Volume 98, Issue 4

Crystals constitute potential sources of information on magmatic timescales, conditions, and processes. One could say that the ecology of crystals in magmas is very rich, including phenocrysts, microlites, xenocrysts, and antecrysts, to name just a few types. A recent reassessment of thermodynamic equilibrium between crystals in the Bishop Tuff has concluded to the reliability of temperature estimates based on Fe-Ti oxides.

The hydrothermal uranium vein-type deposits of Menzenschwand and Wittichen in the Schwarzwald in southwestern Germany have been investigated with regards to their primary and secondary mineralization. Primary magmatic uraninite I from the host granite of Menzenschwand, primary hydrothermal uraninite II and secondary (supergene) uranyl silicates (uranophane and cuprosklodowskite), uranyl phosphates (torbernite and uranocircite), and uranyl arsenates (zeunerite, heinrichite, nováčekite, walpurgite, and uranospinite) were analyzed for their REE contents by LA-ICP-MS together with uraninite II samples from other known Schwarzwald uraninite II occurrences for comparison. Water samples were taken from drillings and abandoned mines and were analyzed for their major and trace element composition including U and REE. The REE patterns of uraninite show significant Eu anomaly variations: negative Eu anomalies are related to granitic host rocks, whereas positive Eu anomalies imply a gneissic REE source. This is in agreement with the observation of Eu anomalies in granite and gneiss derived waters, which display negative and positive Eu anomalies, respectively. Rare earth element distributions in the secondary uranyl minerals provide information about the sequence of mineral precipitation and the degree of remobilization of U. Cerium anomalies (or their absence) imply that uranyl silicates formed during an earlier stage of weathering under more reduced conditions than the uranyl arsenates and phosphates. The REE patterns of the uranyl silicates in Wittichen are similar to those of uraninite II, suggesting a very local redistribution on the millimeter- to centimeter-scale. In contrast, the REE patterns of uranyl arsenates and phosphates are different from uraninite II patterns and resemble those of the waters. This shows that the uranyl phosphates and arsenates are formed not only by redistribution of U in the hydrothermal veins but are influenced by waters from the host rock implying that U was probably transported over a greater distance. These conclusions are supported by field observations, where uranyl silicates are often found in the vicinity of uraninite II, whereas uranyl phosphates and arsenates are commonly found on fissures in the host rock. To explain specific features in the sulfide-rich (pyrite, chalcopyrite) Menzenschwand deposit, we conducted thermodynamic reaction modeling using the PHREEQC computer code. Sulfide oxidation was modeled by reaction of a U-rich water in equilibrium with atmospheric oxygen with FeS 2 and CuFeS 2 . A water that initially precipitates uranophane gradually evolves to more acidic pH values, thereby reaching torbernite saturation. This is in agreement with observed paragenetic sequences. Ongoing reaction results in a further decrease of pH and in the destabilization of uranophane and torbernite. Since goethite precipitates even at low-pH values, these calculations can explain observed pseudomorphs of goethite after both uranophane and torbernite, which are frequently observed at Menzenschwand. If the reaction proceeds and f O2 in the system is buffered by the surrounding minerals, U 6+ can be reduced to U 4+ and uraninite can form and replace pyrite and earlier formed secondary uranium minerals such as ianthinite. Also these textures have been observed. We show that the combination of REE geochemistry, careful paragenetic observations and thermodynamic modeling allows to reconstruct the formation and weathering of uranium deposits in great detail.

The crystal structure of čejkaite, Na 4 (UO 2 )(CO 3 ) 3 , from the type locality, was determined for the first time by single-crystal X-ray diffraction. In contrast to the previously reported pseudohexagonal triclinic symmetry, the current data indicate čejkaite is monoclinic, triply twinned, and belongs to the space group Cc. Refined unit-cell parameters are a = 9.2919(8), b = 16.0991(11), c = 6.4436(3) Å, β = 91.404(5)°, and V = 963.62(12) Å 3 . The monoclinic unit cell is also supported by the good fit to the powder diffraction data. The structure of čejkaite consists of uranyl tricarbonate clusters, forming sheets sub-parallel to (001) by sharing edges with (NaΦ) polyhedra. Sheets are interconnected through the uranyl O atoms and columns of (Na1Φ) polyhedra that share their trigonal faces. All Na atoms in the structure are in sixfold coordination . The structure refinement yielded R obs = 0.0424 for 1687 observed reflections [I obs > 3σ(I)] and 0.0538 for all 2016 unique reflections. Refinement and bondvalence analysis of the structure confirmed the previously proposed formula Na 4 (UO 2 )(CO 3 ) 3 , Z = 4.

A combination of techniques, including powder X-ray diffraction (XRD), electron microprobe analysis (EPMA), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAFS), is used to characterize the common ferric-arsenate-sulfate compounds, which could result from the pressure oxidation of refractory gold ores at elevated temperatures. Three general types of precipitate are identified; namely, arsenate-bearing basic ferric sulfate [FeSO 4 (OH) and designated as BFS], ferric arsenate-sulfate [an extensive solid solution Fe(AsO 4 ) 0.2-0.7 (SO 4 ) 0.7-0.2 (OH) 0.7-0.2 and designated as FAS], and hydrated ferric orthoarsenate (FeAsO 4 ·0.75H 2 O). The crystal structure of FAS is solved by precession electron-diffraction experiments. The structures of BFS and FAS are constructed from octahedral Fe 3+ chains, which are cross-linked by sulfate and arsenate tetrahedra. Extensive substitution of arsenate for sulfate occurs in both types of compounds with charge neutrality being maintained by variations in the (OH) content. The XAFS spectra indicate that the local structures of both BFS and FAS are made of corner-linked single chains of FeO 6 octahedra where the chains are linked by AsO 4 or SO 4 tetrahedra forming alternating layers of FeO 6 octahedra and AsO 4 or SO 4 tetrahedra. Preliminary toxicity characteristics leaching procedure (TCLP) testing of the precipitates indicates that FAS with a molar ratio As/(As+S) ratio of ≤0.5 could be an acceptable material for disposal in a tailings impoundment, whereas more As-rich FAS and BFS may require further treatment. The results for the laboratory-prepared precipitates are compared with those obtained on three residues from the processing of refractory gold ores. The major As-carrier in one of the residues is FAS, whereas As-bearing goethite and hematite are the dominant As-carriers in the other two residues. Thus, the mineralogical characteristics of the residues dictate the appropriate arsenic management and disposal options in the processing of refractory gold ores.

