Volume 95, Issue 4

Ordering of Fe 3+ and Fe 2+ cations between octahedral and tetrahedral sites in synthetic members of the magnetite-ulvöspinel (Fe 3 O 4 -Fe 2 TiO 4 ) solid-solution series was determined using Fe L 2,3 -edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L 2,3 -edge and Fe K-edge X-ray absorption spectroscopy (XAS), and unit-cell parameters. Microprobe analyses, cell edges, and chemical FeO determinations showed that bulk compositions were stoichiometric magnetite-ulvöspinel solid solutions. XMCD showed that the surface was sensitive to redox conditions, and samples required re-equilibration with solid-solid buffers. Detailed site-occupancy analysis gave Fe 2+ /Fe 3+ XMCD-intensity ratios close to stoichiometric values. L 2,3 -edge XAS confirmed that Ti 4+ was restricted to octahedral sites. XMCD showed that significant Fe 2+ only entered the tetrahedral sites when Ti content was >0.40 atoms per formula unit (apfu), whereas Fe 2+ in octahedral sites increased from 1 apfu in magnetite to a maximum of ~1.4 apfu when Ti content was 0.45 apfu. As Ti content increased, a steady increase in Fe 2+ in tetrahedral sites was observable in the XMCD spectra, concurrent with a slow decrease in Fe 2+ in octahedral sites. Calculated magnetic moments decreased rapidly from magnetite (4.06 μ B ) to USP45 (1.5 μ B ), then more slowly toward ulvöspinel (0 μ B ). Two synthesized samples were maghemitized by re-equilibrating with an oxidizing buffer. XMCD showed that Fe 2+ oxidation, with concomitant vacancy formation, was restricted to octahedral sites. Through the direct measurement of Fe oxidation states, XMCD results can be used to rationalize the magnetic properties of titanomagnetites, along with oxidized titanomaghemitized analogs, in Earth’s crustal rocks.

The mineral wadalite (ideal and simplified formula: Ca 6 Al 5 Si 2 O 16 Cl 3 ) has been discovered for the first time in a meteorite, specifically in coarse-grained, igneous type B calcium-aluminum-rich inclusions (CAIs) from the CV carbonaceous chondrite Allende. We report the results of electron microprobe, scanning electron microscopy, and transmission electron microscopy analyses of wadalite-bearing assemblages in the Allende CAIs and propose that wadalite formed by metamorphic reaction between åkermanitic melilite and anorthite, likely mediated by chlorine-bearing fluids. Petrographic relationships support the likelihood of multistage alterations by fluids of different chemistries interspersed or coinciding with thermal metamorphic episodes on the Allende parent asteroid. Fluid involvement in metamorphism of Allende CAIs implies that these objects experienced open-system alteration after accretion into the CV chondrite parent asteroid, which may have resulted in disturbances of their oxygen- and magnesium-isotope systematics.

Synthetic rubies were grown by the optical floating zone method and characterized by cathodoluminescence (CL) microscopy and spectroscopy in combination with microprobe analysis. The measured intensity of the Cr 3+ -induced CL of corundum is influenced by different parameters such as the sample thickness and orientation, the intensity of the beam current, and the quality of the sample surface. Furthermore, the spectral distribution of the CL emission changes with different concentrations of Cr in the corundum structure. Due to these effects, it is usually not possible to estimate the Cr content directly by using the measured CL intensity. However, we have developed an indirect method that is not significantly affected by these parameters. By using intensity ratios instead of measured intensities, a level of quantification of Cr in natural and synthetic corundum is now possible. Our study shows that the intensity ratio between the N-line at 701.6 nm and the sideband maximum at 713.2 nm displays a nearly linear dependence on the Cr concentration from ca. 100-300 ppm up to ca. 1.1 mol% Cr 2 O 3 . These results give a possible approach to develop an optical-microscope-based CL spectroscopy for quantitative measurements of Cr in natural rubies and synthetic α-Al 2 O 3 materials.

