Band 98, Heft 8-9

Carbonaceous chondrites make up a majority of asteroids in the asteroid belt, yet are relatively rare in meteorite collections at approximately 5%. The rare CR carbonaceous chondrites are providing a wealth of new discoveries including (1) abundant organic compounds required for biochemical processes, (2) pre-solar mineral grains that hold isotopic records of stellar processes, (3) inclusions, chondrules, and chondrule rims that contain information about the early Solar System, (4) hydrous minerals that formed during aqueous alteration of the matrix and other components, and (5) foreign clasts that formed at slightly higher pressures than normally expected in the early Solar System. All of these discoveries suggest these carbonaceous meteorites will continue providing new information and help to revise our understanding of the broad range of conditions existing in the early Solar System.

The external detector method (EDM) is a widely used technique in fission track thermochronology (FTT) in which two different minerals are concomitantly employed: spontaneous tracks are observed in apatite and induced ones in the muscovite external detector. They show intrinsic differences in detection and etching properties that should be taken into account. In this work, new geometry factor values, g, in apatite, were obtained by directly measuring the ρ ed /ρ is ratios and independently determined [GQR] ed/is values through the measurement of projected lengths. Five mounts, two of which were large area prismatic sections and three samples composed of random-orientation pieces have been used to determine the g-values. A side effect of applying EDM is that the value of the initial confined induced fission track, L 0 , is not measured in routine analyses. The L 0 -value is an important parameter to quantify with good confidence the degree of annealing of the spontaneous fission tracks in unknown-age samples, and is essential for accurate thermal history modeling. The impact of using arbitrary L 0 -values on the inference of sample thermal history is investigated and discussed. The measurement of the L 0 -value for each sample to be dated using an extra irradiated apatite mount is proposed. This extra mount can be also used for determining the g value as an extension of the ρ ed /ρ is ratio method. Eight apatite samples from crystalline basement, with grains at random orientation, were used to determine the g-values. The results found are statistically in agreement with the values found for apatite samples (from Durango, Mexico) measured in prismatic section and also measured at random orientation. There was no observable variation in efficiency regarding crystal orientation, showing that it is relatively safe using non-prismatic grains, especially in samples with paucity of grains, as it is the case of most basin samples. Implications for the ζ-calibration and for the calibration of the direct (spectrometer-based) fission-track dating are also discussed.

Chemical Th-U-total Pb isochron method (CHIME) of monazite and xenotime from three leucogranites in far-eastern Nepal revealed the presence of Carboniferous inherited grains of monazite and intense intrusion of leucogranites around 18-16 Ma in the High Himalaya. In garnet-bearing sillimanitemuscovite- biotite leucogranite, most monazite grains have inherited cores with the chemical dates of ~504-418 and ~342-272 Ma, which were overgrown by Early Miocene mantles and rims of 17.9 ± 1.5 Ma. Early Ordoviciain and Carboniferous ages are rarely found in the same euhedral-subhedral cores. In addition to the previously recognized Early Paleozoic magmatism, monazite cores with Carboniferous ages in Early Miocene leucogranites provide evidence for two periods of magmatism at the base of the High Himalaya prior to the Cenozoic Himalayan orogeny. In muscovite-biotite leucogranite, no inherited domains were observed in monazite and xenotime grains. They yielded the CHIME monazite and xenotime dates of 16.1 ± 2.0 and 19.8 ± 6.5 Ma, respectively. Monazite grains adjacent to xenotime have significantly lower concentrations of UO 2 and Y 2 O 3 compared to those isolated from xenotime. These results imply that xenotime influences Y and U contents in monazite, reflecting local equilibrium system. In aplitic leucogranite, monazite grains yielded the mean of apparent chemical date of 18.0 ± 2.2 Ma. The CHIME monazite ages of ~18-16 Ma in three leucogranites reflect the timing of melt crystallization.

Diffusion of helium has been characterized in natural monazite and synthetic LREE phosphates. Polished slabs of natural monazite and flat growth faces of synthetic phosphates were implanted with 100 keV 3 He at a dose of 5 × 10 15 3 He/cm 2 and annealed in 1 atm furnaces. 3 He distributions in monazites following experiments were measured with nuclear reaction analysis using the reaction 3 He(d,p) 4 He. For diffusion in monazite we obtain the following Arrhenius relation for diffusion normal to (100) D = 1.60 × 10 -7 exp(-150 ± 8 kJ/mol/RT) m 2 /s. Diffusion in natural monazite exhibits little diffusional anisotropy, as diffusion normal to (001), (100), and (010) appear similar, as do diffusivities of He in natural monazites from two different localities. Over the investigated temperature range, these diffusivities are similar to those of Farley and Stockli (2002), and values obtained for some of the monazite grains analyzed by Boyce et al. (2005), obtained through bulk-release of He by step heating, and are bracketed by diffusivities obtained by Farley (2007) for monazite-structure synthetic REE phosphates. Helium diffusion measured in synthetic REE phosphates (LaPO 4 , NdPO 4 , SmPO 4 , and EuPO 4 ) with the monazite structure appears to behave systematically, with diffusion in the lighter REE phosphates slightly faster than in the heavier REE phosphates. This trend is in broad agreement with the findings of Farley (2007), but the difference in diffusivities among the REE phosphates is smaller than that reported by Farley (2007). Activation energies for He diffusion in the synthetic LREE phosphates are similar to that for natural monazite measured in this study, but He diffusivities in synthetic LaPO 4 are about an order of magnitude faster than in the natural monazites. The differences in He diffusivities among the natural monazites and synthetic REE phosphates may be a consequence of the density and distribution of interstitial apertures in the crystal structure, which may result in differences in He migration rates. The similarities in He diffusivities for natural monazites containing varying mixtures of rare-earth elements, as well as differences in Th contents, however, indicate that the above Arrhenius relation is most relevant for describing He diffusion in monazite in nature. We use this Arrhenius relation to evaluate He retentivity in monazite experiencing thermal events, using both the closure temperature formulation of Dodson (1973) and recently developed expressions (Watson and Cherniak 2013) for prograde thermal events and heating and cooling trajectories. Calculations indicate that monazite is relatively retentive of He compared with accessory mineral phases.