Due to its strong chalcophile character and the influence of its s 2 lone-pair electrons on the crystal structure trivalent bismuth is extremely rare in silicate minerals, with Bi-contents in common silicates typically below 1 ppm. In the present paper, we report on an exceptionally Bi-rich variety of the rock-forming mineral vesuvianite with up to ca. 20 wt% Bi 2 O 3 , occasionally in combination with enhanced Pb contents up to ca. 5 wt% PbO. The mineral occurs as small (≤300 μm) idiomorphic, black crystals in a sulfide-free silicate skarn in the Långban Mn-Fe deposit, central Sweden. The major skarn minerals comprise Ba-rich potassium feldspar, albitic plagioclase, Pb-rich scapolite, and phlogopite, while Pb-rich epidote, vesuvianite, and calcic garnets are minor phases. The vesuvianite grains are intensely zoned displaying Bi-rich cores surrounded by thinner Bi-poor rims. Although generally high in bismuth, the crystal cores invariably show oscillatory zoning. In addition to high Bi- and Pb-contents, the crystals are occasionally enriched in copper, cerium, antimony, and arsenic, thus reflecting the complex chemistry and evolution of the Långban mineralization. Chemical analyses demonstrate a strong negative correlation between Ca and Bi, hence confirming that Bi replaces Ca at X sites of the vesuvianite structure. Concentrations of Si and Al are lower, while Fe and Ti contents are somewhat enhanced in the Bi-rich cores. Maximum Bi and Pb contents analyzed in the present vesuvianite crystals correspond to 3.19 and 0.87 atoms per formula unit, respectively. This exceeds by far previous reports in the literature. X-ray single-crystal diffraction studies of a crystal splinter with intermediate Bi-content (1.08 apfu) show that the space group P4/nnc is the most appropriate to describe the crystal structure; the refinement converged to an R1 index of 0.0493. The recorded unit-cell parameters, a = 15.7018(6), c = 11.8648(6) Å, and V = 2925.2(2) Å3, are to our knowledge the largest ones observed so far for P4/nnc vesuvianite. Bismuth was demonstrated to order at the X3′(Bi) site that is only 0.46 Å distant from the nearest X3(Ca) site. Consequently, the X3 and X3′ sites cannot be simultaneously fully occupied.

This study reports on the structural relationship between graphite, lonsdaleite, and diamond extracted from the Goalpara ureilite and propose a model for the formation of lonsdaleite and diamond in these stony meteorites. The study is based on data from reflected-light microscopy and laser Raman spectroscopy of a polished thin section (PTS) of the Goalpara ureilite and X-ray powder diffraction (XRPD) analyses of the grains taken out of it. Selected-area electron diffraction (SAED) analyses and high-resolution TEM (HRTEM) observations were carried out in the three unique directions of pristine graphite with two thin slices prepared from a carbon grain directly taken out of a PTS. SAED patterns reveal the relative crystal-axes orientations between graphite (Gr), lonsdaleite (Lo), and diamond (Di) as (001) Gr //(100) Lo //(111) Di , [210] Gr //[001] Lo //[21̅1̅] Di , and (12̅0) Gr //(1̅20) Lo //(02̅2) Di . The shapes of diffraction spots in the SAED patterns reveal that the transformation of graphite to lonsdaleite and diamond is initiated by sliding of hexagonal carbon planes of graphite along the [210] of the graphite structure. These results suggest that lonsdaleite and diamond in ureilites formed directly from graphite through boat-type buckling and chair-type puckering of hexagonal carbon planes of graphite, respectively. The results of this study confirm the shock origin of diamond in ureilites.

Banded iron formations (BIF) have recently been used as proxies for tracking the chemical changes associated with the transition from an anoxic to oxic atmosphere around 2.45 billion years ago, known as the Great Oxidation Event (GOE). The timing of the GOE has been ascribed to both the collapse of a methane greenhouse and a decreased overall demand for oxygen due to the production of more oxidizing gases associated with greater subaerial volcanism. The latter is a byproduct of a period of high mantle plume activity and the formation of new continental crust between 2.51 to 2.45 Ga. Here we report unique mineral evidence for momentary subaerial volcanism recorded in hematite-rich layers of the 2.48 Ga BIF from Dales Gorge, Hamersley of Western Australia. The BIF contains euhedral quartz nanocrystals (QNC), which only occur on the surfaces or in cavities of hematite breccias exhibiting soft-sediment features and an exogenous source. These QNCs with an average size of 170-100 nm are distinct to the amorphous chert in the BIF mineral assemblage and have the smallest crystal sizes of well-crystallized quartz ever reported. We suggest that QNCs represent pyroclastic materials that were transported as dust particles to the BIF depositional setting. Although the analysis of one specific BIF unit does not provide proof of changing modes of volcanism during the Archean-Paleoproterozic transition, this high-resolution petrological study does confirm that subaerial volcanism existed at that time.

