Band 97, Heft 8-9

Uranium deposits older than about 2200 Ma are generally hosted within quartz-pebble conglomerates and are a source of economic uranium. The genesis of these deposits is controversial and genetic models include hydrothermal, detrital/placer, and modified placer. Petrography of the uranium mineralogy from the Pele Mountain quartz-pebble conglomerate uranium deposit of the Elliot Lake district, Canada, shows that the dominant uranium minerals are thorite [(Th,U)SiO 4 ] and brannerite [(U,Ca,Ce) (Ti,Fe) 2 O 6 ]. Uranium-lead (U-Pb) and lead-lead (Pb-Pb) isotopic analyses of thorite, brannerite, and galena were obtained using secondary ion mass spectrometry. Thorite has U-Pb age of 2489 ± 24 in the quartzose conglomerate and 280 ± 67 Ma in the quartz arenite. Brannerite has a U-Pb age of 2403 ± 120 Ma. Thorite Pb-Pb ages range from 521 ± 19 to 248 ± 8 Ma in the arenite and from 2453 ± 12 to 935 ± 27 Ma in the conglomerate, whereas brannerite Pb-Pb ages are between 1335 ± 0.11 and 848 ± 13 Ma. Galena has a Pb-Pb age of 2659 ± 8 Ma, which likely represents the age of the source formation that produced detrital galena. The rounded texture and U-Pb age of the conglomerate thorite corresponds to the depositional age of the host conglomerate bed of the Matinenda Formation, suggesting a detrital origin. The young U-Pb age of the thorite in the arenite represents the age of a resetting event. The brannerite, which replaces rutile, has a U-Pb consistent with fluid events associated with the Blezardian or Penokean Orogenic events, and is likely of hydrothermal origin. Therefore, based on the textures and variable ages of the uranium and sulfide minerals, the Pele Mountain quartz-pebble conglomerate uranium deposit is interpreted to be a modified placer-type deposit.

Celadonite is a common alteration product of basalts in marine environments. It has been argued that marine fluids are necessary for celadonite formation, possibly by providing a source of K and other dissolved cations. Laterally extensive deposits of celadonite occur in basalts of the Grande Ronde Basalt of the Columbia River Basalt Group. The celadonite is found in scoriaceous flow tops of layered basalt flows, where it fills vesicles and replaces the surrounding groundmass. Evolved interstitial glasses are present in the basalts and dissolution of these glasses may provide sufficient K for celadonite formation, whereas dissolution of groundmass augite provides a source of Mg and Fe. These observations show that alteration by seawater or any other external source of dissolved ions is not necessarily required for celadonite formation.

The surface enthalpies of manganese oxide phases, hausmannite (Mn 3 O 4 ), bixbyite (Mn 2 O 3 ), and pyrolusite (MnO 2 ), were determined using high-temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The energy for the hydrous surface of Mn 3 O 4 is 0.96 ± 0.08 J/m 2 , of Mn 2 O 3 is 1.29 ± 0.10 J/m2, and of MnO 2 is 1.64 ± 0.10 J/m 2 . The energy for the anhydrous surface of Mn 3 O 4 is 1.62 ± 0.08 J/m 2 , of Mn 2 O 3 is 1.77 ± 0.10 J/m 2 , and of MnO 2 is 2.05 ± 0.10 J/m 2 . Supporting preliminary findings (Navrotsky et al. 2010), the spinel phase (hausmannite) has a lower surface energy than bixbyite, whereas the latter has a smaller surface energy than pyrolusite. Oxidation-reduction phase equilibria at the nanoscale are shifted to favor the phases of lower surface energy-Mn 3 O 4 relative to Mn 2 O 3 and Mn 2 O 3 relative to MnO 2 . We also report rapidly reversible structural and phase changes associated with water adsorption/desorption for the nanophase manganese oxide assemblages.

In spite of the amount of research that has been done on grossular, Ca 3 Al 2 Si 3 O 12 , there is still uncertainty regarding its exact thermodynamic properties. Because of insufficient sample characterization in the various published calorimetric studies, it is difficult to analyze conflicting C P and S ○ results. To resolve the discrepancies, a detailed and systematic multi‑method investigation was undertaken. Three synthetic grossular samples and four natural grossular‑rich garnets were characterized by optical microscopy, electron microprobe analysis, IR, and MAS 29 Si and 27 Al NMR spectroscopy, and X-ray powder diffraction methods. Two of the natural grossulars, crystallized at relatively low temperatures, are optically anisotropic and two from the higher temperature amphibolite faces are isotropic. The natural garnets have between 94 and 97 mol% grossular with minor fractions of other garnet components, as well as small amounts of OH in solid solution. 29 Si and 27 Al MAS NMR spectra indicate that synthetic grossular crystallized at high‑P and high‑T conditions is ordered with respect to Al and Si. Heat-capacity measurements between 5 and 300 K were made using relaxation calorimetry and between 282 and 764 K using DSC methods. For the three synthetic grossulars, the C P data yield an average S ○ value of 260.23 ± 2.10 J/(mol·K). The S ○ values for the four natural grossular‑rich garnets, adjusted to end‑member grossular composition, range between 253.0 ± 1.2 and 255.2 ± 1.2 J/(mol·K). The results of this work thus confirm earlier low‑temperature adiabatic calorimetric studies that show small, but experimentally significant, differences in S° between natural and synthetic grossular samples. The difference in terms of heat-capacity behavior between synthetic and natural samples is that the latter have lower C P values at temperatures between 20 and 100 K by up to about 20%. Above 298 K, C P for grossular is given by C P J/(mol·K) = 556.18(±12) - 1289.97(±394)⋅T -0.5 - 2.44014(±0.24)⋅10 7 ⋅T -2 + 3.30386(±0.39)⋅10 9 ⋅T -3 . Applying mathematical programming, published high‑P‑T results on the reaction 3anorthite = grossular + 2kyanite + quartz were analyzed thermodynamically. The calculations yield best‑fit values of Δ f H ○ = -6627.0 kJ/mol and S ○ = 258.8 J/(mol·K) for grossular. It is concluded that S ○ ≈ 260 J/ (mol·K) is the best value for end‑member grossular. Variations in structural state and composition in natural samples, as well as assumptions used in correcting for solid‑solution and OH groups, appear to be the most important factors that could account for the smaller S ○ values of 253-257 J/(mol·K).