The crystal structure of a high-pressure phase of calcium hydroxide, Ca(OH) 2 (portlandite), was clarified for the first time using the combination of in situ single-crystal and powder X‑ray diffraction measurements at high pressure and room temperature. A diamond-anvil cell with a wide opening angle and cell-assembly was improved for single-crystal X‑ray diffraction experiments, which allowed us to successfully observe Bragg reflections in a wide range of reciprocal space. The transition occurred at 6 GPa and the crystal structure of the high-pressure phase was determined to be monoclinic at 8.9 GPa and room temperature [I121; a = 5.8882(10), b = 6.8408(9), c = 8.9334(15) Å, β = 104.798(15)°]. The transition involved a decrease in molar volume by approximately 5.8%. A comparison of the structures of the low- and high-pressure phases indicates that the transition occurs by a shift of CaO 6 octahedral layers in the a-b plane along the a-axis, accompanied by up-and-down displacements of Ca atoms from the a-b plane. The crystal structure of this high-pressure phase is considered to be an intermediate state between the low-pressure phase and the high-pressure-high-temperature phase. The complicated diffraction patterns of the high-pressure phase suggest that the phase transition occurred toward three directions around the c-axis of the low-pressure phase. This explains the difficulties encountered in previous structural analyses. The present results will provide key information for discussing the behavior of hydrogen bonds in these hydrous minerals under pressure.

In this study, deuteric coarsening of a lamellar cryptoperthite to a patch perthite has been experimentally induced for the first time. An homogeneous alkali feldspar, bulk composition Ab60Or40, was produced by molten salt ion-exchange with a gem quality orthoclase from Madagascar. This was then annealed isothermally for 32 days at 550 °C. This resulted in a coherent lamellar cryptoperthite (periodicity 28 nm) probably by spinodal decomposition and subsequent coarsening. The cryptoperthite was then reacted with 20 wt% H 2 O or 0.1 M HCl at 400 and 500 °C at 200, 400, and 1000 MPa for 96 h and for 192 h at 1500 MPa. At 1000 and 1500 MPa and 500 °C, the cryptoperthitic crystal fragments were partially replaced by a mosaic of albite and sanidine subgrains in the form of a patch perthite. Other than the Gibb’s free energy of reaction as derived from the chemical potentials of the reactant and product phases, the second principle mechanism responsible for driving this reaction is interface-coupled dissolution-reprecipitation driven by a reduction in the coherency strain energy stored within the initial cryptoperthite. The main indicator of this is a change in the microtexture due to the destruction of the coherent lamellar cryptoperthite and its replacement by an assemblage of incoherent subgrains. Detailed mass balancing, based on XRD and EPMA data, indicate that conversion of cryptoperthite to patch perthite is not isochemical. Experimental formation of patch perthite appears to take place in two steps. Between the original cryptoperthite and the patch perthite, TEM investigation identified a “transition zone,” characterized by a significant coarsening of the preexisting lamellar microstructure, without any sign of a loss in lattice coherency. This zone is probably the result of hydrous species diffusing into the cryptoperthite causing the H 2 O-enhanced diffusion of Na and K and hence lamellar coarsening. In the second step, this coarsened microstructure is replaced by the patch perthite. The polycrystalline patch perthite is characterized by a very complex distribution of albite and sanidine sub-grains. Even the larger albite or sanidine patches are polycrystalline. This inevitably results in a three-dimensional network of grain boundaries characterized by high defect densities resulting from the structural misfit between the albite and sanidine sub-grains. An increase in average grain size with distance from the interface indicates that secondary coarsening of the initially, very fine-grained intergrowth occurred. This was driven by the reduction of interface and surface energy similar to the coarsening observed in multiphase ceramics. In addition, the newly formed patch perthite is highly porous despite the fact that the results obtained in this study show that the replacement of cryptoperthite by patch perthite is nearly isovolumetric (ΔV ∼ -0.6%). The observed reaction-induced porosity results from differences in relative solubilities between the cryptoperthite and the patch perthite. Porosity abundance has been estimated at approximately 10 to 11 vol% based on the amount of quartz formed during the experiments. Three-dimensional investigations via FIB serial sectioning indicate almost no interconnectivity between pores. A consequence of albite and sanidine sub-grain coarsening is the constant redistribution of the porosity and the sub-grain boundaries within the patch perthite mosaic over time. This suggests that a dynamically evolving porosity could provide an important transport mechanism for fluid transport through rocks even when the abundance and interconnectivity of the reaction-induced porosity is low.