Actinolite and glaucophane are the principal amphiboles in greenschist- and blueschist-facies metamafic rocks, respectively, and constitute an important mineral pair for deducing the conditions of medium- and high-pressure metamorphism. Here we present the crystal-chemical properties of amphiboles synthesized along the tremolite-glaucophane join as an important starting point for the more chemically complex samples occurring in nature. Amphiboles were synthesized in 10 mol% increments at conditions ranging from 840 °C and 0.6 GPa for tremolite-rich to 750 °C and 2.5 GPa for glaucophane-rich amphiboles. The amphibole yields were generally high (~95 wt%), though minor quartz, pyroxene, and amorphous material (either glass or quenched solute) were often present. Electron microprobe analysis of the amphibole showed that deviations from the nominal or intended compositions occurred, with the observed compositions showing solid solution primarily toward the katophorite component and reaching a maximum near the middle of the join. Unit-cell dimensions showed a pronounced positive deviation from ideality, even after (linear) correction for the non-join components, suggesting a strong tendency toward exsolution. Single-crystal refinements of a couple of selected amphiboles confirmed the deviation from nominal composition and confirmed the presence of A Na, T Al, and C Al, the latter disordered between the M(2) and M(3) sites. Infrared spectra in the OH-stretching region were measured on samples that were heated to 250-350 °C to remove absorbed moisture. The spectra consist of a main band centered in the 3675-3660 cm -1 region and two minor absorptions on both sides of the dominant central peak, which are centered at 3720 and 3640 cm -1 and can be attributed to OH next to A Na and CAl, respectively. The main band is evidently composed of several overlapping components due to local arrangements of B cations typical of tremolite, CaCa, glaucophane, NaNa, and cummingtonite, MgMg. Examination of several intermediate amphiboles by both single-crystal XRD and by high-resolution TEM analysis could not identify evidence of reflections or ordered domains that would violate the C2/m symmetry of the end-member amphiboles. Comparing the compositions of the synthetic amphiboles from this study with winchite-rich amphiboles in nature shows that deviations from the tremolite-glaucophane join are common. The inability to make pure winchite or even winchite-rich amphiboles on the tremolite-glaucophane join suggests that this structure is unstable.

The pressure dependence of single-crystal elastic moduli of a natural Mn-rich franklinite, (Mn 0.4 0Fe 2+ 0.16 Zn 0.37 Mg 0.03 )(Fe 3+ 1.9 4Al 0.08 )O 4 , has been determined by GHz-ultrasonic interferometry in a diamond-anvil cell to 9.8 GPa. The room-pressure elastic constants of franklinite are C 11 = 244(3) GPa, C 12 = 142(4) GPa, and C 44 = 77(2) GPa. Linear pressure derivatives of C 11 and C 12 are 4.3(3) and 3.8(3), respectively, whereas the C 44 modulus exhibits softening, fitted in the P ≤ 10 GPa pressure range to C 44 = 77(2) + 0.29(2)P - 0.018(2)P 2 GPa. The average of Hashin-Shtrikman bounds on the adiabatic bulk modulus (K S0 ) of franklinite is 175(3) GPa, with pressure derivative K S ′ = 4.3(3), and the shear modulus G 0 = 66(2) GPa with G′ = 0.09(3). The isothermal compressibility of franklinite was determined from a separate high-pressure, single-crystal X-ray diffraction experiment to 7.8 GPa, yielding K T0 = 173.5(7) GPa fitted with a fixed pressure derivative of K T ′ = 4. When K′ is fixed to the ultrasonic value of 4.3, we obtain KT0 = 172.2(7) GPa. In contrast to iron-free gahnite (ZnAl 2 O 4 ), franklinite exhibits pressure-induced mode softening of C 44 similar to magnetite (Fe 3 O 4 ). Between end-member compositions ZnFe 2 O 4 (franklinite) and MnFe 2 O 4 (jacobsite), the bulk modulus decreases linearly with increasing %Mn, however we observe non-linear behavior in other elastic moduli, especially C 44 , which displays a pronounced negative anomaly for the mid-range Mn composition. Applying Birch’s law to AB 2 O 4 -type spinels reveals that oxide spinels containing transition metals on both A and B sites follow a distinct trend from other spinels.

Magnetite is commonly found at sites on Mars explored by robotic spacecraft, yet is rare in martian meteorites and in experimental studies of martian magma compositions. Iron redox systematics of the high-FeO shergottitic liquids are poorly known, yet have a fundamental control on stability of phases such as magnetite, ilmenite, and pyroxenes. We undertook experiments to constrain the Fe 3+ / ΣFe in high-FeO (15-22 wt%) glasses as a function of f O2 , melt P 2 O 5 , temperature and pressure. We also performed a series of sub-liquidus experiments between 1100 and 1000 °C and FMQ+0.5 to FMQ-1 to define magnetite stability. Run products were analyzed for Fe 3+ and Fe 2+ by Mössbauer spectroscopy and micro-X-ray absorption near edge structure (micro-XANES) spectroscopy. One bar liquids equilibrated at FMQ-3 to FMQ+3 show a much lower Fe 3+ /ΣFe than terrestrial basalts at the same conditions. As melt P 2 O 5 contents increase from 0 to 3 wt% (at fixed pressure, temperature, and f O2 ), Fe 3+ /ΣFe decreases from 0.07 to 0.05, but this is within error on the measurements. Temperature increases between 1200 and 1500 °C cause little to no variation in Fe 3+ /ΣFe. Pressure increases from 1 to 4 GPa cause a 0.06 decrease in Fe 3+ /ΣFe. The trends with pressure and temperature are in agreement with results of previous studies. Combining our new series of data allows derivation of an expression to calculate Fe 3+ /Fe 2+ for high-FeO melts such as martian magmas. ln(X Fe3+ /X Fe2+ ) = a lnf O2 + b/T + cP/T + dX FeO + eX Al2O3 + fX CaO + gX Na2O + hX K2O + iX P2O5 + j This expression can be used to show that decompressed melts become slightly more oxidized at the surface (compared to 4 GPa). Magnetite stability is suppressed by the lower Fe 3+ /Fe 2+ of the high- FeO melts. Magnetite stability is a function of Fe 2 O 3 and temperature and is stable ~50 °C lower than typical terrestrial basalt. Difficulty in producing magnetite as a liquidus phase in magmatic systems suggests either that many martian basalts are more oxidized than FMQ (but not represented among meteorite collections), that the titano-magnetite only forms upon cooling below ~1000 °C at FMQ, or that the magnetite has a secondary origin.

