A and amorphous precursor of zeolite A regarding the following reactions up to 100 °C, where T is temperature in Kelvin. The enthalpy of formation from elements, Gibbs free energy of formation from elements, and standard entropy derived for zeolite A and the amorphous form of zeolite A with the chemical formulas mentioned above at 25 °C and 1 bar are -2738 ± 5 kJ/mol, -2541 ± 2 kJ/mol, 373 ± 10 J/(K·mol); and -2642 ± 3 kJ/mol, -2527 ± 2 kJ/mol, and 648 ± 10 J/(K·mol), respectively. The enthalpy of formation from elements for zeolite A derived in this study based on solubility experiments in hydrothermal solutions agrees well with those obtained by calorimetric measurements and by theoretical calculations.

We report the first characterization of the internal structural features within rhombohedral nanocrystals of hematite (α-Fe 2 O 3 ), specifically nanoscale pores (nanopores) within these crystals observed by high-angle annular dark-field scanning transmission electron microscopy tomography. Threedimensional observations of the internal structure of hematite nanocrystals suggest that the nanopores are formed due to a large reduction in solid volume during the transformation of a poorly crystalline precursor [aggregates of ferrihydrite: Fe 8.2 O 8.5 (OH) 7.4 ·3H 2 O], which results in the formation of pores between grain boundaries. This formation mechanism is different from those previously reported, such as hollow cores originating from screw dislocations. We also discuss dissolution experiments of the hematite nanocrystals in ascorbic acid solution, in which we demonstrated that the nanopores are reactive sites for dissolution and enlarged by preferential etching. Our findings are of fundamental importance to understanding how certain crystal morphologies, internal structures, defects, and reactive sites occur in nanocrystals formed from a poorly crystalline precursor.

The interaction between metallic iron and kaolinite was studied in conditions relevant to those that may be encountered in a high-level radioactive waste disposal facility in geological formation. Experiments were carried out under anoxic atmosphere at 90 °C and in chloride solutions to simulate conditions close to disposal facilities. KGa-2 kaolinite was put in contact with powdered metallic iron in batch experiments for durations of 1, 3, and 9 months. Solutions extracted from the end-products were analyzed (pH, Eh, conductivity, and cation concentrations). End-products were characterized by a set of chemical (oxide analyses, CEC, EDXS) and mineralogical techniques (SEM, TEM, XRD, and FTIR), textural analyses (nitrogen adsorption and low-pressure argon adsorption), XPS, and Mössbauer spectroscopy. In another set of experiments the system was changed from anoxic to oxic conditions to evaluate the stability of the system in the presence of O 2 . The interaction between metallic iron and kaolinite led to a fast initial reaction as major modifications took place during the first month. The partial oxidation of metallic iron resulted in a pH increase and negative Eh values. Iron was not found in solution but in two new Fe-rich phases: magnetite in very low amounts and a Fe-rich clay phase, belonging to the berthierine family. The Si and Al of the berthierine are derived from the partial alkaline dissolution of kaolinite, mostly along edge faces. TEM-EDXS local analyses showed that the composition of resulting particles consisted in mixtures of berthierine and kaolinite layers. Clay particles became thicker with the epitaxial growth of berthierine layers on the basal surfaces of pristine kaolinite. Neoformed berthierine was not stable in the presence of O 2 at 90 °C. Berthierine layers dissolved, iron was mobilized to form iron oxides and oxyhydroxides while kaolinite layers recrystallized from released Al and Si.

The elastic and structural behavior of natural cordierite compressed in aqueous medium up to 6 GPa was studied by means of in situ synchrotron powder diffraction with a diamond-anvil cell. In the range between 1-4 GPa the elastic behavior is regular and slightly anisotropic, with linear compressibilities β a :β b :β c = 4:4:5, the most rigid a-b plane coinciding with the orientation of 6-membered rings. A distinct decrease of compressibility in the range of 4-5 GPa indicates a pressure-induced hydration (PIH), which is confirmed by the structure refinements. The addition of about 60% of the initial water content into the cordierite channels proceeds through positional disordering of the H 2 O sites inside the channel cavity and a stepwise filling of the H 2 O position inside the 6-membered rings, leading to the phase transition at about 4.7 GPa. The appearance of H 2 O molecules inside 6-membered rings prevents their contraction and even causes their slight enlargement along the a direction, apparently related to the orientation of H-bonds. This results in an anisotropic deformation of the unit cell and an increase of the a parameter in the HP phase at 4.9 GPa, as well as a decrease of linear compressibility along a upon the further compression up to 6 GPa (β a :β b :β c = 5:9:10).

