Volume 93, Issue 7

Using the CaIrO 3 -type structure model (space group Cmcm), lattice parameters and atomic positions of the MgGeO 3 post-perovskite (pPv) phase were determined based on Rietveld refinements at 78-109 GPa and first-principles calculations based on density functional theory. The reproducibility of structural parameters obtained for different samples, consistency with theoretical calculations, and good agreement with expected bond lengths based on structurally similar materials all provide evidence for both validity of CaIrO 3 -type structure model for the pPv phase in MgGeO 3 exceeding 1 Mbar and reliability of structural parameters obtained by Rietveld refinements approaching 1 Mbar. The MgGeO 3 pPv phase exhibits strong anisotropy in axial compressibility, with the b-axis being most compressible. The polyhedral bulk modulus for the GeO 6 octahedron is 1.9× larger than that for the MgO 8 hendecahedron. Examination of neighboring O-O distances shows that the O-O distance aligned along the a direction is one of the longest and that aligned along c is one of the shortest, and these may be related to the lower compressibility along c compared with a. Comparison of structural features of MgGeO 3 pPv with those for MgSiO 3 , NaMgF 3 , and CaIrO 3 pPv show that MgSiO 3 pPv has more similarity with NaMgF 3 and MgGeO 3 pPv than with CaIrO 3 pPv in such parameters as degree of octahedral distortion, implying that both NaMgF 3 and MgGeO 3 pPv are better analogs to MgSiO 3 pPv than CaIrO 3 pPv.

High-temperature, ammonium-rich white mica has been identified for the first time in deep Paleozoic (and probably older) polymetamorphic schists from the Internal Zone of the Betic Cordillera (Spain). Ammonium-rich white mica has been characterized by optical microscopy, X-ray diffraction, infrared spectroscopy, elemental analysis, electron microprobe, and scanning and transmission electron microscopy. High-temperature, ammonium-rich white mica shows some significant chemical differences with tobelite formed in hydrothermal and low-temperature metamorphic rocks. Although the average formula, Ca 0.09 Na 0.01 K 0.15 (NH 4 ) 0.75 (Al 1.70 Ti 0.01 Fe 0.26 Mg 0.13 )(Si 2.99 Al 1.01 )O 10 (OH) 2 , is typical of a dioctahedral mica, the chemical plots reveal a clear deviation toward the trioctahedral field. Thus, the increase in Fe + Mg contents is not accompanied by the parallel increase of Si contents, characteristic of the phengitic substitution, which is characteristic of low-pressure conditions of formation. Chemical differences are also accompanied by notable differences in the optical properties, both features suggesting that the term tobelite is not appropriate for this mica. Ammonium-rich white mica relics only persist in some graphite-rich microdomains, defining the first schistosity. Textural relations indicate that this mica formed during an older pre-Alpine metamorphic episode.

Crystalline CsAlSiO 4 was synthesized from a stoichiometric mixture of Al 2 O 3 + SiO 2 + Cs 2 O (plus excess water) in Ag-capsules at hydrostatic pressure of 0.1 GPa and temperature of 695 °C. The duration of synthesis was 46 h. The crystal structure of CsAlSiO 4 was investigated by single-crystal X-ray diffraction. The structure is orthorhombic with Pc2 1 n space group and lattice parameters: a = 9.414(1), b = 5.435(1), and c = 8.875(1) Å. Because of the orthohexagonal relation between b and a (a ≈ b√3), within the standard uncertainty on the lattice parameters, a hexagonal superlattice exists, which is responsible for twinning. The crystals are twinned by reflection, with twin planes (110) and (310): twinning in both cases is by reticular merohedry with twin index 2 and hexagonal twin lattice (LT). The transformation from the lattice of the individual (L ind ) to L T is given by: a T = a ind - bind, b T = 2b ind , and c T = c ind . The refinement was initiated using the previously published atomic coordinates for RbAlSiO 4 . The final least-square cycles were conducted with anisotropic displacement parameters. R 1 = 3.04% for 66 parameters and 2531 unique reflections. For a more reliable crystallographic comparison the crystal structure of RbAlSiO 4 is reinvestigated here adopting the same data collection and least-squares refinement strategy as for CsAlSiO 4 . The crystal structure of the CsAlSiO 4 feldspathoid is built on an ABW framework type, showing a fully ordered Si/Al-distribution in the tetrahedral framework. The only extra-framework site is occupied by Cs, lying off-center in the 8mR-channels. CsAlSiO 4 is more likely to retain Cs when immersed in a fluid phase, relative to several other Cs-bearing zeolites. The topological configuration of the Cspolyhedron (and its bonding environment), the small dimension of the pores and the high flexibility of the ABW framework type would imply a better thermal and elastic stability of CsAlSiO 4 than those of the zeolitic Cs-aluminosilicates. In this light, CsAlSiO 4 can be considered as a functional material potentially usable for fixation and deposition of radioactive isotopes of Cs and can also be considered as a potential solid host for a 137 Cs γ-radiation source to be used in sterilization applications.