Ringwoodite [(Mg,Fe) 2 SiO 4 ] is the high-pressure polymorph of olivine stable in the upper mantle between ~525 to 660 km. Information on its temperature-dependent water content and Fe-oxidation state bears important implications on the hydrogen cycle and oxidation state of the Earth’s interior. We conducted several multi-anvil experiments to synthesize iron-bearing (0.11 ≤ x Fe ≤ 0.24) hydrous ringwoodite under oxidizing and reducing conditions. The experiments were performed at 1200 °C and pressures between 16.5 and 18.3 GPa. The incorporation of hydrogen and iron in ringwoodite was studied using Fourier transform infrared (FTIR), Mössbauer (MB), ultraviolet-visible (UV-VIS), and electron energy loss (EEL) spectroscopy. For MB spectroscopy, ringwoodite enriched in 57Fe was synthesized. The IR spectra of ringwoodite show a broad OH band around 3150 cm -1 and two shoulders on the high-energy side: one intense at 3680 cm -1 and one weak at around 3420 cm -1 . The water content of the samples was determined using FTIR spectroscopy to have a maximum value of 1.9(3) wt% H 2 O. UV-VIS spectra display a broad band around 12 700 cm -1 and a shoulder at 9900 cm -1 representing the spin-allowed dd-transitions of VIFe 2+ . The weaker band around 18 200 cm -1 is a distinct feature of Fe 2+ -Fe 3+ intervalence charge transfer indicating the presence of Fe 3+ in the samples. EEL spectra yield Fe 3+ fractions ranging from 6(3)% at reducing conditions to 12(3)% at oxidizing conditions. We performed heating experiments up to 600 °C in combination with in situ FTIR spectroscopy to evaluate the temperature-dependent behavior of ringwoodite, especially with respect to hydrogen incorporation. We observed a color change of ringwoodite from blue to green to brown. The heat-treated samples displayed hydrogen loss, an irreversible rearrangement of part of the hydrogen atoms (FTIR), as well as oxidation of Fe 2+ to Fe 3+ evidenced by the appearance of the spin-forbidden dd-transition band for Fe 3+ and the ligand-metal (O 2- -Fe 3+ ) transition band in the optical spectra. An increased Fe 3+ fraction was also revealed by EEL and MB spectroscopy (up to 16% Fe 3+ /ΣFe). Analyses of MB data revealed the possibility of tetrahedral Fe 3+ in the annealed ringwoodite. These results lead to a reinterpretation of the broad OH band, which is a combination of several bands, mainly [VMg(OH) 2 ] x ), a weaker high-energy band at 3680 cm -1 ([VSi(OH) 4 ] x ) and a shoulder at 3420 cm -1 ([(Mg/Fe)Si(OH) 2 ] x ).

Multiple substitution mechanisms for hydrogen in γ-(Mg,Fe) 2 SiO 4 , ringwoodite, lead to broad, overlapping, and difficult-to-interpret FTIR spectra. Through combined low-temperature, high-pressure synchrotron-based FTIR spectroscopy, the multiple bonding sites become evident, and can be traced as a function of temperature and compression. Multiple OH stretching bands can be resolved in iron-bearing and iron-free samples with 0.79-2.5(3) wt% H 2 O below 200 K at ambient pressure, with cooling to 5 K at 35 and 23 GPa resulting in the resolution of possibly as many as 5 OH stretching bands traceable at room temperature from 23 GPa down to 8 GPa. A distribution of defect mechanisms between ‴ Mg ″+2(H·) at 3100, 3270, and possibly 2654 cm -1 , ‴ Si ″′+4(H·) at 3640 cm -1 , and Mg Si ″+2(H·) at 2800 cm -1 can then be resolved. These multiple defect mechanisms can therefore explain the higher electrical and proton conductivity in ringwoodite when compared to wadsleyite, and therefore may be applied to resolve spatial variations in water storage in the Earth’s transition zone.

The shifts in wavenumber of the ν3 (SiO 4 ) (~1008 cm -1 ) Raman band of fully crystalline synthetic zircon with changing pressure (P) and temperature (T) were calibrated for application as a Raman spectroscopic pressure sensor in optical cells to about 1000 °C and 10 GPa. The relationship between wavenumber (ν) of this band and T from 22 to 950 °C is described by the equation ν (cm -1 ) = 7.54·10 −9 ·T 3 - 1.61·10 −5 ·T 2 - 2.89·10−2·T + 1008.9, where T is given in °C. The pressure dependence is nearly linear over the studied range in P. At ~25 °C, the ∂ν/∂P slope to 6.6 GPa is 5.69 cm -1 /GPa, and that to 2 GPa is 5.77 cm -1 /GPa. The ∂ν/∂P slope does not significantly change with temperature, as determined from experiments conducted along isotherms up to 700 °C. Therefore, this pressure sensor has the advantage that a constant ∂ν/∂P slope of 5.8 ± 0.1 cm -1 /GPa can be applied in experiments to pressures of at least about 6.6 GPa without introducing a significant error. The pressure sensor was tested to determine isochores in experiments with H 2 O+Na 2 Si 3 O 7 and H 2 O+NaAlSi 3 O 8 fluids to 803 °C and 1.65 GPa. These pressures were compared to pressures calculated from the equation of state (EoS) of H 2 O based on the measured vapor dissolution or ice melting temperature for the same experiment. Pressures determined from the zircon sensor in runs in which NaAlSi 3 O 8 melt dissolved in aqueous fluid were close to or lower than the pressure calculated from the EoS of H 2 O using the vapor dissolution or ice melting temperature. In experiments with H 2 O+Na 2 O+SiO 2 fluids, however, the pressure obtained from the Raman spectrum of zircon was often significantly higher than that estimated from the EoS of H 2 O. This suggests that the pressures along some critical curves of water-silicate melt pseudobinary systems should be revised.