Isobaric heat capacities (C P ) of Mg 2 SiO 4 forsterite and ringwoodite were measured by differential scanning calorimetry in the temperature range of 306-833 K. The measured C P of Mg 2 SiO 4 forsterite was consistent with those reported by previous studies. On the other hand, the present C P of Mg 2 SiO 4 ringwoodite was about 3-5% larger than those measured by previous researchers. The calorimetric data of Mg 2 SiO 4 ringwoodite were extrapolated to 2500 K using a lattice vibrational model calculation, which well reproduced the low-temperature C P data measured by thermal relaxation method. The calculated C P shows good agreement with the present calorimetric data. The obtained C P was expressed by the polynomial of temperature: C P = 164.30 + 1.0216 × 10 −2 T + 7.6665 × 10 3 T −1 - 1.1595 × 10 7 T −2 + 1.3807 × 10 9 T −3 [J/(mol·K)] in the range of 250-2500 K.

An in situ high-temperature X-ray powder diffraction study of the thermal behavior of realgar (α-As 4 S 4 ) has been carried out. Data, measured in transmission geometry on a non-hermetically sealed capillary, indicate that the realgar → β-As 4 S 4 phase transition starts at 558 K and is completed at 573 K due to kinetics. Melting starts at 578 K and is completed at 588 K. Thermal expansion of realgar is significant and fairly isotropic. In fact, the a- and b-parameters expand almost at the same rate, whereas the c-parameter is slightly softer against heating. Moreover, the β-angle contracts as temperature is raised. The geometry of the As 4 S 4 molecule is largely independent from heating. The lengthening of a few As-S and As-As contacts above or near the sum of the As,S van der Waals radii represents the driving force of the phase transition. In addition, the thermal behavior of arsenolite As 2 O 3 and nonstoichiometric As 8 S 8+x crystals produced from As 4 S 4 melt recrystallization has been investigated. Two members located along the β-As 4 S 4 -alacranite (As 8 S 9 ) series joint were identified at RT: a term close to the β-As 4 S 4 end-member (As 8 S 8+x +x: x = ca. 0.1) and one term of approximate As 8 S 8.3 composition. The thermal expansion of β-As 4 S 4 is significantly anisotropic following the α b > α a > α c relationship. This is clearly the result of the different packing scheme of the As 4 S 4 cages in β-As 4 S 4 with respect to realgar. The dependence of cell parameters and volume of As 8 S 8.3 is more complicated. In fact, a strong discontinuity on the dependence of cell parameters and volume is observed in the 403-443 K thermal range, i.e., that at which As 8 S 8.3 converts partly to realgar. A significant volume expansion is observed as a result of a change of composition to As 8 S 8.7 .

The thermodynamics of mixing and its dependence on cation distribution in the Fe 3 O 4 -Fe 2 TiO 4 (magnetite-ulvöspinel) spinel solid solution were studied using high-temperature oxide melt solution calorimetry and a range of structural and spectroscopic probes. The enthalpies of formation of ilmenite and ulvöspinel from the oxides and from the elements were obtained using oxidative drop solution calorimetry at 973 K in molten sodium molybdate. The enthalpy of mixing, determined from the fit to the measured enthalpies of drop solution calorimetry, is endothermic and represented by a quadratic formalism, ΔH mix = (22.60 ± 8.46)x(1 - x) kJ/mol, where x is the mole fraction of ulvöspinel. The entropies of mixing are more complex than those for a regular solution and have been calculated based on average measured and theoretical cation distributions. Calculated free energies of mixing show evidence for a solvus at low temperature in good agreement with that observed experimentally.

The Raman spectra of eight clinopyroxenes synthesized along the join diopside-clinoenstatite (Di- Cen, CaMgSi 2 O 6 -Mg 2 Si 2 O 6 ) were measured. The splitting of the 670 cm -1 mode of the Ag symmetry, observed in the composition Di 52 En 48 to clinoenstatite, was interpreted as evidence of a C2/c-P2 1 /c phase transition. The transition was also revealed by deviation from the linear dependence of the peak position vs. composition and by the appearance of several new peaks in the samples richer in clinoenstatite. Analysis of peak positions vs. structural changes suggests that for the M2 polyhedron, in which Ca substitution for Mg occurs, a different deformation mechanism acts in Ca richer and poorer P2 1 /c pyroxenes, and that Ca richer P2 1 /c pyroxenes deform with the same mechanism as C2/c pyroxenes. The frequency of the peak at 670 cm -1 was found to change linearly with the kinking angle of the tetrahedral chains for C2/c and of the B chain for P2 1 /c, whereas the position of the peak ascribed to the A chain was little affected by the kinking angle. Peak broadening in C2/c Ca-rich homogeneous pyroxenes was interpreted to be a consequence of the positional disorder of the Ca and Mg in the M2 cavity: peak broadening increases with increasing Mg content for peaks assigned to M2-O vibrations, but it changes little for peaks assigned to chain bending, which suggests that cation substitution in the M2 cavity occurs with little interaction with the silicate chain. Furthermore peak broadening was observed in intermediate pyroxenes as a consequence of mottled textures, antiphase domains, and compositional inhomogeneity.