This paper describes a multi-technique approach to the complete crystal-chemical characterization of a gadolinite-(Y) sample found in a volcanic holocrystalline ejectum near the Vico lake (Latium, Italy). Gadolinite-(Y) occurs as poly-twinned crystals forming rounded short-prismatic aggregates (generally 0.1-0.3 mm in size, with the largest ever found >1 mm), associated with zircon, thorite, danburite, betafite, and tourmaline. Both the chemical and the structural characterization of gadolinite-(Y) from Vico required nonstandard procedures. After correction for (100) twinning, the structure of a crystal with unit-cell dimensions a = 4.7708(4) Å, b = 7.6229(7) Å, c = 9.8975(9) Å, β = 90.017(7)°, and V = 359.95(6) Å 3 was refined in the P2 1 /c space group down to R = 2.3%. Electron microprobe (EMP) analyses failed to give accurate quantification of major elements, due to the presence of light and volatile elements as well as of rare earth elements (REE) and actinides. Secondary ion-mass spectrometry (SIMS) analysis done with accurate calibrations on well-characterized minerals allowed quantification of light, volatile, REE, and actinide elements, and also of Ca and Si. The derived chemical composition was interpreted with reference to the site-scattering values obtained from single-crystal structure refinement. The resulting unit formula is (Ca 0.81 REE 0.66 Y 0.39 Th 0.13 U 0.02 )Σ 2.01 (Fe 2+ 0.29 Li 0.14 Fe 3+ 0.12 Mn 0.02 Mg 0.01 ) Σ0.58 (Si 1.98 Be 1.09 B 0.81 Li 0.12 ) Σ4.00 O 8 (O 1.20 F 0.51 OH 0.29 ) Σ2.00 , which yields a calculated density of 4.267 g cm -3 . Fourier transform infrared spectroscopy (FTIR) single-crystal spectrum of gadolinite-(Y) shows several absorptions in the OH-stretching region that can be assigned to the different local configurations involving Ca and (REE,Y) at the A site and Be, B, and Li at the Z site. Lithium incorporation in gadolinite-group minerals is proposed to occur according to the exchange vectors: (1) X Fe 2+ + A Y → X Li + A (Th +U) and (2) Z Be + X Fe 2+ → Z Li + X Fe 3+ ; the maximum amount of Li allowed in the gadolinite structure is 1.0 apfu. This work provides the first evidence that Li is a significant component in gadolinite-group minerals, particularly in geochemical environments enriched in actinides. This conclusion suggests that materials having the composition of Li-rich gadolinite may be considered as possible forms for radioactive waste disposal.

Four synthetic crystals belonging to the jadeite (Jd, NaAlSi 2 O 6 )-hedenbergite (Hd, CaFeSi 2 O 6 ) solid solution were investigated by X-ray diffraction in situ at high pressure using a diamond anvil cell to P max = 10.6 GPa. The samples exhibited space group symmetry C2/c throughout the investigated pressure range and did not show any phase transformations. V 0 , K T0 , and K' were simultaneously refined by fitting a third-order Birch-Murnaghan equation of state to pressure-volume data for all samples. The following relationship between bulk modulus and molar fraction of jadeite is observed: K T0 = 108.7(2) (GPa) + 0.191(9) × [% molar Jd] + 0.0006(1) × [% molar Jd] 2 The bulk modulus of hedenbergite is 19% lower than jadeite with a strong axial anisotropy that increases with increasing the Hd content. In particular, the compressibility along the b axis (the most compressible in pyroxenes) increases by about 35% going from Jd to Hd while along the c axis the increase in compressibility is about 24%. The a axis does not show any variation in the deformation rate along the join. The analysis of the crystal structure behavior with pressure for all samples clearly indicates that the main cause of the strong anisotropy on the b-c plane is related to the narrowing of the M1 octahedral chain and to anion-anion interactions increasing the packing efficiency of the anion skeletons of the crystals going from Jd to Hd.

Samples of beachrock from Lakes Butler and Fellmongery, near Robe, South Australia, were examined by neutron and X-ray powder diffraction. Considerable variation in phase composition was observed, although all samples possessed monohydrocalcite, calcite, aragonite, and magnesiancalcite. The Rietveld refinement of the structure of monohydrocalcite showed significant changes in the orientation of water molecules and a major change in the orientation of a carbonate group with respect to the currently accepted structure. The current refinement shows that the hydrogen-bonding structure consists of three distinct networks, with each of the three carbonate groups being bonded to a single water molecule via one linear and one bifurcated hydrogen bond.

The theoretical infrared spectrum of hematite (α-Fe 2 O 3 ) was computed using ab initio quantum mechanical calculations. Frequencies of the normal vibrational modes and Born effective charges were computed using the density functional theory (DFT) with and without the addition of a Hubbard U correction. The infrared reflection spectra of a single crystal of hematite were calculated as well as the infrared powder absorption spectrum using an electrostatic model that takes into account the shape of hematite particles. The theoretical behavior of the absorption bands is in agreement with experimental observations and provides a firm basis for the interpretation of the bands in term of vibrational modes. Overall, results suggest that the use of DFT + U, which is necessary to describe correctly the electronic and magnetic properties of hematite, does not improve noticeably the prediction of vibrational properties.