Elastic properties are an important issue in explaining the behavior of seisms and to ascertain the mineralogical composition of the Earth’s shells through which seismic waves pass. Computational methods can yield an additional, detailed, free-of-heterogeneity model knowledge of the mineral series of interest. Therefore, a computational study on the influence of the interlayer cation in the muscovite-paragonite (Ms-Pg) series on the crystal cell, internal geometry, and the elastic properties was made to shed light on the mineralogical, geophysical, and geochemical properties of the series. These properties have been calculated by means of Density Functional Theory (SIESTA2.0.2 code). The crystal structure and internal geometry agreed with the range of experimental values in the literature. In general, elastic stiffness constants (EC) agreed with the known experimental values. ECs of different interlayer cation configurations for the middle concentration sample showed very similar values, except for C 33 , The majority of ECs, with the exception of C 33 and C 66 , decreased as a function of Na′ [Na/(Na+K)], many of which showed ideal crystalline solution behavior, and some showed mixing terms. The polycrystalline bulk modulus registered similar values for the end-members of the series and a minimum at Na′ = 0.5, although an estimate of the value at room temperature made the Pg stiffer than Ms; while the shear modulus showed a decreasing trend as a function of the Na′. Velocities of the sound waves lowered as a function of Na′. Local deformabilities were also studied, where the highest deformability was found for the interlayer space. The results are discussed in the framework of the mineralogical, geochemical, and geophysical knowledge of the series.

Transition-metal L 2,3 XANES spectra are widely used to determine coordination and valence of the target ion. For decades, experimental fingerprinting, i.e., the comparison with spectra obtained from known reference compounds was the way to interpret spectral features. This approach was based on the common understanding that only anions in the first coordination sphere would determine the nearedge structure, and crystalline references were selected accordingly. Using ab initio charge-transfer multiplet calculations, we demonstrate that there is also a significant impact on spectral features from the second-nearest neighbor cations. This finding is exemplified for three fresnoite-type vanadates, namely Ba 2 VSi 2 O 8 (BVS), K 2 VV 2 O 8 (KVV), and Rb 2 VV 2 O 8 (RVV). The theoretical treatment provides evidence that for the three compounds studied it is not variable bond lengths or bond angles between vanadium and oxygen that make the V-L 2,3 XANES spectra different, but the interaction of the target vanadium ions with its neighboring cations (Si for BVS, V for KVV and RVV), which dominates. Therefore, we conclude that simple fingerprinting can result in misleading interpretations when interactions with second-nearest neighboring cations are not taken into account. Ab initio charge-transfer multiplet calculations of spectral shapes (theoretical fingerprinting) should be employed instead to get a deeper understanding of structure-spectra relationships, or the choice of reference spectra should take into account second-nearest neighbors. Our findings are similarly important for the interpretation of electron energy-loss near-edge (ELNES) spectra.

To measure the uptake of radioactive Cs in soils that are rich in Mn oxides, we applied time-resolved synchrotron X-ray diffraction (TR-XRD), inductively coupled plasma-mass spectrometry (ICP-MS), and analytical scanning electron microscopy (SEM) to determine the kinetics and mechanisms of aqueous Cs substitution in the phyllomanganate hexagonal H-birnessite at pH values ranging from 3 to 10. We observed that the rate of Cs cation exchange into hexagonal H-birnessite exhibited only a weak dependence on pH, but the total amount of Cs loading in the interlayer region increased dramatically above pH 6.5. The increase in Cs content at higher pH may be attributed to the increasingly negative charge on the Mn-O octahedral sheets and perhaps to a structural change toward triclinic symmetry with high pH. Dissolution at low pH may have inhibited Cs sequestration. Our work supports delamination-reassembly as a mechanism of cation exchange.

We evaluate a temperature-dependent coefficient for grain-boundary diffusion in quartz aggregates using grain size data from a contact aureole, based on the coupling of a numerical model for the temperature-time history of the contact aureole with a model for the kinetics of diffusion-controlled grain growth. The metachert samples were collected from the contact aureole of the Hanase-Bessho quartz diorite at Hanase Pass, Kyoto, Japan. The quartz grain sizes vary systematically with distance from the quartz diorite. We calculated the temperature-time history using a one-dimensional thermal model, validated by peak metamorphic temperature estimates that are based on the degree of graphitization of carbonaceous material in metapelites, as characterized by Raman microspectroscopy. To minimize the sum of the squares of the errors between the measured and calculated grain sizes, based on the normal grain growth law together with the temperature-time history, we estimated the activation energy and pre-exponential factor in the α-quartz field to be 208 kJ/mol and 1.1 × 10 -8 m 2 /s, respectively, assuming a grain-boundary width of 1 nm. The grain-boundary diffusion rates for temperatures in the greenschist and amphibolite facies are similar to those determined in natural or laboratory grain-coarsening experiments, but differ significantly from those determined in tracer diffusion experiments. During grain-size-sensitive deformation, “effective” grain-boundary diffusion rates may be intermediate between the rates of diffusion along and across the grain boundary, and would be higher than the grain-boundary diffusion rates estimated by grain-coarsening experiments, and lower than those by tracer diffusion experiments.

There is an understandable desire to use simple unpolarized infrared analysis of unoriented anisotropic samples to extract quantitative information, rather than using more demanding polarized techniques. Owing to the fact that unpolarized infrared absorbance in birefringent media deviates from the Beer-Lambert law, previous studies have either warned against using unpolarized spectroscopy for quantitative purposes, or have used flawed error analysis to justify using simple averages of integrated absorbance of multiple absorbance bands as a proxy for total integrated polarized absorbance in the principal spectra. It is shown here that unpolarized infrared absorbance is correctly calculated by averaging in the transmission domain. The errors in estimates of principal absorbance by averaging of unpolarized absorbance spectra are evaluated using correct theory of unpolarized infrared transmission. Correction schemes for integrated absorbance based on linear-absorbance error calculations are shown to be inappropriate. A theory is developed that allows the sum of the polarized principal absorbance spectra to be estimated from multiple unpolarized measurements of randomly oriented samples. The systematic errors that arise when averaging in the absorbance domain are avoided by use of exact theory rather than an approximation. Numerical simulation shows that applying the new procedure to 10 unpolarized measurements of OH stretching bands in olivine results in convergence of the estimated total integrated principal polarized absorbance to within 10% of the true value for a sample size of 10 measurements, but the technique is limited to spectral regions that do not contain absorption bands that are simultaneously intensely absorbing and strongly anisotropic.