The sulfur compounds released by volcanic eruptions, generally believed to be in the form of SO 2 and H 2 S, may cause global cooling of the atmosphere. However, several recent field and experimental studies suggested that under moderately oxidized conditions hexavalent sulfur species may coexist with SO 2 in magmatic fluids and may later be directly emitted at volcanic vents, which contradicts some thermodynamic predictions. We have investigated sulfur speciation in magmatic-hydrothermal fluids by loading different amounts of dilute sulfuric acid into a hydrothermal diamond-anvil cell and performing in situ Raman spectroscopy at temperatures up to 700 °C. Upon heating SO 2- 4 disappeared beyond 100 °C, and SO 2 formed at >250 °C probably due to reduction by the rhenium or iridium gasket. With high-fluid densities (such as >0.9 g/mL), the initial acid and air bubble homogenized into the liquid phase and most sulfur was present in the form of either HSO - 4 or H 2 SO 4 (the rest being SO 2 ) within investigated T-P conditions (with pressures up to 10 kb). With low-fluid densities (such as <0.2 g/mL), the system homogenized into the vapor phase and molecular H 2 SO 4 appeared to dominate (with pressures less than 1 kb). These observations strongly suggest that hexavalent sulfur is stabilized by hydration in magmatic fluids.

The structural state of metamict titanite was studied by Raman spectroscopy, complementary high-resolution transmission electron microscopy, and single-crystal X-ray diffraction. The results show that Raman scattering collected from metamict titanite is highly anisotropic, which is typical of single crystals. But surprisingly, the observed Raman-scattering dependence on the sample orientation is much more pronounced for heavily metamict than for weakly metamict titanite samples. These radiation-induced anisotropic effects are related to the specific atomic arrangements in metamict titanite. The Raman spectra collected in backscattering geometry from a plane nearly perpendicular to the chains of corner-sharing TiO 6 octahedra arise predominantly from phonon modes in crystalline nanoregions with radiation-induced defects, whereas the contribution of atomic vibrations in radiation-induced amorphous nanoregions is better pronounced in the Raman spectra collected from a plane containing TiO 6 chains. This difference provides a unique opportunity to study separately, the structural transformations of the crystalline and amorphous fractions in metamict titanite. The results show that the radiation-induced periodic faults in the crystalline matrix are related to the disturbance of SiO 4 -TiO 6 -SiO 4 -TiO 6 rings comprising TiO 6 octahedra from different chains, whereas the radiationinduced amorphization is related to the partial change of Ti coordination from octahedral to pyramidal and/or tetrahedral, which in turn violates the Ti-O-Ti intrachain linkages. This indicates that the plane containing Si-O-Ti-O bond rings is less susceptible to a self-accumulation of radiation-induced defects resulting in the development of amorphous regions as compared to the perpendicular plane containing Ti-O bond chains. Sample-orientation-dependent Raman spectroscopy was further applied to annealed metamict titanite to give further insight into the temperature-driven recovery processes in the crystalline and amorphous nanoregions. Multistep annealing by 50 K for 2 h per step gradually suppresses the structural defects in the crystalline fraction as the improvement of the SiO 4 -TiO 6 connectivity within planes nearly perpendicular to the TiO 6 chains reaches saturation near 900 K. The annealing-induced recrystallization of the radiation-induced amorphous nanoregions takes place in the temperature range between approximately 650 and 950 K, with a maximum near 750 K. Raman scattering shows that multistep annealing up to 1173 K is insufficient to recover the crystalline structure of the studied metamict titanite sample, which has an accumulated radiation dose of 1.2 × 10 18 α-event/g.

Carbonado, a variety of natural polycrystalline diamond whose origin remains unknown, differs notably in the properties from common diamonds of mantle origin. In this study, infrared spectroscopic and microscopic analyses were conducted on carbonado from the Central African Republic. Stepwise heating followed by infrared spectroscopic measurements indicated that liquid H 2 O is enclosed within diamond single crystals in carbonado. Transmission electron microscope observation revealed a negative crystal that is interpreted as a primary fluid inclusion in a diamond single crystal. Observations by field-emission scanning electron microscope and electron backscatter diffraction analysis show an absence of lattice preferred orientation of diamond crystals, {111} growth steps along grain boundaries, and the crystal-size distribution of diamond similar to those of crystals formed in liquid media. In addition, the redox conditions of carbonado formation is inferred to be ~3 log units below the quartz-magnetite-fayalite buffer, which is the prevailing condition in cratonic upper mantle. These lines of evidence suggest that the carbonado crystallized in C-O-H fluid, supporting the hypothesis of a mantle-depth origin of carbonado.