We report the first natural occurrence of keatite, also known as silica K, discovered as a precipitate in the core of ultrahigh-pressure (UHP) clinopyroxene (Cpx) within garnet pyroxenite from the Kokchetav Massif, Kazakhstan. High-resolution transmission electron microscopy and electron diffraction demonstrate that sub-micrometer and nano-scale keatite precipitates have a definite crystallographic relationship with the host pyroxene (diopside = ~Di 90 ) Cpx (100) || keatite (100) and Cpx (010) || keatite (001). Clinopyroxene provides a template for keatite nucleation due to the close structural relationship and excellent lattice match between the diopside and keatite. We propose that keatite micro-precipitates are formed in localized low-pressure micro-environments produced as a result of exsolution of extra silica and vacancies held within UHP host diopside and stabilized by the pyroxene lattice. Low-density metastable keatite and its relationship to the host pyroxene likely reflects the important influence of pyroxene/precipitate interfacial energy on the micro- and nano-scales in controlling the nature of exsolved phases in exhumed UHP minerals.

High-pressure micro-Raman spectroscopic measurements of calcium ferrite-type MgAl 2 O 4 and CaAl 2 O 4 were made using a diamond-anvil cell high-pressure apparatus. The pressure dependence of frequencies of 18 Raman peaks for calcium ferrite-type MgAl 2 O 4 and 26 Raman peaks for calcium ferrite-type CaAl 2 O 4 were determined up to 20 GPa at ambient temperature. The mode Grüneisen parameter for each observed Raman mode was obtained from the pressure dependence of frequencies. Vibrational mode calculations by first principles using density functional theory were also performed for assignment of Raman peaks and for estimating frequencies of Raman inactive modes. From the obtained mode Grüneisen parameters and the results of the vibrational mode calculations, thermal Grüneisen parameters were determined to be 1.50(5) for calcium ferrite-type MgAl 2 O 4 and 1.31(3) for calcium ferrite-type CaAl 2 O 4 . These thermal Grüneisen parameters were applied to heat capacity and vibrational entropy calculations using Kieffer model.

To understand the fate of the host phase for potassium subducted into the deep Earth’s interior, we have studied the high-pressure stability and elastic properties of KAlSi 3 O 8 hollandite (K-hollandite) by means of the first-principles computation method. Based on experimental observations, the tetragonal K-hollandite I phase was found to undergo a ferroelastic second-order phase transition to the monoclinic K-hollandite II phase at 14.9 GPa. This K-hollandite II phase was mechanically stable up to 150 GPa (i.e., entirely in the Earth’s lower mantle), being consistent with previous studies. The Born’s elastic stability criteria indicate that the tetragonal mechanical instability occurs at similar pressure of 16.9 GPa with shear softening. This causes anomalous pressure dependence of the wave velocities across the instability. Taking a Clapeyron slope of 7 MPa/K and a temperature of 1800 K, the transition pressure becomes ~28 GPa corresponding to about 770 km depth, which would be seismologically detectable and could be comparable to seismic scatterers observed at the shallowest lower mantle. Next, we studied the solid-solution effect of sodium to K-hollandite, indicating that it is very limited on the phase stability, although the Na-end-member phase was found to be metastable. Elasticity demonstrates strong anisotropy around 15 GPa due to its ferroelastic nature.