Dissolution behavior of muscovite under alkaline conditions at 25-50 °C was investigated using in situ atomic force microscopy (AFM) with an air/fluid heater system to derive reliable dissolution rates and determine the dissolution mechanism and the effect of temperature on the dissolution rates. The muscovite dissolution took place only at the edge surfaces that are less than a few percent of the total surface area (TSA), while the basal surfaces were completely unreactive. During the initial stage of the experiments, some rough edge surfaces of muscovite dissolved much faster at the reactive site and appeared to straighten. The straightened edge surfaces seemed to retreat with a lower constant rate, which may correspond to the actual dissolution rate of muscovite, in the late stage. The edge surface area (ESA)-normalized dissolution rate at a certain pH and temperature condition, therefore, has a constant value independent of the size of etch pit or island (particle). The ESA-normalized dissolution rates derived from this AFM study were consistent with the dissolution rates renormalized to the estimated ESA of the earlier studies. In contrast, the TSA-normalized dissolution rates varied with the size of etch pit or island. The activation energy for muscovite dissolution under alkaline conditions was very close to that for montmorillonite and illite dissolution. A model dissolution rate equation, which simultaneously includes the effect of pH and temperature, was deduced from the effect of pH on the activation energy, the rate equation of muscovite dissolution at 25 °C, and the Arrhenius equation. The dissolution rates estimated from the model were in good agreement with the experimental rates from 25 to 70 °C. The dissolution reaction order with respect to hydroxyl activity (or pH) increases with temperature.

This study of melt and fluid inclusions in graphic pegmatite quartz from the Island of Elba, Italy has identified silicate melts and coexisting aqueous fluid from the final stage of pegmatite formation, which contain extremely high concentrations of boron (up to 19.8 mass% B 2 O 3 ), cesium (>5 mass% Cs 2 O), and rubidium (>0.5 mass% Rb 2 O). Several boron-bearing minerals have been identified in melt and fluid inclusions, including sassolite (H 3 BO 3 ) and santite (KB 5 O 8 ·4H 2 O). In addition, we have found two new minerals observed as daughter mineral phases, and these have been accepted by the IMA, Commission on New Minerals, Nomenclature and Classification (CNMNC) as ramanite-(Cs) (2007-007), monoclinic β-cesium pentaborate tetrahydrate (CsB 5 O 8 ·4H 2 O) with space group C2/c, and ramanite-(Rb) (2007-006), orthorhombic rubidium pentaborate tetrahydrate (RbB 5 O 8 ·4H 2 O) with space group Aba2. At the moment, given sample constraints, we cannot directly determine some of the properties traditionally reported for new minerals; however, both phases have Raman spectra identical to spectra obtained on synthetic Cs and Rb pentaborates, which were studied for comparison. Although difficult to recognize optically, ramanite-(Cs) and ramanite-(Rb) have distinctive Raman spectra allowing simple and definitive identification from other translucent phases found in many hypersaline fluid inclusions. Although currently only identified from a limited number of occurrences, given the availability of a definitive means of identification, we suspect that ramanite-(Rb) and ramanite-(Cs) may in fact be common, at least in boron-rich pegmatites.

X-ray diffraction measurements of natural talc plus water at combined pressures and temperatures of 0-15 GPa and 23-400 °C reveal the presence of a structural change that could be interpreted as a new high-pressure phase at 4.0 (±0.5) GPa, and raise the possibility that the newly inferred phase transition takes place in cold subducting slabs as a precursor to appearance of the 10 Å phase of talc.

Analytical electron microscopic observations have been carried out on a garnet peridotite from the Maobei area, Sulu ultrahigh-pressure terrane. The results showed that olivine in this garnet peridotite (5.3-6.6 GPa; 853-957 °C), contains precipitates of chromian magnetite and chromian-titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and the olivine host, viz. [101] Mt //[001] Ol , [110] Mt //[01̄1] Ol , and [01̄1] Mt //[011] Ol ; and [0001] Hm //[100] Ol and [101̄0] Hm //[001] Ol . These oriented oxides are not associated with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation. It is postulated that these magnetite and hematite precipitates most likely have resulted from dehydrogenation-oxidation of nominally anhydrous mantle olivine during rock exhumation. In view of the contrasting diffusion rates of H and Fe in the olivine lattice, it is suggested that the formation process might actually take place in steps. Hydrogen diffusion with concomitant quantitative oxidation of Fe 2+ to Fe 3+ in olivine occurred early during initial rock exhumation and was followed by slow Fe diffusion forming magnetite/hematite at stacking faults and dislocations within the olivine lattice. Two requirements are essential under such a scenario: an ample amount of H content of the olivine, and an appropriate exhumation rate, probably in the range of 6-11 mm/year, of the host rock. It is also noted that such dehydrogenation-oxidation processes may hamper a correct estimate of the actual P-T conditions and mantle oxidation state based on mineral chemistries present in mantle eclogite/peridotite. The present study demonstrates that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through a series of more complicated reaction mechanisms.