Cation ordering in the magnesioferrite-qandilite (MgFe 2 O 4 -Mg 2 TiO 4 ) solid solution has been investigated using an interatomic potential model combined with Monte Carlo simulations. The dominant chemical interaction controlling the thermodynamic mixing behavior of the solid solution is a positive nearest-neighbor pairwise interaction between tetrahedrally coordinated Fe3+ and octahedrally coordinated Ti 4+ (J TO FeTi ). The predicted cation distribution evolves gradually from the Néel-Chevalier model to the Akimoto model as a function of increasing J TO FeTi , with J TO FeTi = 1000 ± 100 K providing an adequate description of both the temperature and composition dependence of the cation distribution and the presence of a miscibility gap. Although Mg is a good analog of Fe 2+ in end-member spinels, a comparison of model predictions for MgFe 2 O 4 -Mg 2 TiO 4 with observed cation ordering behavior in titanomagnetite (Fe 3 O 4 -Fe 2 TiO 4 ) demonstrates that the analog breaks down for Fe 3 O 4 -rich compositions, where a value of J TO FeTi closer to zero is needed to explain the observed cation distribution. It is proposed that screening of Ti 4+ by mobile charge carriers on the octahedral sublattice is responsible for the dramatic reduction in J TO FeTi . If confirmed, this conclusion will have significant implications for attempts to create a realistic thermodynamic model of titanomagnetite.

A 3T mica polytype from Kasenyi (southwest Uganda), kamafugite, was studied by electron probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD), micro-Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) to characterize its crystal chemistry and the relationships with phlogopites from the same rock but showing different stacking sequence and to get insights into factors affecting polytypism in Ugandan phlogopites. EPMA data gave: SiO 2 = 38.7(2), Al 2 O 3 = 13.08(9), MgO = 20.4(2), TiO 2 = 4.8(1), MnO = 0.03(3), FeOtot = 5.51(9), Cr 2 O 3 = 0.90(7), NiO = 0.11(5), SrO = 0.03(3), ZnO = 0.04(3), ZrO 2 = 0.01(2), K 2 O = 9.64(5), Na 2 O = 0.29(1), BaO = 0.15(5), F = 0.13(5), and Cl = 0.01(1) wt%. The analyzed sample may be classified as a Ti-rich phlogopite. X-ray photoelectron spectroscopy provided Fe 3+ /Fe 2+ and O 2- /OH equal to ~0.75 and 7.14, respectively, which are in agreement with the results of previous Mössbauer investigation on the BU1 phlogopites from the same rock and with the structural formula of the studied crystal. Infrared spectra showed a shoulder at ~3660 cm -1 in the OH − stretching region (~3740-3600 cm -1 ), which is assigned to MgMgFe 3+ -OH − -K-O 2- local configurations. No evidences of vacancy substitutions were observed. Structure refinement based on single-crystal X-ray diffraction data was performed in space group P3 1 12 using anisotropic displacement parameters and converged to R 1 = 4.34 and wR 2 = 3.33%. Unitcell parameters are: a = b = 5.3235(3) and c = 30.188(2) Å. Geometrical and chemical considerations point to a disordered cation distribution over T1 and T2 tetrahedral sites, whereas partial cation ordering characterizes the octahedral sites with high-charge cations preferentially located as expected on M2 and M3. Tetrahedral bond length distortion and angular variance parameters describe more distorted polyhedra in 3T polytype than those found in coexisting 1M and 2M 1 polytypes. Finally, the overall crystal-chemical features indicate the occurrence of the following substitution mechanisms in the studied sample: Ti-oxy [ VI M 2+ +2(OH) - ↔ VI Ti 4+ +2(O 2- )+H2↑] and Al, Fe 3+ , Croxy [ VI M 2+ +(OH)- ↔ VI M 3+ +O 2- +½(H 2 )↑]; Al, Fe 3+ -Tschermak [ VI M 2+ + IV Si 4+ ↔ VI (Al 3+ , Fe 3+ )+ IV Al 3+ ]; XII K + + IV Al 3+ ↔ IV Si 4+ + XII ⃞; tetraferriphlogopite [ IV Fe 3+ ↔ IV Al].

The previously published crystal structure study suggested that haiweeite is orthorhombic, Cmcm, with a = 7.125(1), b = 17.937(2), c = 18.342(2) Å, and V = 2344.3(7) Å 3 , and an ideal chemical formula Ca[(UO 2 ) 2 Si 5 O 12 (OH)2]·3H 2 O, with Z = 4. Using single-crystal X-ray diffraction and electron microprobe analysis we re-examined haiweeite from the Teofilo Otoni, Minas Gerais, Brazil. Our diffraction experiment provided weak reflections responsible for doubling of the b cell parameter (for the current space-group settings), leading finally to the choice of a different space group. Haiweeite is thus orthorhombic, the space group Pbcn, with the unit-cell parameters a = 18.3000(5), b = 14.2331(3), c = 17.9192(5) Å, V = 4667.3(2) Å 3 , and an ideal formula Ca[(UO 2 )2(SiO 3 OH) 2 (Si 3 O 6 )]·6H 2 O (6.25 H 2 O inferred from the thermal analysis; 7.50 H 2 O from the structure model), with Z = 8. The structure refinement yielded R 1 = 0.0512 for 2498 observed reflections [I obs > 3σ(I)] and wR 2 = 0.1286 for all 6117 unique reflections. Structure solution confirmed by subsequent refinement provided a structure model with full occupancies for U, Si, and Ca atoms, contrasting to previous average structure model. Although the general topology of our structure resembles that reported previously, all Si and O sites in our structure are fully occupied, in contrast to the previous structure determination.