Oxy-dravite, Na(Al 2 Mg)(Al 5 Mg)(Si 6 O 18 )(BO 3 ) 3 (OH) 3 O, is a new mineral of the tourmaline supergroup. The holotype specimen originates from the locality of Osarara (Narok district, Kenya) and occurs in quartz-muscovite schist. Crystals of oxy-dravite are dark red, partially translucent with a vitreous luster, a pink streak, and conchoidal fracture. It has a Mohs hardness of approximately 7, and a calculated density of 3.073 g/cm 3 . In plane-polarized light, oxy-dravite is pleochroic (O = orange and E = pink) and uniaxial negative: ω = 1.650(5), ε = 1.620(5). Oxy-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 15.9273(2) and c = 7.2001(1) Å, V = 1581.81(4) Å 3 , Z = 3. Chemical characterization based on electron microprobe analysis, single-crystal structure refinement, Mössbauer, and optical spectroscopy, resulted in the empirical structural formula: X(Na 0.83 □ 0.15 K 0.02 ) ∑1.00 Y (Al 1.34 Fe 3+ 0.58 Mg 1.03 Fe 2+ 0.03 Ti 0.02 ) ∑3.00 Z (Al 4.95 Mg 1.03 Fe 3+ 0.02 ) ∑6.00 T (Si 5.98 Al 0.02 O 18 ) ∑6.00 B (BO 3 ) 3 V (OH) 3 W (O 0.76 OH 0.24 ) ∑1.00 . While the end-member formula of oxy-dravite may be formalized as Na Y (Al 3 )Z(Al 4 Mg 2 ) Si 6 O 18 (BO 3 ) 3 (OH) 3 O, the most representative structural formula is Na Y (Al 2 Mg) Z (Al 5 Mg) Si 6 O 18 (BO 3 ) 3 (OH) 3 O. The difference between these two formulas is solely in Al-Mg order-disorder, i.e., there is no difference in chemical composition. Although the Mg-Al disorder over the Y and Z sites is controlled by the short-range bond-valence requirements of O 2- at the O1 (≡W) site, the amount of Mg at the Z site is a function of the degree of cation size mismatch at Z. The crystal structure of oxy-dravite was refined to statistical index R1 of 1.17% using 1586 equivalent reflections collected with MoKα X-radiation. Oxy-dravite is chemically related to dravite (and fluor-dravite), NaMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 (OH,F), by the heterovalent substitution Al 3+ + O 2- → Mg 2+ + (OH,F) 1- .

We have investigated experimentally the partitioning of Au between solid and liquid sulfide phases and basaltic melts at 200 MPa, at redox conditions close to the sulfide-sulfate transition, over temperatures between 1050 and 1200 °C, which span the monosulfide solid solution (MSS) - sulfide liquid (SuL) solidus. The measured MSS/basalt partition coefficient of Au (D MSS-sil Au ) is about 100-200, whereas the partition coefficient of sulfide liquid/basalt (D MSS-sil Au ) is approximately 10 times larger at 2200. Although we find that temperature, pressure, and oxygen fugacity (ƒ O₂ ) exert relatively weak controls on Au partitioning, they exert major indirect influences on Au behavior by controlling the identity of the condensed sulfide phase and by affecting S solubility. These observations have important implications for the behavior of Au in the processes of partial melting in the mantle and magma crystallization in the crust. The occurrence of natural magmas with elevated concentrations of Au and presumably other highly siderophile and chalcophile elements requires predominance of MSS over SuL in the source or/and oxidizing conditions close to or above the sulfide-sulfate transition in the magma.

Much of the bioavailable and geochemically reactive iron in aerobic, circumneutral settings is frequently found in the form of nanoscale particles of a hydrated iron(III) oxyhydroxide phase known as ferrihydrite. Developing useful structural descriptions of defective nanophases such as ferrihydrite has long posed significant challenges. Recently, Michel et al. (2007, 2010) proposed a structural model for ferrihydrite in place of the long-accepted model of Drits et al. (1993). Both models reproduce to high accuracy certain forms of X‑ray scattering data from powdered ferrihydrite. However, discrepancies remain that we hypothesized are due to forms of structural disorder not easily represented by existing models. To test this hypothesis, we performed a novel structural analysis of total X‑ray scattering data acquired from 6-line ferrihydrite. We generated three candidate whole-nanoparticle models of ferrihydrite composed of a two-phase Drits model, the Michel model, and a hybrid phase based on a single-phase Drits model that incorporated tetrahedral Fe sites, creating a lattice in which the Michel model was one of many possible topologies. We implemented a reverse Monte Carlo (RMC) approach to explore alternative configurations of iron occupancies plus structural disorder, and to refine the nanoparticle structure using both the reciprocal and real-space forms of the X‑ray scattering data. We additionally used oxygen K-edge X‑ray absorption spectroscopy to semi-quantitatively assess the ratio of protonated:non-protonated oxygen sites in an iron(III) oxides. This analysis provides independent evidence for a significantly lower OH:O stoichiometric ratio for ferrihydrite than for goethite, further constraining the RMC models. The hybrid structure model gave better agreement to the experimental total scattering data than nanoparticles based upon either the Michel or Drits models. Models that incorporated tetrahedrally coordinated iron sites consistently achieved better matches to the data than models containing facesharing octahedra. Long-range vacancy disorder was essential for optimum fits to the scattering data, highlighting the utility of whole-nanoparticle models in place of unit-cell models with random distributions of iron vacancies. The RMC-derived structures do not satisfy all experimental constraints on composition and structure. Nevertheless this work illustrates that a suitably constrained RMC method applied to whole-nanoparticle models can be an effective approach for exploring disorder in nanocrystalline materials.