An arsenide (low-Ni, high-Co löllingite) was found in a peridotite xenolith, which is strongly metasomatized by slab-derived melt or fluid from Avacha volcano, located in the Kamchatka arc. This is the first finding of a mantle arsenide within a fresh peridotite xenolith that may have been precipitated from metasomatic fluid/melt ultimately derived from the subducting slab. The löllingite is very low in Ni, suggesting low Ni-Fe partitioning between arsenide and olivine at mantle conditions. This is in contrast to sulfide, which favors Ni over Fe. An As-bearing fluid/melt thus plays some role in the metasomatic Fe enrichment in the mantle wedge. Supply of As is one of the characteristics of the upper mantle beneath the volcanic front.

Recent advances in high-resolution solid-state NMR enabled us to resolve the previously unknown details of atomic configurations around quadrupolar nuclides such as O ( 17 O, spin 5/2) and aluminum ( 27 Al, spin 5/2) in phyllosilicates. Here, we report the 17 O 1D and 2D NMR (MAS and MQMAS) spectra of synthetic kaolinite-montmorillonite mixtures. Various O sites in the mixtures revealed from O-17 NMR include apical O ( VI Si-O-2 IV Al), multiple basal O atoms ( VI Si-O- VI Si and two VI Si-O- VI Al sites) in tetrahedral layers, and hydroxyl group with distinct ranges for NMR parameters (quadrupolar coupling constant, C q and isotropic chemical shift, 17 O δ iso ). In particular, the structurally relevant NMR parameters ( 17 O δ iso , C-, and asymmetry parameter, η) for two resolved IV Si-O- VI Al sites are about 52 ± 2 ppm, 2.6 ± 0.2 MHz, and 0.5 for IV Si-O- IV Al (I) and about 44.5 ± 2 ppm, and 3.1 ± 0.2 MHz, and 0.5 for IV Si-O- IV Al (II), respectively. These sites may stem from crystallographically distinct IV Si-O- IV Al sites of varying bond length and IV Si-O- VI Al bond angle or IV Al-O-2 VI Al site in the montmorillonite.

The crystal structure of a new synthetic Na 8-r Al 8-r Si 8+r O 32 (r = 0.15) modification has been solved by direct methods and refined to an R index of 0.0289 for 11 643 independent reflections and 764 parameters. The compound is monoclinic with space group symmetry P112 1 , a = 9.9897(6) Å, b = 9.9622(6) Å, c = 24.979(2) Å, and γ = 119.788(4)°. The twinning of three individuals related by rotation around the c direction for 120° was accounted for in the calculation. The phase was named monoclinic trinepheline because the length of its c lattice parameter is 3× the length of the c parameter in nepheline, whereas the a parameter is almost the same in both structures, similarly to a previously investigated trinepheline of hexagonal symmetry. Actually, the present compound represents an intermediate state between these two phases. The characteristic of the crystal structure are layers of six-membered rings built up of regularly alternating AlO 4 and SiO 4 tetrahedra. The layers are perpendicular to [001] and are connected through apical O atoms. The resulting spatial network contains channels in which the Na cations are situated. The structure of monoclinic trinepheline can be described as a consecutive stacking of basic nepheline like modules (containing two tetrahedral sheets) and one sheet corresponding to those observed in the hexagonal trinepheline structure. The topology of the new compound corresponds to that of tridymite. The coordination sequences are identical for all T atoms in the asymmetric unit: 4-12-25-44-67-96-130-170-214-264. The vertex symbols for the tetrahedral centers are 6 2 ·6 2 ·6 2 ·6 2 ·6 2 ·6 2 . The framework density of the structure is equal to 22.2 T atoms/1000 Å 3 .

Fresh and weathered granite from drill cores in Tono, Gifu, Japan, was examined to understand weathering products and the mechanisms of chlorite and biotite weathering under low P O₂ conditions. A fresh sample from 365 m depth, a slightly weathered light-green sample from 367 m depth, and a nearly fresh sample from 369 m depth (but with brown stains on fractures), were investigated. The XRD, SEM, EMPA, and TEM analysis of green grains present within chlorite, biotite, and plagioclase grains and in veins was found to be Fe 2+ -rich corrensite [about 40 wt% FeO with Fe/(Fe + Mg) = 0.94]. The corrensite is interpreted to have formed from chlorite and biotite via a dissolution-precipitation mechanism. The <2 μm fraction of the weathered sample had an Fe 2+ /ΣFe value of 0.69, which, when combined with the presence of amorphous Fe 3+ (hydr)oxides confirmed by TEM, indicates that the Fe 2+ /ΣFe value of corrensite is >0.69. These results indicate that on dissolution of chlorite and biotite, Fe 2+ was transported as Fe 2+ and precipitated as Fe 2+ -rich corrensite and a part of the dissolved Fe 2+ was oxidized to amorphous Fe 3+ (hydr)oxides under low P O₂ conditions. The formation of Fe 2+ -rich corrensite and that of Fe 2+ -rich smectite or vermiculite in the laboratory at 1 atm of P CO ₂ and ≤3 × 10 -5 atm of P O₂ (Murakami et al. 2004) suggest that a possible Fe 2+ -bearing product during Precambrian weathering is Fe 2+ -rich sheet silicates but not siderite.