Density functional theory (DFT) calculations were performed to study the crystallographic properties of the cis-vacant form of dioctahedral 2:1 phyllosilicates. Samples with different layer charges are studied; only tetrahedrally charged, only octahedrally charged, or mixed octahedrally/tetrahedrally charged. The isomorphous cation substitutions were explored in different relative positions with substitutions of octahedral Al 3+ by Mg 2+ or Fe 3+ , tetrahedral substitution of Si 4+ by Al 3+ , and different interlayer cations (IC) (Na + , K + , and Ca 2+ ). X-ray diffraction patterns of cis-vacant and trans-vacant forms of phyllosilicates were simulated and compared. The experimental observation of clustering tendency of Fe 3+ and dispersion tendency of Mg 2+ in the octahedral sheet is reproduced and explained with reference to the relative energies of the octahedral cation arrangements observing the same tendency that in the trans-vacant forms. These energies are higher than those due to the IC/tetrahedral and IC/octahedral cations relative arrangements. The tetrahedral and octahedral substitutions that generate charged layers tend also to be dispersed. The energy difference between the cis-vacant and trans-vacant polymorphs is smaller than that of cation arrangements.

The structural phase transition of the titanomagnetite (Fe 3-x Ti x O 4 ) solid solution under pressures up to 60 GPa has been clarified by single-crystal and powder diffraction studies using synchrotron radiation and a diamond-anvil cell. Present Rietveld structure refinements of the solid solution prove that the prefered cation distribution is based on the crystal field preference rather than the magnetic spin ordering in the solid solution. The Ti-rich phases in 0.734 ≤ x ≤1.0 undergo a phase transformation from the cubic spinel of F̅d̅3̅m to the tetragonal spinel structure of I4 1 /amd with c/a < 1.0. The transition is driven by a Jahn-Teller effect of IV Fe 2+ (3d 6 ) on the tetrahedral site. The c/a < 1 ratio is induced by lifting of the degeneracy of the e orbitals by raising the d x2-y2 orbital below the energy of the dz 2 orbital. The distortion characterized by c/a < 1 is more pronounced with increasing Ti content in the Fe 3-x Ti x O 4 solid solutions and with increasing pressure. An X-ray emission experiment of Fe 2 TiO 4 at high pressures confirms the spin transition of FeKβ from high spin to intermediate spin (IS) state. The high spin (HS)-to-low spin (LS) transition starts at 14 GPa and the IS state gradually increases with compression. The VI Fe 2+ in the octahedral site is more prone for the HS-to-LS transition, compared with Fe 2+ in the fourfold- or eightfold-coordinated site. The transition to the orthorhombic post-spinel structure with space group Cmcm has been confirmed in the whole compositional range of Fe 3-x Ti x O 4 . The transition pressure decreases from 25 GPa (x = 0.0) to 15 GPa (x = 1.0) with increasing Ti content. There are two cation sites in the orthorhombic phase: M1 and M2 sites of eightfold and sixfold coordination, respectively. Fe2+ and Ti4+ are disordered on the M2 site. This structural change is accelerated at higher pressures due to the spin transition of Fe 2+ in the octahedral site. This is because the ionic radius of VI Fe 2+ becomes 20% shortened by the spin transition. At 53 GPa, the structure transforms to another high-pressure polymorph with Pmma symmetry with the ordered structure of Ti and Fe atoms in the octahedral site. This structure change results from the order-disorder transition.

The OH stretching vibration frequencies of crystalline zircon that contains water have been investigated by quantum mechanical calculation using CRYSTAL09 and several hybrid functionals. Incorporation mechanisms considered for H in zircon include: (1) hydrogarnet and partial hydrogarnet-type substitution; (2) H-compensated trivalent substitution at the Si site; and (3) H-compensated trivalent substitution at the Zr site. The results provide a clear picture of the H locations in zircon and their associated IR stretching vibrations. Based on the results of structural relaxation and corresponding OH stretching calculations, we can assign the 3420 cm -1 band to a hydrogarnet or partial hydrogarnet-type substitution with polarization dependent on the extent of local relaxation (i.e., closer to E||c for partial hydrogarnet and less polarized for hydrogarnet). These results also show that the 3385 cm -1 (E⊥c) IR band originates from H incorporation that accompanies a trivalent cation substitution at the Si-site. As an example of such a substitution, the [AlO 4 /H] defect was investigated. H-incorporation coupled with a trivalent cation substitution at the Zr-site appears to be responsible for generating the 3180 cm -1 (E⊥c) IR band. The ab initio calculation on [YO 8 /H] and [LaO 8 /H] structures show that the 3180 cm-1 band represents an average between two distinct bands at ~3220-3250 and ~3100 cm-1 attributed to H locations close to or further away from the Zr-substituted atom. The energetics of these substitutions demonstrate that the overlapping of the two IR bands depends on the substituting cation, and this is controlled, in part, by the thermal history of the sample and/or by the experimental conditions under which the IR spectra are obtained. Furthermore, these results predict for the first time the existence of a peroxy-type O bond associated with the partial hydrogarnet substitution in the zircon structure that now accounts for the ~800 cm -1 bands.

In this work, the vibrational spectra of hexagonal hydroxylapatite OHAp (space group P6 3 ) and type A carbonated apatite [Ca 10 (PO 4 ) 6 (CO 3 ), space group P1] have been calculated with an ab initio approach by the density function method using the hybrid B3LYP functional and an all-electron polarized double-ζ quality Gaussian-type basis set using the CRYSTAL09 computer program. The effect on the vibrational properties due to improving the Ca pseudopotential, usually adopted in previous studies on hydroxylapatite, toward the present all-electron basis set has also been briefly addressed. The anharmonic correction for hydroxyl groups in OHAp has also been considered. The results of the modeling are in good agreement with the available FTIR and Raman data presented in the literature and can be useful to experimental researchers to assign unequivocally the bands in infrared and Raman spectra to specific fundamental vibrational modes.