The effects of progressive metamorphism (Grampian orogeny) and later tectonic activity on detrital zircon in the Barrovian zones of Scotland were studied using secondary ion mass spectrometry (SIMS) and backscattered electron imaging (BSE). Fifteen samples recording progressive metamorphism from the chlorite through the sillimanite-K-feldspar zones were investigated by: (1) SIMS U-Pb depth profiling into rims of unpolished zircon grains to analyze sub-micrometer-scale features, and (2) conventional spot analysis on sectioned and polished grains. Spot analyses of zircon interiors yield pre-metamorphic detrital ages for all metamorphic grades. Most are in the range ca. 600 to ca. 2000 Ma, but some stretch back to the Archean. Younger ages are recorded in zircon rims, but zircon rim alteration at lower metamorphic grades occurs over much shorter length scales (the outer ~80 nm to ~1 μm of the grain) than in the upper amphibolite to granulite facies (rims of 10 to 30 μm). For example, Grampian (~470 Ma) metamorphism from the garnet and kyanite zones affected only the outermost rims (<1 μm) of detrital zircon grains. Thicker, 10 to 30 μm rims that could be dated by conventional spot analysis developed only at high grades in the sillimanite and sillimanite-K-feldspar zones, probably in the presence of partial melt. The mean Grampian zircon age from spot and depth profile analyses is 472 ± 4 Ma (n = 19). In addition to Grampian ages, the zircon depth profiles reveal ages related to five main events that postdate the Grampian Orogeny: decompression melting at ca. 450 Ma; subduction and I-type granite intrusion at ca. 420 Ma; granite intrusion at ca. 384 Ma; extensionrelated volcanism and vein mineralization at ca. 335 Ma, and further rifting and basaltic magmatism at ca. 250 Ma. These events are recorded only in the very narrow rims (<1 μm) of zircons, and are thus undetectable with conventional spot analysis. We conclude that: (1) zircon interiors were able to retain detrital ages up to and including the highest grade of Barrovian metamorphism (sillimanite-Kfeldspar zone), and (2) the <1 μm thick zircon rims may preserve a rich history of metamorphic and post-metamorphic events that can be dated using SIMS U-Pb depth profiling techniques.

Saltonseaite, K 3 NaMn 2+ Cl 6 , is a new mineral from the Salton Sea, Imperial County, California, U.S.A., which formed as the result of the evaporation of geothermal (hydrothermal) brines enriched in K, Na, Mn, and Cl. It occurs as lozenge-shaped and bladed crystals to about 10 cm that are composites of parallel-grown {012} rhombohedra. It is associated with large, well-formed crystals of sylvite and halite. Crystals are transparent and colorless, but appear light orange due to inclusions of akaganéite. The streak is white and the luster is vitreous to oily, the latter being due to deliquescence. The Mohs hardness is about 2½, the tenacity is brittle, the fracture is irregular, and crystals exhibit one very good cleavage on {110}. The mineral has an astringent taste and is markedly hygroscopic. The measured and calculated densities are 2.26(1) and 2.297 g/cm 3 , respectively. Saltonseaite is soluble in water at room temperature and crystallizes from solution above 52 °C. Optically, saltonseaite is uniaxial positive, with ω = 1.577(1) and ε = 1.578(1) (white light) and is non-pleochroic. Energy-dispersive spectroscopic analyses (average of 5) provided: K 28.79, Na 5.35, Mn 13.48, Fe 0.24, Cl 52.19, total 100.05 wt%. The empirical formula (based on 6 Cl atoms) is: K 3.00 Na 0.95 Mn 2+ 1.00 Fe 2+ 0.02 Cl 6 . Saltonseaite is trigonal, R3̄c, with cell parameters a = 12.0966(5), c = 13.9555(10) Å, V = 1768.48(16) Å 3 , and Z = 6. The nine strongest lines in the X-ray powder diffraction pattern are [dobs in Å(I)(hkl)]: 5.83(61) (012); 3.498(25)(300); 2.851(68)(131); 2.689(32)(312); 2.625(62)(214); 2.542(100)(223); 1.983(32) (324); 1.749(20)(600), and 1.384(22)(multiple). The structure of saltonseaite (R 1 = 1.08% for 558 F o > 4σF) contains face-sharing chains of alternating Mn 2+ Cl 6 octahedra and NaCl 6 polyhedra along c . The chains are joined via bonds to eight-coordinated K atoms. Saltonseaite is isostructural with rinneite, K 3 NaFe 2+ Cl 6 , and very similar in structure with chlormanganokalite, K 4 Mn 2+ Cl 6 . Existing chemical analyses for saltonseaite and rinneite fail to confirm a solid-solution series between them; experimental studies are needed.