Uranium is one of the main heat sources in the Earth, as about 25% of the total heat is produced by the radioactive decay of U. The location of U in the deep mantle is then essential for a better understanding of the geodynamics and thermal behavior of the Earth. For the first time, the crystal structure of natural simple dioxide UO 2 uraninite has been studied by X-ray diffraction with synchrotron radiation (ESRF, Grenoble, France), in situ in a laser-heated diamond-anvil cell at pressures and temperatures relevant to the deep Earth’s mantle. Fluorite-type UO 2 displays a new sequence of phase transitions at high P and T, with a cubic modified fluorite Pa3 observed at 18 GPa, and an orthorhombic Pbca structure from 33 GPa up to 82 GPa. Using a second-order Birch-Murnaghan equation of state, we calculated room-pressure bulk modulus K 0 = 166(7) GPa with pressure derivative K′ 0 = 4.0 for the Pa3̄ structure, and K 0 = 225(8) GPa with K′ 0 = 4 for the Pbca structure. The expected Pnma cotunnite structure was not observed but is not excluded at pressures higher than 82 GPa. Since UO 2 displays a Pbca structure stable up to 82 GPa and presents a density much higher than the average density of the surrounding mantle, UO 2 could be a host of U in the deep lower mantle.

To gain new structural insights into phase D and superhydrous B, two phases of potential mantle water reservoir, we have applied a range of one- (1D) and two-dimensional (2D) 1H and 29 Si NMR techniques, as well as Raman spectroscopy, to samples synthesized at 24 GPa and 900~1100 °C. These data have revealed that phase D is characterized by disordered and varying local structures around both Si and H. The 29 Si NMR spectra of phase D contain a nearly symmetric, broad peak near -177.7 ppm, attributable to octahedral Si with local structural disorder. The high-resolution 1 H CRAMPS spectra of phase D contain a main broad peak near 12.6 ppm with shoulders near 10 and 7 ppm, suggesting a distribution of hydrogen bonding distances. For superhydrous B, our comprehensive 2D 1 H and 29 Si NMR results have clearly revealed that it contains dissimilar hydrogen (H1-H2) pairs and one tetrahedral Si site, consistent with space group Pnn2.

Mid-IR spectra (4000-400 cm -1 ) of synthetic calcic amphiboles in the tremolite-pargasite series and of various natural calcic amphiboles have been investigated. The pargasite substitution, a combination of the Tschermak (= [4] Al [6] Al [6] Mg -1 [4] Si -1 ) and edenite (= [4] Al [A] Na [4] Si -1 [A] □ -1 ) substitutions, causes the following features in the region 1200-600 cm -1 . (1) Weak [4] Al-O stretching bands appear at 895 and 815 cm -1 that are distinct from the 955 and 925 cm -1 Si-O stretching bands in tremolite. (2) There is a reduction in the intensity and frequency of the Si-O-Si symmetric bending band (=“chain breathing” mode) at 750 cm -1 in tremolite, and there is an appearance of medium-strong composite bands having a weak shoulder on the high-frequency side near 690 cm -1 . These bands are assigned to Si-O-Al deformation bands. (3) Two OH-libration bands at 690 and 650 cm -1 become weak and broad composite bands from 720 to 610 cm -1 . And (4) because the intensity and frequency of the band at 640 cm -1 in tremolite is affected neither by deuteration nor by the pargasite substitution, this band is ascribed to O- [T2] Si-O bending. Even in pargasite, most T2 sites are occupied by Si, so that the O2- [T2] Si-O4 bending mode will be dominant in this amphibole. The same behavior occurs for the synthetic fluoro-gallian tremolite-pargasite series but with larger downward frequency shifts-Ga-O stretching bands appear at 880 and 780 cm -1 , and an Si-O-Ga bending band appears at 605 cm -1 . The major T-O-T and O-T-O deformation bands in synthetic amphiboles are readily apparent in natural calcic amphiboles whose compositions are near the tremolite-pargasite join.

Ferrimagnetic nanoparticles have many uses in industry including in magnetic recording media and transformers, however these particles are often expensive to synthesize. In this study, the Fe 3+ - reducing bacteria Geobacter sulfurreducens and Shewanella oneidensis were used to synthesize spinel ferrite nanoparticles of the general chemical formula M x Fe 3-x O 4 , where M is either Co, Ni, Mn, Zn, or a combination of Mn and Zn. This was done at ambient temperatures through the dissimilatory reduction of Fe 3+ -oxyhydroxides containing the appropriate substitutional cations. A combination of L-edge and K-edge X-ray absorption spectroscopy (XAS) and L-edge X-ray magnetic circular dichroism (XMCD) was used to determine the site occupancies, valence, and local structure of the Fe and substitutional cations within the spinels. The Ni and Co ferrites produced using each bacterium were very similar and therefore this study concludes that, despite the difference in reduction mechanism of the bacteria used, the end-product is remarkably unaltered. Nickel ferrites contained only Ni 2+ , with at least 80% in Oh coordination. Cobalt ferrites contained only Co 2+ but with a significant proportion (up to 45%) in Td coordination, showing a slight preference for Td sites. The Mn-ferrites contained Mn 2+ only on the Oh sites but a mixture of Mn 2+ and Mn 3+ on Td sites when the amount of Mn exceeded 3% (compared to the amount of Fe) or some Zn was also present. This study successfully produced a range of nanoparticulate ferrites that could be produced industrially using relatively environmentally benign methodologies.