Dolomite crystals in partially dolomitized limestone from the Platteville Formation are both compositionally and microstructurally heterogeneous. A single dolomite crystal usually contains three phases: the host Ca-rich dolomite [Ca 1.14 Mg 0.86 (CO 3 ) 2 ], an Fe-bearing dolomite [Ca 1.06 Mg 0.80 Fe 0.14 (CO 3 ) 2 ], and calcite inclusions. These three phases show similar orientations. The Ca-rich dolomite exhibits modulated microstructures with wavelength ranging from 7 to 30 nm. The modulated microstructures are not evident in Fe-bearing dolomite. Modulations in the Ca-rich dolomite have three predominant orientation ranges in the studied sample: from (205) to (104), from (001) to (1̅01), and ~(110), which are consistent with previous studies. Bright-field (BF) and high-angle annular dark-field (HAADF) images confirm that these modulations are due to chemical variation rather than strain or diffraction contrast. The Ca-rich lamellae are Mgrich calcite with compositions ranging from Ca 0.85 Mg 0.15 CO 3 to Ca 0.70 Mg 0.30 CO 3 . The observed results indicate that these Ca-rich exsolution lamellae formed during diagenesis. In this study, three kinds of “c”-reflections, which are weak spots in the halfway position between the principal reflections along the (104)*, (112)*, and (110)* directions, have been found in the diffraction patterns of some Ca-rich dolomite. Mg-Ca ordering in x-y planes was not observed directly in Z-contrast images. FFT patterns from the Z-contrast images do not show “c”-reflections. STEM images confirm that the “c”-reflections could result from multiple diffraction between the host dolomite and twinned Mg-calcite nano-lamellae under TEM imaging and diffraction modes

Karenwebberite, Na(Fe 2+ ,Mn 2+ )PO 4 , belongs to the triphylite group of minerals and corresponds to the Fe-equivalent of natrophilite or to the Na-equivalent of triphylite. It occurs in the Malpensata pegmatite dike, Colico, Lecco Province, Italy. Karenwebberite is found as late-magmatic-stage exsolution lamellae up to 100 μm thick, hosted by graftonite and associated with Na-bearing ferrisicklerite and with a heterosite-like phase. Lamellae are pale green, with very pale grayish-green streak. The luster is greasy to vitreous, and lamellae are translucent (pale green) to opaque (dark green). Optically, the mineral is anisotropic, biaxial (+), α = 1.701(2), β = 1.708(2), γ = 1.717(2) (for λ = 589 nm), 2V meas = 87(4)°, 2V calc = 41°, Z = b. Pleochroism is moderate with X = dark gray, Y = brown, and Z = yellow. The mineral is brittle with a Mohs hardness of 4.5; in thin section it displays a perfect cleavage along {001} with an irregular fracture. Karenwebberite is non-fluorescent either under short-wave or long-wave ultraviolet light, and its calculated density is 3.65 g/cm3. The mean chemical composition, determined by the electron microprobe from 16 point analyses (wt%), is: P 2 O 5 41.12, Fe 2 O 3 * 7.00, FeO* 25.82, MgO 0.23, ZnO 0.11, MnO 9.31, CaO 0.10, Na 2 O 14.66, total 98.41 (*: calculated values). The empirical formula, calculated on the basis of 1 P atom per formula unit from, is (Na 0.817 Ca 0.003 □ 0.180 ) Σ1.000 (Fe 2+ 0.622 Mn 2+ 0.228 Fe 3+ 0.151 Mg 0.010 Zn 0.002 ) Σ1.013 PO 4 . Karenwebberite is orthorhombic, space group Pbnm, a = 4.882(1), b = 10.387(2), c = 6.091(1) Å, V = 308.9(1) Å3, and Z = 4. The mineral possesses the olivine structure, with the M1 octahedra occupied by Na, and the M2 octahedra occupied by Fe and Mn. The eight strongest lines in the X-ray powder pattern are [d in Å (intensities) (hkl)]: 5.16 (50) (020), 4.44 (90) (110), 3.93 (80) (021), 3.56 (90) (120), 3.04 (80) (002), 2.817 (100) (130), 2.559 (100) (131), and 1.657 (50) (061). The mineral is named in honor of Karen Louise Webber, Assistant Professor Research at the Mineralogy, Petrology and Pegmatology Research Group, Department of Earth and Environmental Sciences, University of New Orleans, Louisiana, U.S.A.

The crystal structure of ramdohrite, Pb 5.9 Fe 0.1 Mn 0.1 In 0.1 Cd 0.2 Ag 2.8 Sb 10.8 S 24 , from the Chocaya mine, Potosí, Bolivia, determined by Makovicky and Mumme from film data in 1983, was refined from single-crystal diffractometer data to the R value 0.060, based on 5230 reflections [I > 2σ(I)] from a twinned crystal. Lattice parameters are a = 8.7348(3), b = 13.0543(4), c = 19.3117(6) Å, and β = 90.179(2)°, space group P2 1 /n. Two bicapped trigonal prismatic sites of lead bridge and unite adjacent (311) PbS slabs. These slabs contain five distinct coordination pyramids of Sb with trapezoidal cross sections, a mixed and disordered Sb-Ag-Cd-(Pb) site, refined as 0.39 Sb + 0.61 Ag, a pure Ag site with a very open, irregular tetrahedral coordination, and an octahedral site occupied by Pb. The (311) PbS slabs contain large lone electron pair micelles formed by four distinct antimony sites in alternation with small such micelles formed by a single Sb site. The geometric arrangement of these slabs is not based on crankshaft chains of short, strong Me-S bonds but on a chess-board arrangement of (predominantly) Sb pairs that share two common S atoms via short bonds. Relationships to, and differenced from, fizelyite and uchucchacuaite are described and discussed.