In this study, we carried out Pb/Pb dating of tranquillityite in high-Ti mare basalts 10044, 75055, and 74255, using a Cameca NanoSIMS 50 at a spatial resolution of ~3 μm. The analyses yielded 207 Pb/ 206 Pb dates of 3722 ± 11 Ma for sample 10044, 3772 ± 9 Ma for sample 75055, and 3739 ± 10 Ma for sample 74255, at 95% confidence level. These dates are consistent with previously determined crystallization and emplacement ages of these samples using different radiogenic systems. These high-precision ages allow refinement of the timing of some of the high-Ti basaltic volcanism on the Moon. Crystallization ages of three different high-Ti basalt units, integrating these new Pb/Pb ages with previous Rb-Sr and Sm-Nd age determinations, are consistent with previous estimates but associated with uncertainties 3 to 5 times lower. In addition, the data obtained in this study confirm that tranquillityite contains very low amounts of initial common Pb and has a high-Pb ionization efficiency, making it an excellent candidate for Pb/Pb dating by ion microprobe. The higher spatial resolution afforded by NanoSIMS 50 and the recent discovery of tranquillityite in several terrestrial mafic rocks opens up a new area of research allowing an independent and rapid age dating of basaltic rocks in polished sections.

We explore a method to utilize the stoichiometry of iron sulfide to determine the sulfur fugacity in experiments containing CO-CO 2 -SO 2 gas mixtures. The Fe-S phase diagram shows that the stoichiometry of iron sulfide melts is closely related to the sulfur fugacity ƒs 2 at a given temperature and ambient pressure. We derive equations that relate the sulfur fugacity to the mole fraction of sulfur X s in the iron sulfide from available literature data, and a solution model using the Redlich-Kister approximation for the excess Gibbs energy of mixing. We test the method by exposing iron sulfide to CO-CO 2 -SO 2 gas mixtures and subsequently analyzing the “Fe-S monitor” for its stoichiometry. Most sulfur fugacities calculated from the equilibrium gas composition agree within 5-10% (1200 °C) and 1-4% (1400 °C) with those derived from the sulfur mole fraction in the monitor and literature data calibration, which is consistent with the spread observed in literature data. There were no suitable literature data for a full calibration at 1300 °C, so we combined the available literature and our data to find the sulfur fugacity as a function of mole fraction sulfur. Overall the Fe-S monitor technique is a convenient method to determine the sulfur fugacity in high-temperature experiments containing CO-CO 2 -SO 2 gas mixtures as long as oxygen fugacities remain below that of the iron-wüstite or iron-magnetite buffer.

The temperature variations of the ferric and ferrous hyperfine fields in natural samples of strunzite, ferristrunzite, and ferrostrunzite are examined by Mössbauer spectroscopy between 4.2 K and their respective magnetic transition temperatures. The spectra of all three strunzite species are composed of two dominant sextets with equal contribution and with hyperfine fields at 4.2 K of approximately 48.5 and 48.0 T, respectively. They are assigned to Fe 3+ cations in the Fe(1) and Fe(2) sites in the crystallographic structure. A third magnetic component, with field of 54.8 ± 0.1 T, is well resolved in the 4.2 K spectrum of ferristrunzite and is attributed to Fe3+ in the Mn sites of the structure. This third ferric contribution also appears in spectra of strunzite and ferrostrunzite, however with smaller relative spectral areas as compared to ferristrunzite, i.e., ∼10% and ∼3% of total spectral area, respectively, where it is 30% for ferristrunzite. For ferrostrunzite, two additional magnetic components are recognized. They are both attributed to Fe 2+ substituting in the Mn sites. Their hyperfine fields at 4.2 K are 21.8 ± 0.5 T and 28.6 ± 0.5 T, respectively, and their relative contributions 11% and 15%. With increasing temperature, enhanced asymmetric broadening of the absorption lines is observed for all three samples. This phenomenon is due to relaxation of the magnetic moments, thus causing gradually broader distribution ranges of the respective hyperfine fields. The spectra have been fitted with superpositions of model-independent hyperfine-field distributions. The temperature dependencies of the maximum-probability hyperfine fields of the two ferric subspectra associated with the Fe(1) and Fe(2) sites are interpreted in terms of the molecular field theory taking into account the occurrence of exchange magnetostriction. The variations with temperature of the magnetic hyperfine fields acting at the Mn sites indicate that the magnetization of the Mn sublattice shows re-entrant spin glass behavior. This feature is most prominent for the strunzite sample.

Binary and ternary sulfides in the Fe-Ni-S system are of major fundamental and practical interest in environmental, geological, and planetary science. Despite extensive thermodynamic modeling of phase equilibria, the enthalpies of formation of solid solutions in this system have not been measured directly as functions of both nickel/iron and metal/sulfur ratios. This communication reports enthalpies of formation of Fe-Ni monosulfide solid solutions [mss = (Fe 1-x Ni x ) 1-z S, 0 ≤ x ≤ 1; 0.875 ≤ 1-z ≤ 1] quenched from 750 °C, representing disordered structural states. Measurements were performed on three series of solid solutions using oxidative solution calorimetry in molten sodium molybdate solvent at 702 °C. The measured enthalpies of formation of Ni 1-z S solid solutions from elements are consistent with prior data reported for a few compositions. The enthalpies of formation of ternary compositions, (Fe 1-x Ni x ) 1-z S, are reported for the first time. Within their uncertainties, the enthalpies of formation of the solid solutions, quenched from high temperature, with respect to the end-members (enthalpies of mixing) at 25 °C are zero. An experimentally derived equation is proposed for the calculation of enthalpies of formation of all high-temperature monosulfide solid solutions in the Fe-Ni-S system. The thermodynamic stabilization of these disordered solid solutions arises not from energetic terms but from the configurational entropy.