Three types of metamorphic titanite have been noted in a metarodingite pod within the tectonized and serpentinized peridotite portion of the Penjwin ophiolite sequence within the Iraqi Zagros Thrust Zone (IZTZ), northeastern Iraq. Type I metamorphic titanite occurs as subhedral to anhedral finegrained disseminated crystals within chlorite that was formed during chloritization of biotite as a result of low-temperature (T = 330-340 °C) ocean-floor metamorphism or rodingitization of plagiogranite under a reducing environment. It is characterized by intermediate Al 2 O 3 (Avg: 3.61 wt%), high FeO T (Avg: 0.89 wt%), and intermediate TiO 2 (Avg: 34.7 wt%). Type II metamorphic titanite occurs as a thin rim around ilmenite and has high Al 2 O 3 (Avg: 4.8 wt%), intermediate FeO T (Avg: 0.6 wt%), and low TiO 2 (Avg: 33.7 wt%); it represents a reaction product between grossular and ilmenite, which resulted from an oxidizing high-pressure-high-temperature (P = 1.4-1.6 GPa and T > 750 °C) metamorphic event involving plagiogranite. Type III metamorphic titanite occurs as very coarse, highly fractured grains up to 0.5 mm, with inclusions of ilmenite, and surrounded by albitic plagioclase, analcime, and chlorite. This titanite is characterized by low Al 2 O 3 (Avg: 1.23 wt%), low FeO T (Avg: 0.30 wt%), and high TiO 2 (Avg: 36.98 wt%). It formed during extensive titanitization of ilmenite by a reaction with Ca-plagioclase during moderate pressure and temperature (P < 1.6 GPa and T < 750 °C) conditions, as a result of albitization of Ca-plagioclase in plagiogranite.

The temperature dependence of the partitioning of Al 2 O 3 between forsterite-rich olivine and Crrich spinels has been experimentally calibrated at 100 kPa between 1250 and 1450 °C under reducing conditions. For spinel with values of Y Cr between 0.07 and 0.69, and containing <0.1 Fe 3+ atoms and <0.025 Ti atoms per 4 O atoms the experimental data can be fit to an equation: where Y Cr = Cr/(Cr + Al) in spinel in atomic proportions and K D = Al 2 O 3 ol /Al 2 O 3 sp in wt%. This equation reproduces the temperature of the experiment for the calibration data set within ±22 °C. Empirical tests suggest that this geothermometer has little pressure dependence within the range of pressures encountered in the crust and upper mantle.

Single crystals of CaIrO 3 were prepared via flux growth method. Crystal structure parameters, including the anisotropic displacement parameters, are determined based on a single-crystal X-ray diffraction experiment. The unit-cell dimensions are a = 3.147(2), b = 9.866(6), and c = 7.302(5) Å. The structure is a three-dimensional dense structure with small vacant spaces. The CaIrO 3 structure can be described as a pseudo-one-dimensional oxide and is compared with Ca 4 IrO 6 structure. The IrO 6 octahedra are significantly distorted, in contrast to other octahedral Ir 4+ compounds. The O-O distances for faces and edges shared between polyhedra are shorter than other non-shared edge distances. These effects are explained by Pauling’s rules and occur to decrease the repulsion between the cations. Thermal vibrations of Ca and Ir atoms are significantly anisotropic. Thermal vibrations of Ca and Ir atoms are restricted in orientation toward the shared face, shared edges, and shortest cation-cation directions. The single-crystal experiment shows that CaIrO 3 crystals grow fastest along the a axis and that they assume a prism or needle shape. Strongly preferred orientation of such prism shaped CaIrO 3 -type post perovskite MgSiO 3 crystals may develop under the shear flow in the Earth’s mantle.