The presence of graphite in natural environments is linked to the redox and thermal conditions of C-H-O fluid/graphite equilibrium in hydrothermal veins and metasomatic contacts. A time-series experimental study was performed to investigate the graphite undersaturated C-H-O system at 600 °C and 1000 MPa, and with ƒ O₂ ranging from highly reducing (10 −23 ) to highly oxidizing (10 4 ). A nonvolatile intermediate carbon phase exhibiting the Raman spectral features of poorly ordered graphite was formed as the system evolves toward equilibrium as a function of run duration. The thermometric empirical expressions using the G and D bands in the spectra of graphite failed to accurately estimate the experimental temperature. Thus, the existing Raman geothermometers appear inadequate to address graphite formation under conditions of metastable equilibria and to account for kinetic effects such as, for example, the degree of crystallinity. The presence of poorly ordered graphitic carbon at all the redox conditions investigated suggests that the disordered structure of the mineral attains an extensive thermodynamic stability field, and that it may be more readily deposited than crystalline graphite. Metastable graphitic carbon could, therefore, function as a precursor and substrate for the formation of the well-ordered phase. Such metastable graphite may provide an intermediate state that facilitates subduction of carbonaceous material, while imposing constraints on the formation mechanisms and the 13 C/ 12 C isotopic systematics of deep seated carbonaceous fluids and minerals such as diamonds.

Biogenetic biosilica displays intricate patterns that are structured on a nanometer-to-micrometer scale. At the nanoscale, it involves the polymerization products of silica, apparently mediated by the interaction between different biomolecules with special functional groups. In this paper, using tetraethyl orthosilicate [TEOS, Si(OCH 2 CH 3 ) 4 ] as a silica source, phospholipid (PL) and dodecylamine (DA) were introduced as model organic additives to investigate their influence on the formation and morphology of silica in the mineralization process. Morphology, structure, and composition of the products were characterized using a range of techniques including FESEM, TEM, SAXRD, TG-DTA, solid-state 29 Si NMR, FTIR, and nitrogen physisorption. The FESEM and TEM analyses demonstrate that increasing PL concentrations at constant DA content leads to the formation of siliceous elongated structures. Localized enlargement can also be observed during further growth of elongated structures, displaying some features of the earliest recognizable stage of valve development in diatoms. In addition, in the presence or absence of PL, a series of control experiments using ammonia instead of DA show that no elongated structures are obtained, suggesting that the formation of elongated silica structures results from the cooperative interactions between PL and DA molecules. Because both organic amines (e.g., long-chain polyamines, LCPA) and phospholipid membranes (e.g., silicalemma) are of special importance for biosilicification in diatoms and sponges, our results imply that phospholipids are involved in the formation of organic aggregates, and thus influence the amines-mediated silica deposition. As such, our results may provide a new insight into the mechanism of biosilicification

The structural relaxation around the Co 2+ ion along the gahnite (ZnAl 2 O 4 )-Co-aluminate (CoAl 2 O 4 ) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn 1-y Co y Al 2 O 4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co 2+ for Al 3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co 2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm -1 with increasing cobalt amount, while the Racah B parameter is in the 744-751 cm -1 range (C ∼3375 cm -1 ). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ε = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn 2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility.

Fe 2+ - and Mn 2+ -rich tourmalines were used to test whether Fe 2+ and Mn 2+ substitute on the Z site of tourmaline to a detectable degree. Fe-rich tourmaline from a pegmatite from Lower Austria was characterized by crystal-structure refinement, chemical analyses, and Mössbauer and optical spectroscopy. The sample has large amounts of Fe 2+ (~2.3 apfu), and substantial amounts of Fe 3+ (~1.0 apfu). On basis of the collected data, the structural refinement and the spectroscopic data, an initial formula was determined by assigning the entire amount of Fe 3+ (no delocalized electrons) and Ti 4+ to the Z site and the amount of Fe 2+ and Fe 3+ from delocalized electrons to the Y-Z ED doublet (delocalized electrons between Y-Z and Y-Y): X (Na 0.9 Ca 0.1 ) Y (Fe 2+ 2.0 Al 0.4 Mn 2+ 0.3 Fe 3+ 0.2 ) Z (Al 4.8 Fe 3+ 0.8 Fe 2+ 0.2 Ti 4+ 0.1 ) T (Si 5.9 Al 0.1 )O 18 (BO 3 ) 3 V (OH) 3 W [O 0.5 F 0.3 (OH) 0.2 ] with a = 16.039(1) and c = 7.254(1) Å. This formula is consistent with lack of Fe 2+ at the Z site, apart from that occupancy connected with delocalization of a hopping electron. The formula was further modified by considering two ED doublets to yield: X (Na 0.9 Ca 0.1 ) Y (Fe 2+ 1.8 Al 0.5 Mn 2+ 0.3 Fe 3+ 0.3 ) Z (Al 4.8 Fe 3+ 0.7 Fe 2+ 0.4 Ti 4+ 0.1 ) T (Si 5.9 Al 0.1 )O 18 (BO 3 ) 3 V (OH) 3 W [O 0.5 F 0.3 (OH) 0.2 ]. This formula requires some Fe 2+ (~0.3 apfu) at the Z site, apart from that connected with delocalization of a hopping electron. Optical spectra were recorded from this sample as well as from two other Fe 2+ -rich tourmalines to determine if there is any evidence for Fe 2+ at Y and Z sites. If Fe 2+ were to occupy two different 6-coordinated sites in significant amounts and if these polyhedra have different geometries or metal-oxygen distances, bands from each site should be observed. However, even in high-quality spectra we see no evidence for such a doubling of the bands. We conclude that there is no ultimate proof for Fe 2+ at the Z site, apart from that occupancy connected with delocalization of hopping electrons involving Fe cations at the Y and Z sites. A very Mn-rich tourmaline from a pegmatite on Elba Island, Italy, was characterized by crystal-structure determination, chemical analyses, and optical spectroscopy. The optimized structural formula is X (Na 0.6 □ 0.4 ) Y (Mn 2+ 1.3 Al 1.2 Li 0.5 ) Z Al 6 T Si 6 O 18 (BO 3 ) 3 V (OH) 3 W[F 0.5 O 0.5 ], with a = 15.951(2) and c = 7.138(1) Å. Within a 3σ error there is no evidence for Mn occupancy at the Z site by refinement of Al ↔ Mn, and, thus, no final proof for Mn 2+ at the Z site, either. Oxidation of these tourmalines at 700-750 °C and 1 bar for 10-72 h converted Fe 2+ to Fe 3+ and Mn 2+ to Mn 3+ with concomitant exchange with Al of the Z site. The refined Z Fe content in the Fe-rich tourmaline increased by ~40% relative to its initial occupancy. The refined Y Fe content was smaller and the <Y-O> distance was significantly reduced relative to the unoxidized sample. A similar effect was observed for the oxidized Mn 2+ -rich tourmaline. Simultaneously, H and F were expelled from both samples as indicated by structural refinements, and H expulsion was indicated by infrared spectroscopy. The final species after oxidizing the Fe 2+ -rich tourmaline is buergerite. Its color had changed from blackish to brown-red. After oxidizing the Mn 2+ -rich tourmaline, the previously dark yellow sample was very dark brown-red, as expected for the oxidation of Mn 2+ to Mn 3+ . The unit-cell parameter a decreased during oxidation whereas the c parameter showed a slight increase.