A new zeolitic aluminosilicate mineral species, rongibbsite, ideally Pb 2 (Si 4 Al)O 11 (OH), has been found in a quartz vein in the Proterozoic gneiss of the Big Horn Mountains, Maricopa County, Arizona, U.S.A. The mineral is of secondary origin and is associated with wickenburgite, fornacite, mimetite, murdochite, and creaseyite. Rongibbsite crystals are bladed (elongated along the c axis, up to 0.70 × 0.20 × 0.05 mm), often in tufts. Dominant forms are {100}, {010}, {001}, and {101̄}. Twinning is common across (100). The mineral is colorless, transparent with white streak and vitreous luster. It is brittle and has a Mohs hardness of ~5; cleavage is perfect on {100} and no parting was observed. The calculated density is 4.43 g/cm 3 . Optically, rongibbsite is biaxial (+), with n α = 1.690, n β = 1.694, n γ = 1.700, c Z = 26°, 2V meas = 65(2)°. It is insoluble in water, acetone, or hydrochloric acid. Electron microprobe analysis yielded an empirical formula Pb 2.05 (Si 3.89 Al 1.11 )O 11 (OH). Rongibbsite is monoclinic, with space group I2/m and unit-cell parameters a = 7.8356(6), b = 13.913(1), c = 10.278(1) Å, β = 92.925(4)°, and V = 1119.0(2) Å 3 . Its structure features an interrupted framework made of three symmetrically distinct TO 4 tetrahedra (T = Si + Al). The framework density is 17.9 T per 1000 Å 3 . Unlike many known interrupted frameworks in zeolite-type materials, which are usually broken up by OH or F, the framework in rongibbsite is interrupted by O atoms. There are various corner-shared tetrahedral rings in the framework of rongibbsite, including two types of 4-membered, three 6-membered, and one 8-membered rings. The extraframework Pb and OH reside alternately in the channels formed by the 8-membered rings. The Pb cations are disordered over two split sites, Pb and Pb′, with site occupancies of 0.8 and 0.2, respectively, and a Pb-Pb′ distance of 0.229 Å, providing a structural explanation for the two strong Raman bands (at 3527 and 3444 cm -1 ) attributable to the O-H stretching vibrations. The average bond lengths for the T1, T2, and T3 tetrahedra are 1.620, 1.648, and 1.681 Å, respectively, indicating that the preference of Al for the three tetrahedral sites is T3 >> T2 > T1. Rongibbsite represents the first natural aluminosilicate with Pb as the only extraframework cation.

A re-investigation of the crystal structure of quadratite, ideally AgCdAsS 3 , was undertaken using a single crystal from the type locality, Lengenbach, Binntal, Switzerland. The average of five electron microprobe analyses led to the empirical formula (Ag 0.994 Cd 0.738 Pb 0.231 Cu 0.006 Tl 0.005 Mn 0.003 Fe 0.004 Zn 0.002 Cr 0.001 ) Σ=1.984 (As 0.955 Sb 0.003 ) Σ=0.958 S 3.058 . A single-crystal structure refinement (R1 = 4.84% for 558 observed reflections) shows that quadratite crystallizes in the space group P4 3 22 and exhibits an atomic arrangement similar to that of the recently approved new mineral manganoquadratite, AgMnAsS 3 . Like manganoquadratite, quadratite adopts a galena-derivative framework, with metal atoms occupying all the available octahedral interstices, although only M1 and M2 cations, occupied mainly by Cd, adopt a fairly regular octahedral coordination; the M3 cation, occupied by Ag, is located outside the center cavity in a square-pyramidal coordination, whereas Pb at the split position M3′ coordinates six S atoms. Arsenic also adopts a 3 + 3 asymmetrical coordination, thus forming the AsS 3 pyramidal groups that typically occur in sulfosalts. The structure can be also described as a stacking of BAB slabs [A: (Cd,Ag)CdS 2 atomic plane; B: (Ag,Pb)AsS 2 atomic plane] along [001]. The rectangular unit cell of these slabs is oriented diagonally to the a axes of quadratite and consecutive slabs are related via interlayer twofold rotation operations parallel either to [100] or to [010]. This ambiguity leads to an OD structure with various possible stacking sequences, from which the tetragonal space group P4 3 22 was observed.