Calcite (CaCO 3 ) powder was replaced by hydroxylapatite [Ca 5 (PO 4 ) 3 OH] in hydrothermal experiments at temperatures between 120 and 180 °C, and times between 2 and 12 h, in 1.0 M (NH 4 ) 2 HPO 4 solutions. Additional experiments using concentrations of up to 2.0 M NH 4 Cl as an additional component were carried out to examine the influence of Cl on the process. X‑ray powder diffraction and Rietveld refinements were used to determine the proportion of parent and product phases after each experiment. The resultant data on the fraction transformed as a function of temperature and time were plotted according to the Avrami equation and an activation energy of 118 kJ/mol was determined for the replacement process without Cl in solution, whereas with 0.5-2.0 M Cl in solution, the activation energy varied from 131 to 145 kJ/mol. No simple correlation between the chlorinity of the solution and the value of the activation energy was observed. However, the data indicate that the mechanism controlling the reaction changes between 140 and 160 °C, correlating with systematic differences in abundance, size, and geometry of the porosity in the apatite formed in the replacement process. At lower temperature, the pores are small and oriented parallel to the reaction interface, whereas at higher temperature, the pores are larger and orientated perpendicular to the interface. Changes in the porosity during the replacement process make it problematic to determine a meaningful value for the activation energy for such replacement processes, since the mechanism controlling the reaction depends on factors other than the temperature, such as the availability and form of the pathways allowing the fluid to reach the unreacted core of the single crystals.

Fe 2+ and Mg distribution on octahedral M1 and M2 sites of the orthopyroxene structure is an indicator of the cooling rate and closure temperature of the mineral. It is generally obtained by single-crystal X‑ray diffraction, which is limited in spatial resolution. In this work, we determine the cationic distribution at a submicrometer scale in a transmission electron microscope using precession electron diffraction. Two orthopyroxene samples coming from the same metamorphic rock are studied, a naturally ordered one and a disordered one. The latter was obtained from the ordered sample by annealing at high temperature and rapid quenching. Both samples have been first studied in X‑ray diffraction and then in precession electron diffraction. Intensities recorded in zone-axis precession electron diffraction experiments have been quantitatively analyzed and compared to simulations, taking into account dynamical interactions between diffracted beams. Our structure refinement results are in good agreement with those obtained by single-crystal X‑ray diffraction. They enable to distinguish between the ordered sample and the disordered one in terms of the observed molar fractions of Fe at M1 and M2 sites. We discuss the sensitivity of the method as a function of experimental parameters. The larger dispersion of the results obtained on the ordered specimen is attributed to structural heterogeneities inherent to the sample.

Organoclays, unlike natural clays with inorganic cations that often have a hydration shell of H 2 O molecules, are organophylic and less adsorptive of H 2 O. These clays, therefore, are potentially important to remove organic contaminants from water; they are of great interest in industry for herbicide manufacture and as the basis for nanocomposite development; and they are of general interest in agriculture and in understanding soils. However, nothing is known about the positions of H 2 O in the interlayer when these molecules intercalate along with medium-sized hydrocarbon molecules, such as tetramethylammomium (TMA) cations. Even the positions of the TMA cations in the interlayer have been questioned recently. To resolve these issues, the orientation and position of TMA and H 2 O in the interlayer of vermiculite and montmorillonite were investigated by using atomistic computational methods. Interlayer H 2 O content, layer charge, and location of layer charge were considered. For both vermiculite and montmorillonite and where the number of H 2 O molecules is sufficient, TMA cations are located alternating between two planes in the interlayer. Each TMA cation is located near a tetrahedral-ring cavity of a 2:1 layer bordering the interlayer, and the H 2 O molecules are disordered. In the absence of H 2 O, TMA cations occur in one plane at the center of the interlayer. The major difference between vermiculite and montmorillonite is that the center of the TMA molecule in montmorillonite is 0.87 Å from the center of the interlayer as compared to 1.22 Å in vermiculite. Thus, the TMA cation is located closer to the tetrahedral-ring cavity in vermiculite, and this is a result of the greater tetrahedral charge of vermiculite. In fluorohectorite, which is similar in layer charge and origin of layer charge as the montmorillonite composition studied, the position of the TMA is expected to be similar to the montmorillonite results. These computational models are consistent with single-crystal, X‑ray diffraction experiments for hydrated TMA-exchanged vermiculite and dried fluorohectorite.

The rare evaporite minerals hanksite, Na 22 K(SO 4 ) 9 (CO 3 ) 2 Cl, and tychite, Na 6 Mg 2 (CO 3 ) 4 (SO 4 ), are excellent case studies for the high-pressure behavior of ionic groups since their structures combine ionic complexity and high symmetry (hexagonal P6 3 /m and cubic Fd3, respectively). Here we investigate the structure and compressibility of hanksite up to 20 GPa in the diamond-anvil cell using Raman spectroscopy and X‑ray diffraction and of tychite up to 17 GPa in the diamond cell using X‑ray diffraction and first-principles modeling. At ambient pressure, the Raman spectrum of hanksite has a single sulfate ν 1 frequency at 992 cm -1 with a lower-frequency shoulder. As pressure is increased, this mode splits into two distinct peaks, which arise from two distinct local environments for the sulfate tetrahedra within the hanksite structure. Below 10 GPa, the mode Grüneisen parameter of the dominant sulfate ν 1 frequency is 0.27(1); the mode Grüneisen parameter of the lower frequency shoulder is 0.199(7). X‑ray diffraction data of hanksite indicate a 5% volume drop between 8-10 GPa with no apparent change of symmetry. A Birch-Murnaghan fit to the data below 8 GPa yields an isothermal bulk modulus of 66(1) GPa for hanksite and 85(1) GPa for tychite, with K′ fixed at 4.