Optical absorption spectra of flux-grown single crystals in the spinel s.s.-magnesiochromite and gahnite-zincochromite solid solutions were recorded with the aim of exploring variations in local Cr-O bond distance as a function of composition. With increasing Cr contents, the crystals vary in color from pale red to intensely red to dark greenish. These variations are reflected in the optical spectra by the position and intensity of the two spin-allowed electronic d-d transitions in six-coordinated Cr 3+ at ~18 000 (ν 1 ) and 25 000 cm -1 (ν 2 ). From the shift of the ν 1 band position, a decrease in crystal field splitting, 10Dq, for six-coordinated Cr 3+ with increasing Cr contents is evident in both solid-solution series. Based on published Cr-O bond distances for the CrO 6 polyhedra in magnesiochromite and zincochromite of 1.995 and 1.991 Å, respectively, and applying the ligand field relationships, local Cr-O bond distances in gahnite and spinel with Cr contents at trace levels are determined to be 1.974(2) and 1.960(3) Å, respectively. These local Cr-O distances result in relaxation parameters (ε) equal to 0.69(2) and 0.60(3) for Cr-O bonds in the Mg(Al 1-x Cr x ) 2 O 4 and Zn(Al 1-x Cr x ) 2 O 4 series, respectively. However, the presently obtained Racah B values indicate increasing Cr-O bond covalency with increasing Cr 3+ contents. This suggests that color changes and accompanying 10Dq variations may be due to variations in Cr-O bond covalency along the two solid-solution series, without or with very minor local Cr-O bond distance variation. Consequently, the ε values obtained from the present optical absorption spectra should be regarded as minimum values.

57 Fe Mössbauer annealing studies of fully metamict gadolinite from Ytterby (Sweden) have been carried out in air from 373 to 1473 K. The Mössbauer spectrum of the untreated sample, dated at 1795 Ma and absorbed α-dose 1.07 × 10 16 α-decay/mg, is characterized by two broad peaks solely from Fe 2+ components in octahedral coordinations. The corresponding quadrupole splitting distribution (QSD) shows two distinct maxima at 1.59 and 2.16 mm/s, which reflect the post-metamictization distributions of Fe 2+ positions up to heating at 773 K. Changes in the Mössbauer hyperfine parameters are observed only after heating to 873 K, at which point the first Fe 3+ component from the oxidation of Fe 2+ appears. The relative contribution of Fe 3+ /ΣFe reaches a maximum of 0.54 at 1073 K and then decreases with increasing temperature. A new Fe 3+ doublet is observed in the samples heated from 1173 to 1473 K. This doublet is characterized by extremely high quadrupole splitting ranging from 2.51 to 2.66 mm/s, implying extreme distortion of the coordination octahedra in partially and fully crystalline gadolinite. The transition from metamict to crystalline structure begins at 1133 K where the Mössbauer spectrum is represented by one Fe 2+ doublet and one Fe 3+ doublet. Both doublets suggest a uniform transitional octahedral site over the whole structure. With increasing annealing temperature, the line widths of the Fe 2+ doublet decrease, and, at high temperatures, an asymmetry of the absorption peaks is observed. The sample becomes completely recrystallized after annealing at 1373 K, and at this point the Mössbauer spectra are nearly exact superpositions of the pure Fe 2+ and Fe 3+ (present in minor proportion) in synthetic gadolinites.