The crystal structure of hereroite, a new complex lead oxychloride mineral from the Kombat Mine, Grootfontein, Namibia, has been solved by direct methods and refined to R 1 = 0.054 for 6931 unique observed reflections. The mineral is monoclinic C2/c, a = 23.139(4), b = 22.684(4), c = 12.389(2) Å, β = 102.090(3)°, and V = 6358.8(18) Å 3 . The structure contains 16 independent Pb sites in strongly asymmetric coordination by O and Cl atoms. There are two tetrahedral sites, from which one (As) is occupied solely by As, whereas the second (T) has the mixed occupancy of [Si 0.48 As 0.29 V 0.15 Mo 0.09 ]. There are in total 21 O sites. The O1-O8 sites belong to the AsO 4 and TO 4 tetrahedral oxyanions. The other O atoms (O9-O20) are tetrahedrally coordinated by Pb atoms, thus being central for the OPb 4 oxocentered tetrahedra. The OPb 4 tetrahedra share edges to form the [O 21 Pb 32 ] 22+ layers that can be described as derivatives of the [OPb] layer from the structure of tetragonal PbO (litharge). The [O 21 Pb 32 ] 22+ layer in hereroite can be obtained from the [OPb] layer by removal of blocks of oxocentered tetrahedra, which results in formation of double-square sevenfold and square fourfold cavities. The cavities are occupied by the AsO 4 and TO 4 tetrahedra, respectively. The topology of the [O 21 Pb 32 ] 22+ layer is complex and can be described as a combination of modules extracted from the layers of OPb4 tetrahedra present in the structures of kombatite and symesite. The topological functions of tetrahedra within the layer are analyzed using the square lattice method, which shows that each symmetry-independent tetrahedron has its own topological function in the layer construction. The structure of hereroite belongs to the 2:1 type of layered Pb oxyhalides and consists of alternating PbO-type layers and Cl sheets oriented parallel to the (010) plane.

The crystal structure of vladkrivovichevite, a new complex lead oxychloride mineral from the Kombat Mine, Grootfontein, Namibia, has been solved by direct methods and refined to R 1 = 0.048 for 3801 unique observed reflections. The mineral is orthorhombic, Pmmn, a = 12.759(1), b = 27.169(4), c = 11.515(1) Å, and V = 3992.0(9) Å 3 . The structure of vladkrivovichevite belongs to a novel type of layered Pb oxychloride structure. The structure contains 12 symmetrically independent Pb sites. All Pb sites have strongly asymmetric coordination. Two B atoms form slightly distorted BO 3 triangles. One symmetrically independent Mn atom forms five Mn-O bonds and one Mn-Cl bond by forming MnO 5 Cl octahedra. The O1, O2, O10, O11, and O12 atoms are tetrahedrally coordinated by four Pb atoms each, forming OPb 4 oxocentered tetrahedra. The O7 site has a remarkable octahedral coordination, consisting of four Pb and two Mn atoms. The O1Pb 4 , O2Pb 4 , O10Pb 4 , and O11Pb 4 tetrahedra share common edges to produce bands interconnected by O12Pb 4 tetrahedra, forming a [O 18 Pb 32 ] 28+ layer. A O7Pb 4 Mn 2 heterometallic oxocentered octahedron serves as the core of the [OPb 4 Mn 2 Cl 2 (BO 3 ) 8 ] 16- clusters that link to the [O 18 Pb 32 ] 28+ layer via BO 3 triangles. The presence of [OPb 4 Mn 2 Cl 2 (BO 3 ) 8 ] 16- clusters is associated with large cross-like vacancies in the [O 18 Pb 32 ] 28+ layer.