Manganese(II) monosulphide crystallizes into three different polymorphs (α-, β-, and γ-MnS). Out of these, α-MnS, also known as mineral alabandite, is considered the most stable and is widespread in terrestrial materials as well as in extraterrestrial objects such as meteorites. In this study, a low-temperature antiferromagnetic state of α-MnS was investigated using macroscopic magnetic measurements as induced and remanent field-cooled (FC) and zero-field-cooled (ZFC) magnetizations and magnetic hysteresis. Both natural alabandite and synthetic samples show: (1) Néel temperatures in a narrow temperature range around 153 K, and (2) a rapid increase of the magnetization around 40 K. An anomalous magnetic behavior taking place at about 40 K was previously ascribed to the magnetic transition from a high-temperature antiferromagnetic to a low-temperature ferromagnetic state documented for non-stoichiometric α-MnS slightly enriched in manganese. However, our detailed microscopic observations and, in particular, oxidation experiments indicate that the anomalous magnetic behavior around 40 K is caused by the presence of an oxide layer of ferrimagnetic hausmannite (Mn 3 O 4 ) on the surface of α-MnS rather than being an intrinsic property of nearly stoichiometric α-MnS.

An Fe-bearing chromo-alumino-povondraite sample from the Sludyanka crystalline complex (Lake Baikal, Russia) was characterized using single-crystal X‑ray diffraction, electron microprobe, Mössbauer, infrared, and optical absorption spectroscopy. The symmetry is rhombohedral, space group R3m, with unit-cell parameters a = 16.0032(2), c = 7.2823(1) Å, V = 1615.15(4) Å 3 , Z = 3. The crystal structure of the Fe-bearing chromo-alumino-povondraite was refined to an R1 index for all reflections of 1.74% using MoKα X‑ray intensity data. Crystal chemical analysis resulted in the empirical structural formula: X (Na 0.76 Ca 0.19 □ 0.03 K 0.02 ) Y (Cr 3+ 1.94 Mg 0.93 Al 0.07 Ti 4+ 0.06 ) Z (Al 3.74 Mg 1.43 Fe 3+ 0.32 V 3+ 0.19 Cr 3+ 0.16 Fe 2+ 0.15 ) [T(Si 5.99 Al 0.01 )O 18 ] B (BO 3 ) 3 V (OH) 3 W [O 0.69 F 0.23 (OH) 0.08 ]. This formula, in which Cr prefers the Y site and Al the Z site, is compatible with the end-member NaCr 3 (Al 4 Mg 2 )(Si 6 O 18 )(BO 3 ) 3 (OH) 3 O. Magnesium is disordered over Y and Z, whereas Fe and V 3+ are ordered at Z. Ideally, chromo-alumino-povondraite is related to oxy-chromium-dravite and oxy-dravite by the homovalent substitution Cr 3+ ↔ Al 3+ . Tourmaline chemical compositions classified as chromoalumino- povondraite can be either Cr-dominant or Al-dominant as a result of the compositional boundaries along the solid solution between Cr and Al that are determined at Y+Z (Cr 5 Al 2 ), corresponding to Na Y (Cr 3 ) Z (Cr 2 Al 2 Mg 2 )Si 6 O 18 (BO 3 ) 3 (OH) 3 O, and Y+Z (Cr 1.5 Al 5.5 ), corresponding to Na Y (Cr 1.5 Al 1.5 ) Z (Al 4 Mg 2 )Si 6 O 18 (BO 3 ) 3 (OH) 3 O.

The stability field of siderite has been determined up to 10 GPa. Decarbonation of siderite occurs at pressures below 6 GPa with a Clapeyron slope of about 0.0082 GPa/K. At higher pressure, we observed direct melting of siderite without decarbonation. The melting temperature is about 1550 °C at 10 GPa. Our experimental results, compared with previous studies on the decomposition curve of magnesite, indicate that Fe has a significant effect on the stability of magnesite-siderite solid solutions under upper mantle conditions. The reaction products are strongly dependent on the oxygen fugacity of the system. The disproportionation reaction during decomposition of siderite might be an important mechanism to explain the stability of carbon as graphite (diamond) in the Earth’s mantle.

Rare-earth silicate oxy-apatites have been found to exhibit high ion-conductivity along channels within their structure, which makes them candidate materials for solid oxide fuel cell electrolytes. It is not understood so far why this high ion-conductivity is restricted to oxide-ion transport and does not occur for halide ions common in apatite type minerals. This study reports on the relationship between the topology of these structural channels and the spatial displacement of the chloride ion in three different structures of natural apatite group minerals [Pb 5 (BO 4 ) 3 Cl (Z = 2) with B = P (pyromorphite), V (vanadinite), and As (mimetite)] using single-crystal X‑ray diffraction. All of these minerals crystallize in the hexagonal chlorapatite structure with space group P63/m with no symmetry lowering or site splitting. The anion channel is built from a face-sharing array of nearly regular Pb26 octahedra running parallel to the c-axis, and the chloride ions were found at the center of each octahedron with bond-valence sums of 1.10 for mimetite and vanadinite and 1.25 for pyromorphite. The mean square displacement (msd) of the chloride ion in [001] was found to be a function of the size of Pb26 octahedron in that direction. This position is also the center of a flat O36 trigonal antiprism. The msds of the chloride ion in the x-y plane were found to be correlated to the size of the antiprism in this plane, namely the distance between the chloride ion and its nearest oxide ions, and the amount of roto-oscillation motion of the BO 4 tetrahedra around the B-O1 axis. While the chloride ion in the channel was bonded to six Pb cations, the repulsion from six neighboring oxygen ions is also apparent and this repulsion restricts the motion of the chloride ion within the x-y plane.