Isothermal compression curves of face-centered cubic iron (γ-Fe) were determined at high temperatures (1273 and 1073 K) up to 27 GPa by in situ X-ray diffraction experiments using synchrotron radiation and the Kawai-type multi-anvil apparatus. Fits of the third-order Birch-Murnaghan equation of state to pressure-volume data yielded V 0 = 48.997 ± 0.040 Å 3 , K T0 = 108.3 ± 2.4 GPa, and K′ T = 5.8 ± 0.2 for 1273 K, and V 0 = 48.600 ± 0.098 Å 3 , K T0 = 88.9 ± 5.1 GPa, and K′ T = 8.9 ± 0.7 for 1073 K, where V 0 , K T0 , and K′ T are unit-cell volume, bulk modulus and its pressure derivative, respectively, at ambient pressure. The relatively large values of K′ T are attributable to successive electronic spin state transitions from mixed-spin at lower pressures to low-spin at higher pressures. When discussing the constituents of Earth’s (or other planets’) solid inner core in terms of density and equations of state, one must carefully consider the influence of the electronic spin state.

Understanding the physical and chemical properties of carbonate minerals at extreme conditions is important for modeling the deep carbon cycle, because they represent likely hosts for carbon in the lower mantle. Previous high-pressure studies have identified a structural and electronic phase transition in siderite using X-ray diffraction and X-ray emission spectroscopy. The Fe end-member of the carbonate group, siderite (FeCO 3 ), exhibits unique high-pressure behavior that we investigated using a combination of in situ Raman spectroscopy, synchrotron X-ray diffraction, and theoretical methods. In this Raman spectroscopy study, we observed the appearance of a new CO 3 symmetric stretching mode at 20 cm −1 lower frequency beginning at approximately 46 GPa. This softening is due to the lengthening of the C-O bonds as a result of a combination of rotation and volume shrinkage of the FeO 6 octahedra while siderite undergoes the isostructural volume collapse and electronic spin transition.

Vaterite is a less stable anhydrous crystalline calcium carbonate than calcite or aragonite and, thus, a rare mineral in geologic settings. However, vaterite is commonly found in biological environments. The mechanisms of crystal nucleation, transformation, and stabilization of vaterite in host materials remain unresolved. Understanding these issues may lead to answer some fundamental questions such as carbonate formation in geological systems and the intriguing occurrence of vaterite in biological systems. This requires an accurate knowledge of the crystal structure of vaterite and its order-disorder transformation. This study employs molecular-dynamics simulations to understand the thermodynamic stability of vaterite and kinetics of the orientational ordering of the carbonate ions. The results show that the potential energy change from disordered to ordered vaterite is about -11 kJ/mol, which significantly changes the relative stabilities of vaterite with respect to other anhydrous calcium carbonate polymorphs, including amorphous calcium carbonate. The heat capacity of vaterite is estimated to be 102.1 ± 0.4 J/(K⋅mol), comparable to an experimental result of 91.5 ± 3.8 J/(K⋅mol). The molecular-dynamics simulations also show similar energies for vaterite with different stacking structures, suggesting possible stacking disordering along the [001] axis. Cyclic high-temperature simulated-annealing molecular-dynamics simulations show that the CO 3 orientational disorder-order transition is thermally activated. The calculated activation energy for the transition is 94 ± 10 kJ/mol with a pre-exponential factor of ∼1.6 × 10 13 s −1 . A good linear fit of the logarithmic transition rate to inverse temperature (the Arrhenius plot) indicates that the transition is controlled by a single activation process that is related to a cooperative rotational motion of CO 3 groups in vaterite.