Phase transformation of amorphous calcium carbonate (ACC) into vaterite and its subsequent stability was investigated at a constant pH (~8.2), ionic strength, and temperature that simulated the biological environment. Solutions containing the same concentrations of CaCl 2 , Na 2 CO 3 , and tris(hydroxymethyl)aminomethane buffer and various concentrations of PO 4 (0-62.5 μM) were prepared, and precipitates in the solutions were sampled at a constant interval to observe the morphology and type of calcium carbonate polymorphs that appeared. The change in the Ca-ion concentration over time, which served as a guide for phase transformation of ACC into crystalline phases, was measured in relation to the PO 4 concentration. The starting time of phase transformation was at the minimum point when the concentration was ~2-3 μM. Vaterite spherulites consisting of needle-like crystals (0.5-2 μm in length) formed only in this PO 4 range and survived the experimental procedure (~2.5 h). In contrast, the starting time of phase transformation increased exponentially with the PO 4 concentration when it was higher than 5 μM. The vaterite spherulites and calcite crystals co-precipitated, and both polymorphs grew over time. The PO 4 was shown to be an accelerator for phase transformation from ACC into vaterite at low concentrations (Ca/PO 4 molar ratio <3000) and an inhibitor for transformation at high concentrations. We investigated the kinetics of vaterite formation in the presence of PO 4 and derived an advanced concept for cluster-based phase transformation. This investigation showed that the appearance and stability of calcium carbonate polymorphs is easily controlled by adjusting the PO 4 concentration.

Eruption of the Bishop Tuff magma preserved equilibrium of exchange components and element concentrations among magnetite, ilmenite, biotite, apatite, zircon, and liquid. Orthopyroxene and clinopyroxene were not in exchange equilibrium with the other MgFe-bearing phases, but they appear to have been in equilibrium among themselves. Internally consistent temperatures recorded by the FeTi-oxide, Ti-in-quartz, and D18O quartz-magnetite thermometers, coupled with evidence for magmatic corrosion of quartz and sanidine, indicate that an initially low-T (≈700 °C), near-H 2 O-saturated, high-SiO 2 rhyolite magma was heated up to ≥800 °C and its crystal cargo partially melted by recharge of hotter melt from below. Oxygen fugacity and compositions of biotite, ilmenite, magnetite, and silicate liquid initially adjusted by internal rearrangement of components and conservation of oxygen. Partial melting of feldspars liberated Sr and Ba back into the melt. Mixing during recharge eventually re-introduced compatible elements (e.g., Mg, Ba, Sr) as well as foreign crystals of euhedral ortho- and clinopyroxene, which evidently never totally re-equilibrated with the rhyolite liquid. Introduction of CO 2 and accompanying reduction in the aH 2 O during recharge raised crystallization temperatures of quartz and sanidine in the rhyolite sufficient to allow marginal regrowth of these phases with enhanced contents Ti, Ba, and Sr.

Davidite (A 1-x M 21 O 38 ) samples from five different geological localities contain approximately 0.2 to 9.5 wt% UO 2 (0.02 to 0.65 atoms per formula unit) and <0.1 to 1.3 wt% ThO 2 (<0.01 to 0.09 atoms per formula unit). Maximum amounts of other notable cations include 3.7 wt% V2O3, 4.1 wt% Cr 2 O 3 , 2.5 wt% Y 2 O 3 , 5.6 wt% La 2 O 3 , 6.0 wt% Ce 2 O 3 , 4.0 wt% MnO, 2.4 wt% ZnO, 2.7 wt% SrO, and 4.9 wt% PbO. As a result of the variation in age and Th-U content, the calculated a decay dose ranges from ~0.2 to 44 × 1016 a/mg (~0.06 to 14.5 dpa). For samples with ages of 275-295 Ma, the critical dose for amorphization based on electron diffraction is ~0.8 × 10 16 α/mg. Natural davidite is commonly altered to rutile, ilmenite, titanite, and other minor phases.