The crystal structure of johnbaumite, ideally Ca 5 (AsO 4 ) 3 OH, was determined and refined using crystals from the Harstigen mine, Värmland, Sweden, on the basis of X‑ray diffraction data. The structure was refined to R 1 = 0.038 in space group P6 3 /m, with unit-cell parameters a = 9.7242(2), c = 6.9657(9) Å, and V = 570.43(2) Å 3 . The chemical composition of the sample, determined by electronmicroprobe analysis, is (Ca 5.02 Pb 0.03 ) Σ5.05 [As 2.91 P 0.04 Si 0.02 ] Σ2.97 O 12 (OH 0.97 Cl 0.03 ). Johnbaumite belongs to the calcium arsenate apatites, together with svabite, Ca 5 (AsO 4 ) 3 F, and turneaureite, Ca 5 (AsO 4 ) 3 Cl. Johnbaumite is topologically similar to the other members of the apatite supergroup: columns of face-sharing M1 polyhedra run along c and are connected through TO 4 tetrahedra, giving rise to a zeolite-like microporous framework, with channels hosting M2 cations and X anions. This is the first published single-crystal structure study of johnbaumite and the first report of this calcium arsenate apatite from the Harstigen mine. Actually, johnbaumite was identified at this locality at the end of the 19th Century but it was described as svabite.

Tuite, the high-pressure γ-form of the Ca 3 (PO 4 ) 2 system, has been synthesized from chlorapatite at 15 GPa and 1300 °C using a multi-anvil apparatus. Its crystal structure was determined with single-crystal X‑ray diffraction. It is isostructural with palmierite, with space group R3̅m and unit-cell parameters a = 5.2522(9) and c = 18.690(3) Å. The structure of tuite is characterized by three distinct polyhedra, PO 4 , Ca1O 12 , and Ca2O 10 , that are translationally interconnected in the sequence of PO 4 - Ca2O 10 -Ca1O 12 -Ca2O 10 -PO 4 along the c axis. Comparison of the CaSiO 3 and Ca 3 (PO 4 ) 2 polymorphic systems shows a striking resemblance in the evolution of atomic packing arrangements as the polymorphic density increases. In both cases, the Ca atoms are progressively incorporated into the (Ca+O) close-packed monolayers, consistent with the hypothesis that close packing is a consequence of volume decrease as density increases. Based on this observation, we predict a possible high-pressure post-tuite phase.

Phase relations in the K 2 CO 3 -MgCO 3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1450 °C temperatures. Subsolidus assemblies comprise the fields K 2 CO 3 +K 2 Mg(CO 3 ) 2 and K 2 Mg(CO 3 ) 2 +MgCO 3 with the transition boundary near 50 mol% MgCO 3 in the system. The K 2 CO 3 -K 2 Mg(CO 3 ) 2 eutectic is established at 1200 °C and 25 mol% MgCO 3 . Melting of K 2 CO 3 occurs between 1400 and 1450 °C. We propose that K 2 Mg(CO 3 ) 2 disappears between 1200 and 1300 °C via congruent melting. Magnesite is observed as a subliquidus phase to temperatures in excess of 1300 °C. At 6 GPa, melting of the K 2 Mg(CO 3 ) 2 +MgCO 3 assemblage can be initiated either by heating to 1300 °C under “dry” conditions or by adding a certain amount of water at 900-1000 °C. Thus, the K 2 Mg(CO 3 ) 2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the K- and Mg-rich carbonatite melts similar to those found as microinclusions in “fibrous” diamonds. The K 2 Mg(CO 3 ) 2 compound was studied using in situ X‑ray coupled with a DIA-type multi-anvil apparatus. At 6.5 GPa and 1000 °C, the structure of K 2 Mg(CO 3 ) 2 was found to be orthorhombic with lattice parameters a = 8.8898(7), b = 7.8673(7), and c = 5.0528(5), V = 353.39(4). No structure change was observed during pressure decrease down to 1 GPa. However, recovered K 2 Mg(CO 3 ) 2 exhibited a trigonal R3̅m structure previously established at ambient conditions.

High-pressure and high-temperature experiments were carried out in a Mg 2 SiO 4 -H 2 system using laser-heated diamond-anvil cells to understand the influence of H 2 fluid on the stability of forsterite. In situ X‑ray diffraction experiments and Raman spectroscopic measurements showed the decomposition of forsterite, and formation of periclase (MgO) and stishovite/quartz (SiO 2 ) in the presence of H 2 after being heated in the range between 2.5 GPa, 1400 K and 15.0 GPa, 1500 K. Transmission electron microscopic observation of the samples recovered from 15.0 GPa and 1500 K showed that the granular to columnar periclase grains maintained the original grain shape of forsterite, indicating that the periclase crystals crystallized under high temperature. On the other hand, euhedral columnar stishovite crystals were found at the boundaries between residual forsterite grains and reacted periclase. This implies that the SiO 2 component was dissolved in H 2 fluid, and that stishovite was considered to have crystallized when the solubility of the SiO 2 component became reduced with decreasing temperature. Additional experiment on a SiO 2 -H 2 system clearly showed the dissolution of quartz in H 2 fluid, while those on a MgO-H 2 system, periclase was hardly dissolved. These lines of evidence indicate that forsterite was incongruently dissolved in H 2 fluid to form periclase crystals in the Mg 2 SiO 4 -H 2 system, which is different from what was observed in the Mg 2 SiO4-H2O system. The results indicate that the stability of forsterite is strongly affected by the composition of coexisting C-O-H fluid.