Metastable vaterite crystals were synthesized by increasing the pH and consequently the saturation states of Ca 2+ - and CO 2- 3 -containing solutions using an ammonia diffusion method. SEM and TEM analyses indicate that vaterite grains produced by this method are polycrystalline aggregates with a final morphology that has a sixfold-symmetry. The primary structure develops within an hour and is almost a spherical assemblage of nanoparticles (5-10 nm) with random orientation, followed by the formation of hexagonal platelets (1-2 μm), which are first composed of nanoparticles and that develop further into single crystals. As determined using transmission electron microscopy, these hexagonal crystallites are terminated by (001) surfaces and are bounded by {110} edges. The hexagonal crystals subsequently stack to form the “petals” (20 μm wide, 1 μm thick) of the final “flower-like” vaterite morphology. The large flakes gradually tilt toward the center as growth progresses so that their positions become more and more vertical, which eventually leads to a depression in the center. Since this sequence encompasses several morphologies observed in previous studies (spheres, hexagons, flowers etc.), they may actually represent different stages of growth rather than equilibrium morphologies for specific growth conditions.

The new mineral argesite, ammonium bismuth chloride (NH 4 ) 7 Bi 3 Cl 16 , was found in a mediumtemperature (~250 °C) active fumarole at La Fossa crater, Vulcano, Aeolian Islands, Sicily, Italy. The mineral occurs on a pyroclastic breccia as pale-yellow crystals up to 0.15 mm in length, in association with bismuthinite, adranosite, brontesite, demicheleite-(Br), demicheleite-(Cl), and panichiite. Argesite is trigonal, space group: R3̅c (no. 167) with Z = 18; the unit-cell parameters are (single-crystal data): a = 13.093(1), c = 102.682(1) Å, and V = 15245(2) Å 3 . The six strongest reflections in the X-ray powder diffraction pattern are: [d obs (Å) (I) (hkl)] 3.164 (100) (0 3 18), 3.808 (44) (2̅ 2 20), 2.742 (78) (2̅ 4 21), 6.14 (16) (1̅ 2 6), 1.906 (16) (0 0 5̅4̅), 1.686 (13) (5̅ 6 34). The mineral is uniaxial (-), with ω = 1.731(2), ε = 1.725(2) (589 nm). The IR spectrum shows absorptions at 3188(vs), 3060(s), and 1397(vs) cm -1 , in agreement with the presence of the ammonium ion. Chemical analyses obtained by EDS electron microprobe gave (average wt%) Bi 42.26, Cl 32.57, Br 13.06, I 0.95, K 2.46, Tl 0.88, NH 4 7.82 (by difference) total 100.00, corresponding to the empirical formula: [(NH 4 ) 6.29 K 0.91 Tl 0.06 ] Σ7.26 Bi 2.93 (Cl 13.33 Br 2.37 I 0.11 ) Σ15.81 . The measured density is 2.88(1) g/cm 3 . The structure was refined, using single-crystal diffraction data, to a final R = 0.0345 for 1289 independent observed reflections [I > 2σ(I)]. It contains Bi 2 Cl 4- 10 and BiCl 3- 6 anions where the Bi atoms are octahedrally coordinated, and NH + 4 cations are partially replaced by K + and Tl + ions.

We present the results of experimental studies on mineral and phase transformations in the model system K 2 O(Li 2 O)-Al 2 O 3 -SiO 2 -H 2 O-HF at 300 to 600 °C and 100 MPa using the method of univariant assemblages. The phase diagrams involve equilibrium curves among topaz, andalusite, muscovite, pyrophyllite, AlF 3 , and K n AlF 3+n built from our experiments, which have allowed us to determine the topaz stability field. Topaz is stable in solutions with HF concentrations from 3⋅10 −3 to 8⋅10 −1 m and with KF concentrations lower than 7.5⋅10 −3 m. As temperature increases, topaz becomes stable at higher HF concentrations. Application of the results to the Akchatau greisen W-Mo deposit provides an explanation of the observed zonation as a manifestation of metasomatic processes and imposes constraints on the mechanism and conditions of formation of the Akchatau deposit as well as on the compositions of the F-rich fluids participating in the greisenization.

The crystal structures of tobelite and NH + 4 -rich muscovite from the sedimentary rocks of the Armorican sandstones (Brittany, France) have been solved for the first time by single-crystal X-ray diffraction. The structural study was integrated by electron probe microanalyses, and X-ray photoelectron and micro-Fourier transform infrared spectroscopy. The crystals belong to the 2M 2 polytype with the following unit-cell parameters: a = 9.024(1), b = 5.2055(6), c = 20.825(3) Å, and â = 99.995(8)° for tobelite and a = 9.027(1), b = 5.1999(5), c = 20.616(3) Å, and β = 100.113(8)° for NH + 4 -rich muscovite. Structure refinements in the space group C2/c converged at R 1 = 8.01%, wR 2 = 8.84% and R 1 = 5.59%, wR 2 = 5.63% for tobelite and NH + 4 -rich muscovite, respectively. X-ray photoelectron spectroscopy revealed nitrogen environments associated either with inorganic (B.E. 401.31 eV) or organic (B.E. 398.67 eV) compounds. Infrared spectra showed, in the OH - - stretching region (3700-3575 cm -1 ), two prominent bands, centered at ~3629 and ~3646 cm -1 , and two shoulders at ~3664 and ~3615 cm -1 that were assigned to Al 3+ Al 3+ □-OH - arrangements having OH - groups affected by different local configurations. In addition, a series of overlapping bands from about 3500 to 2700 cm-1 characteristic of the NH + 4 -stretching vibrations, a main band at ~1430 and a shoulder at ~1460 cm -1 that were associated to the NH + 4 -bending vibration (ν 4 ) were also present. The ammonium concentration was semi-quantitatively estimated in both crystals from the absorbance of the OH - -stretching and NH + 4 -bending vibrations in the infrared spectra. An additional estimate was obtained for the NH + 4 -rich muscovite by considering the normalized peak area between K2p 3/2 and N1s in the X-ray photoelectron spectrum. The obtained values are in agreement with those derived from the interlayer spacing in the simulated X-ray powder diffraction spectra. The results of this integrated approach converged to (K 0.18 Na 0.01 NH + 4 0.62) ∑=0.81 (Al 1.98 Fe 2+ 0.02 ) ∑=2.00 (Si 3.19 Al 0.81 ) ∑=4.00 O 10.00 OH 2.00 for tobelite and to (K 0.46 Na 0.03 Ba 0.01 NH + 4 0.36 ) ∑=0.86 (Al 1.98 Mg 0.01 Fe 2+ 0.01 V 3+ 0.01 ) ∑=2.01 (Si 3.13 Al 0.87 ) ∑=4.00 O 10.00 F 0.08 OH 1.92 for NH + 4 -rich muscovite.