Rengeite, Sr 4 ZrTi 4 (Si 2 O 7 ) 2 O 8 , is a Sr-analog mineral of perrierite and occurs as an accessory mineral in natrolite, pectolite, and itoigawaite veins associated with jadeitic rock from Ohmi-Itoigawa region, Japan. The symmetry of this mineral is reported to be monoclinic. Here, we found an orthorhombic polymorph of rengeite that occurs as micro domains up to 5 pm in width in monoclinic rengeite crystals using electron diffraction and high-resolution TEM analyses. X-ray diffraction analysis us­ing a Gandolfi camera also revealed the presence of the orthorhombic phase in monoclinic rengeite crystals. The unit-cell dimensions of the orthorhombic polymorph are a = 14.0, b = 5.7, c = 21.9 Å, V= 1748 Å 3 , and Z = 4. Although the space group of orthorhombic polymorph of rengeite has not been determined, the possible space group is Pbca or a similar one from theoretical consideration by Ito (1950). The chemical composition was determined by ATEM-EDS analysis to be SiO 2 = 22.5, TiO 2 = 30.2, SrO = 39.4, Nb 2 O 5 = 0.2, Fe 2 O 3 = 0.3, and ZrO 2 = 7.5, totaling 100.1 wt%, and the ideal chemical formula is Sr 4 ZrTi 4 (Si 2 O 7 ) 2 O 8 . Detailed HRTEM imaging revealed that the unit cell of the orthorhombic polymorph of rengeite can be interpreted as a superstructure of monoclinic rengeite, accompanied by repeated twinning on (001) plane. The HRTEM images taken were consistent with the simulated images

The crystal chemistry of two dimorphic hydrated sodium beryllium silicates, epididymite [a = 12.7334(4), b = 13.6298(5), c = 7.3467(3) Å, V = 1275.04 Å 3 , space group Pnma)] and eudidymite [a = 12.6188(10), b = 7.3781(5), c = 13.9940(9) Å, β = 103.762(5)°, V = 1265.47 Å 3 , space group C2/c] from Malosa, Malawi, has been reinvestigated by means of energy dispersive X-ray spectroscopy, thermo-gravimetric analysis, inductively coupled plasma-optical emission spectroscopy and single-crystal neutron diffraction. Two anisotropic structure refinements have been performed with final agreement index R1 = 0.0317 for 137 refined parameters and 2261 unique reflections with F o > 4σ(F o ) for epididymite, and R1 = 0.0478 for 136 refined parameters and 1732 unique reflections with F o > 4σ(F o ) for eudidymite. The analysis of the difference-Fourier maps of the nuclear density of the two dimorphs confirms the presence of extra-framework water molecules in both, and not hydroxyl groups as wrongly reported in previous studies and in several crystal-structure databases. The correct chemical formula of edipidymite and eudidymite is Na 2 Be 2 Si 6 O 15 ·H 2 O (Z = 4). The configuration of the water molecules and the hydrogen bonds are fully described for both the dimorphs. The chemical analyses show that a small, but significant, amount of Al and Fe (most likely substituting for Si in the tetrahedral sites) and K (substituting for Na as an extra-framework cation) occurs in both dimorphs

Although a great deal is known about the interaction between water and rhyolitic glasses and melts at temperatures above the glass transition, the nature of this interaction at lower temperatures is much more obscure. Comparisons between high- and low-temperature diffusion studies suggest that several factors play important roles under lower-temperature conditions that are not significant at higher temperatures. Water concentrations in rhyolitic glasses hydrated at low temperatures are significantly greater than in those hydrated at high temperatures and low pressures. Surface concentrations, which equilibrate quickly with the surrounding environment at high temperature, change far more slowly as temperature decreases, and may not equilibrate at room temperature for hundreds or thousands of years. Temperature extrapolations of high- and low-temperature diffusion data are not consistent, suggesting that a change in mechanism occurs. These differences may be due to the inability of “self-stress,” caused by the in-diffusing species, to relax at lower temperature. Preliminary calculations suggest that the level of stress caused by glass-water interaction may be greater than the tensile strength of the glass. On a microstuctural scale, extrapolations of high-temperature Fourier transform infrared spectroscopy (FTIR) data to lower temperatures suggests that there should be little or no hydroxyl present in glasses hydrated at low temperature. Comparisons of low-temperature hydration results among SiO 2 , obsidian, and albite compositions show distinct differences, and features are present in the spectra that do not occur at high temperature. Analysis of H 2 O and D 2 O diffusion also suggest that mechanistic differences occur between low- and high-temperature diffusive processes.

The heat capacities of two different molecule-containing melanophlogites of approximate composition 46SiO 2 ·1.80CH 4 ·3.54N 2 ·1.02CO 2 from Mt. Hamilton, California, and 46SiO 2 ·3.59CH 4 ·3.10N 2 ·1.31CO 2 from Racalmuto, Sicily, along with a heat-treated (molecule-free) sample of composition SiO 2 , were studied between 5 and 300 K using heat-pulse microcalorimetry. The molecule-free sample was obtained by heating natural Racalmuto crystals at 1173 K for 24 h. The standard third-law entropy of the molecule-free sample is S° = 2216.3 ± 6.6 J/(mol·K) for 46SiO 2 and the natural Mt. Hamilton and Racalmuto samples give S° = 2805.7 ± 8.4 J/(mol·K) and S° = 2956.8 ± 8.9 J/(mol·K), respectively. The entropy and Gibbs free energy for molecule-free melanophlogite relative to quartz at 298 K are ΔS trans = 6.7 J/(mol·K) and ΔG trans = 7.5 kJ/mol, respectively and, thus, it does not have a thermodynamic field of stability in the SiO 2 system. The difference in CP values between molecule-containing and molecule-free melanophlogite is characterized by an increase in C P from 0 to ~70 K, and it then reaches a roughly constant value at 70 K < T < 250 K. The ΔS rxn at 298 K for 46SiO 2 (melan.) + xCH 4 (gas) + yCO 2 (gas) + zN 2 (gas) = 46SiO 2 ·(xCH 4 ) 12 ·(yCO 2 , zN 2 ) 14 is estimated to be about -642 and -802 J/(mol·K) for the Mt. Hamilton and Racalmuto samples, respectively. The thermodynamic data, as well as published results on the occurrence of natural molecule-containing samples suggest that melanophlogite crystallizes metastabily. The occurrence of melanophlogite and the lack of other porosils in nature are probably due to the essential role of molecular structure-directing agents. For melanophlogite they can be CO 2 , N 2 , and CH 4 , whereas the crystallization of other porosils requires more chemically and structurally complex molecules that are not naturally abundant.