The crystal structures of two elbaites from the Paraiba deposit with copper contents of 3.51 and 1.61 wt% CuO [a = 15.881(1), 15.840(3); c = 7.112(1), 7.1028(9), respectively], as well as a synthetic Cu-bearing olenite with a CuO content of 8.39 wt% [a = 15.840(4), c = 7.091(1)] have been refined to R-indices of 2.2, 3.1, and 4.1% using X‑ray single-crystal diffraction. On the basis of original and published data (for six structures) the crystal-chemical relationships of copper-bearing tourmalines has been analyzed. It is shown that copper cations and cations of other 3d elements (Mn, Zn, Fe) occupy almost exclusively the Y site. Such an ordered distribution results in a change of the size of the Y-octahedra. There is an inverse correlation between the content of 3d elements (mainly Cu 2+ cations) and Al 3+ cations in the Y-octahedron and also between the value of <Y-O> and the content of Al 3+ cations in the Y site. The direct correlation between values of <Y-O>-distance in the structures of Cubearing elbaites and the parameters (a, c) of the unit cell have been found. Distortions of polyhedra in the structures of Cu-bearing elbaites are similar, and significantly higher than those present in the structure of the synthetic Cu-bearing olenite.

Eckhardite, (Ca,Pb)Cu 2+ Te 6+ O 5 (H 2 O), is a new tellurate mineral from Otto Mountain near Baker, California, U.S.A. It occurs in vugs in quartz in association with Br-rich chlorargyrite, gold, housleyite, khinite, markcooperite, and ottoite. It is interpreted as having formed from the partial oxidation of primary sulfides and tellurides during or following brecciation of quartz veins. Eckhardite is monoclinic, space group P2 1 /n, with unit-cell dimensions a = 8.1606(8), b = 5.3076(6), c = 11.4412(15) Å, β = 101.549(7)°, V = 485.52(10) Å 3 , and Z = 4. It forms as needles or blades up to about 150 × 15 × 5 mm in size, typically in radial or sub-radial aggregates, but also as isolated needles. The color is light bluish green and the streak is very pale bluish green. Crystals are transparent with vitreous to subadamantine luster. The Mohs hardness is estimated at between 2 and 3. Eckhardite is brittle with an irregular fracture and one likely (but not observed) cleavage on {101}. The calculated density based on the empirical formula is 4.644 g/cm 3 . The mineral is biaxial (-), with indices of refraction of α = 1. 770 (calc), β = 1.860 (calc), and γ = 1.895(5). The measured 2V is 61.2(5)°, dispersion is r < v, perceptible and the optical orientation is Z = b; X ≈ [101]. The pleochroism is: Z (light blue green) < Y (very pale blue green) < X (colorless). The normalized electron microprobe analyses (average of 4) provided: PbO 4.79, CaO 15.90, MgO 0.06, CuO 22.74, Fe 2 O 3 0.06, TeO 3 51.01, H 2 O 5.45 (structure), total 100 wt%. The empirical formula (based on 6 O apfu) is: Ca 0.962 Pb 0.073 Cu 2+ 0.971 Mg 0.005 Fe 3+ 0.002 Te 6+ 0.986 O 6 H 2.052 . The Raman spectrum exhibits prominent features consistent with the mineral being a tellurate, as well as an OH stretching feature confirming a hydrous component. The eight strongest powder X‑ray diffraction lines are [d obs in Å (hkl) I]: 5.94 (101) 100, 3.287 (112) 80, 2.645 (020,2̅13) 89, 2.485 (1̅14,301,014) 48, 2.245 (114,122) 46, 1.809 (223,4̅13,321,4̅04) 40, 1.522 (413,5̅12,421,133) 42, and 1.53 (2̅17,2̅33,4̅06) 43. The crystal structure of eckhardite (R 1 = 0.046 for 586 reflections with F o > 4σF) consists of stair-step-like octahedral layers of Te 6+ O 6 and Cu 2+ O 6 octahedra parallel to {101}, which are linked in the [101̅] direction by bonds to interlayer Ca atoms. The structure can be described as a stacking of stepped HCP layers alternating with chains of CaO 7 polyhedra. The structures of bairdite, timroseite, and paratimroseite also contain stair-step-like HCP polyhedral layers.

The chemical composition and the crystal structure of a gem-quality brazilianite from the Telírio pegmatite, near Linópolis, Minas Gerais, Brazil, [NaAl 3 (PO 4 ) 2 (OH) 4 , a = 11.2448(5) Å, b = 10.1539(6) Å, c = 7.1031(3) Å, b = 97.351(4)°, V = 804.36(7) Å 3 , space group P2 1 /n, Z = 4], have been reinvestigated by means of electron microprobe analysis in wavelength-dispersive mode, single-crystal X‑ray and neutron diffraction. The chemical analysis shows that brazilianite from Telírio Claim approaches almost ideal composition. The neutron anisotropic structural refinement was performed with final agreement index R 1 = 0.0290 for 211 refined parameters and 2844 unique reflections with F o > 4σ(F o ), the X‑ray refinement led to R 1 = 0.0325 for 169 refined parameters and 2430 unique reflections with F o > 4σ(F o ). The building-block units of the brazilianite structure consist of chains of edge-sharing AlO 4 (OH) 2 and AlO 3 (OH) 3 octahedra. Chains are connected, via corner-sharing, by P-tetrahedra to form a three-dimensional framework, with Na atoms located in distorted cavities running along [100]. Five independent H sites were located, here labeled as H(1), H(2a), H(2b), H(3), and H(4). The configuration of the OH groups, along with the complex hydrogen-bonding scheme, are now well defined. The O-H distances corrected for “riding motion” range between ~0.992 and ~1.010 Å, the O···O distances between ~2.67 and ~2.93 Å, and the O-H···O angles between ~151° and ~174°. The H(2a) and H(2b) are only ~1.37 Å apart and mutually exclusive (both with site occupancy factor of 50%). The differences between the crystal structure of brazilianite and wardite [ideally NaAl 3 (PO 4 ) 2 (OH) 4 ·2H 2 O] are discussed. This work fulfills the need for accurate crystal-chemical data for this gem mineral.