Two sets of precipitates collected from stream sediments in the Monte Romero (MR) and Tinto Santa Rosa (TSR) abandoned mine sites-located in the Iberian Pyrite Belt (IPB) of Spain-were identified as the iron oxyhydroxysulfate nanomineral schwertmannite using X-ray diffraction (XRD) and bulk digestion and were further studied in great detail using analytical high-resolution transmission electron microscopy (HRTEM). Extensive HRTEM observations suggest that schwertmannite should not be described as a single-phase mineral with a repeating unit cell, but as a polyphasic nanomineral with crystalline areas spanning less than a few nanometers within an amorphous matrix. The d-spacings measured from lattice fringes within schwertmannite’s needles match with d-spacings of the known transformation products of schwertmannite (goethite and jarosite). This finding implies that the initial stages of schwertmannite transformation occur as a gradual structural reordering at the nanoscale. Energy-dispersive X-ray analysis applied across individual schwertmannite needles with ∼3 nm spot size resolution reveal a decreasing ratio of sulfur to iron and silicon to iron from the surface of the needle to the core with the silicon to iron ratio consistently higher than the sulfur to iron ratio. Amorphous silicon-rich precipitates were identified on the surface of the TSR schwertmannite. All of these observations explain why the measured solubility product of schwertmannite is variable, resulting in calculated stability fields that differ greatly from sample to sample. Arsenic is the most abundant trace element in these samples [MR: 0.218(1) wt% and TSR: 0.53(2) wt%], keeping in mind that schwertmannite has been shown to be a key player in the cycling of this element on a global basis, particularly from the IPB. Furthermore, arsenic in the TSR schwertmannite is associated with crystalline areas within its needle matrix, implying that schwertmannite-derived goethite nanocrystals may be an important host of arsenic.

Wadsleyites with various iron contents were synthesized at ∼12-14 GPa and 1400 °C under oxidizing and hydrous conditions in coexistence with enstatite. The samples were studied using micro-X-ray absorption near edge structure (XANES) and micro-Mössbauer spectroscopy to determine the ferric iron contents in polyphasic samples and secondary ion mass spectrometry (SIMS) to determine the water concentrations. XANES and Mössbauer analyses show that ferric iron content increases with increasing total iron content, and reaches a maximum of ∼30% Fe 3+ /Fe total . Two XANES results were cross-checked by Mössbauer analysis and both methods are in reasonable agreement. The use of Fourier transform infrared spectroscopy reveals a new protonation scheme in wadsleyite, with a significant proportion of protons associated with the high-frequency band at 3611 cm -1 and a new band located at 3500 cm -1 . The intensity of these two bands is higher for Fe 3+ -rich wadsleyite. SIMS analyses show that water contents in wadsleyite vary from 4500 to 9400 ppm H 2 O by weight. Pyroxene water contents range from 790 to 1600 ppm wt H 2 O. The concentration of water in both phases decreases with increasing iron content. The partition coefficient of water between wadsleyite and pyroxene varies between 5 and 9 and increases with increasing Fe-number of wadsleyite [i.e., X Fe /(X Fe + X Mg ) × 100 ratio]. The divalent cation concentrations (i.e., Mg 2+ + Fe 2+ ), the Si as well as the H content in wadsleyite decrease with increasing Fe3+ content, indicating an incorporation mechanism via substitution into the metal (Me = Mg 2+ and Fe 2+ ) and Si sites with a ratio of 5/3 for (Fe 3+ +H + ):Me and of 5/1 for (Fe 3+ +H + ):Si, similarly as in the dry system. Thus, coupled substitution of Fe 3+ and H + does not affect the incorporation mechanism of Fe3 + , but does affect the location of H + , which is partly incorporated at tetrahedral edges forming [Fe Si '-(OH) o ·] X neutral defects substituting for Si. In this model 25% of the ferric iron occupies the tetrahedral sites.