The crystal structure of glaucodot, (Co,Fe)AsS, an important member of the FeAsS-CoAsS-NiAsS system, was determined with single-crystal X-ray diffraction. It is orthorhombic with space group Pn2 1 m and unit-cell parameters a = 14.158(1), b = 5.6462(4), c = 3.3196(2) Å, and V = 265.37(5) Å 3 . The structure is closely related to that of arsenopyrite or alloclasite, and represents a new derivative of the marcasite-type structure. The As and S atoms in glaucodot, which are ordered into six distinct sites (As1, As2, As3, S1, S2, and S3), form three types of layers [S, As, and mixed (S + As) layers] that are stacked along a in the sequence of (S + As)-(S + As)-S-(S + As)-(S + As)-As-(S + As)-(S + As)... In contrast, arsenopyrite contains the mixed (S + As) layers only and alloclasite consists of isolated S and As layers only. There are no As-As or S-S bonds in glaucodot; all dianion units are formed between S and As, like those in arsenopyrite and alloclasite. The (Co + Fe) cations in glaucodot occupy three nonequivalent octahedral sites (M1, M2, and M3), with M1(As 5 S), M2(As 3 S 3 ), and M3(AsS 5 ), which form three distinct edge-shared octahedral chains, A, B, and C, parallel to c, respectively. These chains are arranged along a in the sequence of A-A-B-C-C-B-A-A.... Whereas the configurations of the A and C chains are analogous to those in safflorite and marcasite, respectively, the configuration of the B chain matches that in alloclasite, leading us to propose that the M1, M2, and M3 sites are predominately occupied by Co, (Co + Fe), and Fe, respectively. Our study, together with previous observations, suggests that glaucodot is likely to have an ideal stoichiometry of (Co 0.5 Fe 0.5 )AsS, with a limited tolerance for the variation of the Co/Fe ratio.

Laser-ablation inductively coupled-plasma mass spectrometry (LA-ICPMS) is an established method of analyzing the elemental composition of individual fluid inclusions in minerals. While the method affords good spatial control of ablation pits, we have found that stray light from the UV (193 nm) laser may permanently modify the gas species in adjacent fluid inclusions. We have used Raman spectroscopy to analyze natural fluid inclusions in quartz before and after ablating test pits. New molecular species (CO, H 2 , and O 2 ) appeared in the vapor phases of inclusions near the pits, resulting in complex, disequilibrium gas mixtures (e.g., CO 2 -CH 4 -H 2 O-CO-H 2 -O 2 ) and formation of iron oxyhydroxides in brine inclusions. Ablation for 40 s with a laser fluence of ~24 J/cm 2 on the sample produced the new gas species in inclusions at various distances up to 210 μm from the pits, depending on pit diameter. We attribute the changes in gas speciation to molecular photodissociation induced by stray UV-laser light. We have no evidence that this artifact changes the bulk elemental composition or density of the affected inclusions. The problem for fluid inclusion analysis is that phase-transition temperatures (e.g., clathrate hydrate dissociation, partial homogenization of the carbonic phases, and total homogenization) are highly sensitive to the mixture of gas species present. This means that parameters such as gas compositions, bulk salinity, and minimum entrapment temperatures are likely to be misinterpreted if inclusions are analyzed by microthermometry or Raman spectroscopy in samples that have already undergone laser ablation.

The crystal structures of natrolite and its dehydrated high-temperature phases (α1- and α2- metanatrolite) have been determined from powder X-ray diffraction measurements as a function of temperature and partial pressure of water (P H ₂ O ) to characterize the phase transition behavior. The evolution of crystal structure as a function of temperature shows two different phase transitions, depending on the P H ₂ O , with α1-metanatrolite occurring at elevated P H ₂ O and α2-metanatrolite occurring at low P H ₂ O . Our discovery of α2-metanatrolite implies the existence of more than one transition mechanism, which we correlate with the migration of Na + ions and the rate of evolution of H 2 O molecules. The transition behavior is rationalized in terms of two cooperating mechanisms: (1) dehydration-induced processes, which determine the phase transition temperature; and (2) thermally induced processes, which determine how the framework and its extraframework cations are modified.