This paper reports on the occurrence and the crystal structure of kircherite, a new member of the cancrinite-sodalite group of minerals from Valle Biachella, Sacrofano community (Rome, Latium, Italy). The mineral occurs in association with sodalite, biotite, iron oxides, titanite, fluorite, and a pyrochlore-group mineral. The groundmass of the ejectum consists essentially of K-feldspar with subordinate plagioclase. Kircherite (3 mm as largest size) is observed within miarolitic cavities of the rock and typically occurs as parallel associations of hexagonal, thin, tabular colorless to light-gray transparent crystals; it is non-pleochroic and uniaxial negative, with ω = 1.510(2) and ε = 1.502(2). D calc is 2.457 g/cm 3 . Kircherite is trigonal with a = 12.8770(7), c = 95.244(6) Å, V = 13677(1) Å 3 , Z = 1. The structure has been refined in the trigonal space group R32, obtaining a R-value of 8.5% on 8131 reflections with I/σI > 2. The strongest seven reflections in the X-ray powder pattern are [d in Å (I %) (hkl)]: 3.717 (100) (3 0 0), 2.648 (100) (2 1 28; 0 0 36), 3.232 (65) (2 1 19), 3.584 (60) (1 2 14), 3.604 (53) (1 0 25), 3.799 (52) (1 2 11), 3.220 (38) (2 2 0). The single-crystal FTIR spectrum rules out OH groups and shows the presence of H 2 O and CO 2 molecules in the structural cages of the mineral. Chemical analysis gives (in wt%): SiO 2 32.05, Al 2 O 3 27.13, FeO 0.07, K 2 O 4.38, CaO 8.75, Na 2 O 13.62, MgO 0.01, MnO 0.02, TiO 2 0.01, SO3 12.87, Cl 0.35, F 0.05, total 99.82. The empirical formula calculated on the basis of Σ(Si+Al) = 216 apfu is (Na 89.09 Ca 31.63 K 18.85 Fe 0.20 Mn 0.06 Mg 0.05 Ti 0.03 ) Σ=139.91 [(Si 108.13 Al 107.87 ) Σ=216.00 O 430.00 ](SO 4 ) 32.58 Cl 2.00 F 0.53 ⋅6.86H 2 O, which corresponds to the ideal formula [Na 90 Ca 36 K 18 ] Σ=144 (Si 108 Al 108 O 432 )(SO 4 ) 36 ⋅6H 2 O. The structure can be described as a stacking sequence of 36 layers of six-membered rings of tetrahedra along the c axis. The stacking sequence is ACABCABCABCACBCABCABCABCBABCABCABCAB…, where A, B, and C represent the positions of the rings within the layers. This sequence gives rise to cancrinite, sodalite, and losod cages, alternating along c. Sulfate groups occur within the sodalite and losod cages associated by Na, K, and Ca. H 2 O groups occur within the cancrinite cages, bonded to Ca and Na cations. Anion groups (SO 2- 4 ) in sodalite cages show positional disorder, and so do consequently the extraframework cation sites related to them.

Birnessite and other charged layered manganese oxide minerals exhibit interlayers with variable cation-water behavior that controls many environmentally important cation exchange, adsorption, and redox processes. The occurrence of birnessite phases as fine-grained materials with corresponding high-surface areas makes them effective in controlling soil sediment and groundwater compositions, but difficult to structurally characterize using conventional analytical methods. Molecular simulations provide an alternative approach in which many details of bulk and interlayer structure can be ascertained to supplement and interpret the experimental findings. Classical and electronic structure methods are used to evaluate Na-, K-, and Ba-birnessite phases. Computational results compare favorably with structures obtained by synchrotron X-ray diffraction and difference electron Fourier mapping of the interlayer region. Based on the analysis of the 1 ns atomic trajectories, dynamics of water molecules is enhanced in the interlayer of K-birnessite relative to the limited motion of water molecules and cations in the other birnessite phases. Molecular dynamics simulations of ranciéite, a complex layered manganese oxide having octahedral vacancies, indicate multiple sites for Ca 2+ in the interlayer. In addition to manganese layer charge and layer structure, the hydration enthalpy for the interlayer cation affects the structure and dynamics of the interlayer in birnessite minerals.

Pyrite, the most abundant sulfide on Earth and a common component of gold deposits, can be a significant host for refractory gold. This is the first documentation of pore-attached, composite Autelluride nanoparticles in “arsenic-free” pyrite. Trace elements mapping in pyrite from an intrusionhosted Au deposit with orogenic overprint (Dongping, China) shows trails of tellurides overlapping Co-Ni-zonation. Intragranular microfracturing, anomalous anisotropy, and high porosity are all features consistent with devolatilization attributable to the orogenic event. The pyrite-hosted nanoparticles are likely the “frozen,” solid expression of Te-rich, Au-Ag-Pb-bearing vapors discharged at this stage. Nanoparticle formation, as presented here, provides the “smallest-scale” tool to fingerprint Au-trapping during crustal metamorphism

Although the crustal abundance of tellurium (Te) is about half of that of gold (Au), several classes of Au deposits are highly enriched in Te. Our understanding of the nature of this Au-Te association is hampered by the lack of experimental studies of Te geochemistry at elevated temperature. We characterized the structure of polytelluride solutions from room temperature to 599 °C at 800 bar using in situ X-ray absorption spectroscopy. Both ab-initio XANES and EXAFS fits show that polytellurides are stable up to the highest temperature, with planar structures (four- or threefold coordination of Te) giving way to linear chains (e.g., Te 2- 2 ion) at temperatures above ~200 °C. This is the first experimental confirmation of the thermal stability of polytelluride species. The data show that polytellurides play an important role in Te transport in reduced S-rich or CO 2 -rich solutions and vapors.