This paper reports the occurrence and the crystal structure of fantappièite, a new member of the cancrinite-sodalite group of minerals from Torre Stracciacappe, Trevignano community (Rome, Latium, Italy). The mineral occurs within a volcanic ejectum consisting of dominant sanidine with minor plagioclase, biotite, augitic clinopyroxene, andradite, and iron oxides. Fantappièite (0.7 mm as largest size) is observed within miarolitic cavities of the rock as transparent colorless crystals, showing complex morphologies and striated faces. It is non-pleochroic and uniaxial negative, n ω = 1.5046(5) and n ε = 1.5027(5). D calc is 2.471 g/cm 3 . Fantappièite is trigonal, space group P3̄; the cell parameters are: a = 12.8742(6), c = 87.215(3) Å, V = 12518.8(9) Å 3 , Z = 1. The empirical chemical formula is: (Na 84.12 Ca 30.00 K 15.95 Fe 0.19 Ti 0.13 Mn 0.10 Mg 0.09 )(Si 99.36 Al 98.64 )O 396 (SO 4 ) 30.24 (CO 3 ) 0.29 Cl 0.84 F 0.82 ∙5.18H 2 O, which corresponds to the ideal formula [Na 82.5 Ca 33 K 16.5 ] Σ=132 (Si 99 Al 99 O 396 )(SO 4 ) 33 ·6H 2 O. The five strongest reflections in the X-ray powder pattern are [d in Å (I %) (hkl)]: 3.70 (100) (3 0 0), 3.60 (80) (1 0 23), 2.641 (65) (0 0 33), 6.85 (60) (0 1 10), 6.40 (55) (1 1 0). The single-crystal FTIR spectrum rules out OH groups and shows the presence of H 2 O and CO 2 molecules, as well. The structure can be described as a stacking sequence of 33 layers of six-membered rings of tetrahedra along the c axis. The stacking sequence is ACBACABACBACBACBCACBACBACBABCBACB…, where A, B, and C represent the positions of the rings within the layers. This sequence gives rise to liottite, sodalite, and cancrinite cages, alternating along c. Sulfate groups occur within the liottite cages associated by Na, K, and Ca, while highly disordered sulfate groups are located within the sodalite cages. H 2 O groups occur within the cancrinite cages, bonded to Ca and Na cations. Split positions are found for Na-Ca sites, and are related to disordering of the sulfate groups in the sodalite cages.

The crystal structure of a natural Na-rich gobbinsite from Bundoora, a northern suburb of Melbourne, Victoria, Australia [(Na 4.97 K 0.07 Ca 0.48 ) Σ5.52 (Al 5.62 Si 10.29 ) Σ15.91 O 32 ·11.91H 2 O, Z = 1, a = 10.1035(15), b = 9.7819(10), c = 10.1523(9) Å, and V = 1003.37(20) Å 3 , space group Pmnb (no. 62)], has been investigated by means of single-crystal X-ray diffraction, elemental CHN analysis, and electron microprobe analysis in the wavelength dispersive mode. For the first time, an anisotropic structural refinement has been performed on the basis of single-crystal X-ray diffraction data collected at 298 K, with: R 1 (F) = 0.0608, 1362 unique reflections with Fo > 4σ(Fo), and 114 refined parameters. The residuals in the final difference-Fourier maps are less than ±0.6 e-/Å 3 . Previous structure solutions based on powder diffraction data were performed in the space group Pmn2 1 , with a significantly different description of the channels’ content. In this study, the Si/Al distribution in the tetrahedral framework is found to be highly disordered. A complex configuration of the extra-framework population occurs, with two possible and mutually exclusive sites for cations (Na and Ca) and five independent sites partially occupied by water molecules (W1, W2, W2′, W3, and W3′). The coordination shell of the Na site is represented by a distorted polyhedron with coordination number CN = 6 (4 framework O atoms + 2 H 2 O molecules), with at least five possible mutually exclusive configurations, whereas the coordination shell of the Ca site is represented by a distorted polyhedron with CN = 7 (5 framework O atoms + 2 H 2 O molecules), with at least two possible mutually exclusive configurations.

An As-rich variety of fergusonite-beta-(Y) occurs as greenish yellow pseudo-bipyramidal crystals up to 1 mm in length in centimeter-sized secondary cavities within sub-horizontal pegmatite dikes at Mount Cervandone (Western Alps, Italy). The mineral is associated with quartz, biotite, potassium feldspar, and orange-yellow barrel-shaped hexagonal crystals of synchysite-(Ce) up to 2 mm in length. Fergusonite-beta-(Y) crystallized during the Alpine metamorphism under amphibolite-facies conditions, as a result of interaction between As-enriched hydrothermal fluids, circulating through the pegmatite dikes, and precursor accessory minerals in the pegmatites enriched in high-field-strength elements. These pegmatites are of NYF (niobium-yttrium-fluorine) geochemical type and served as the principal source of Be, Y, Nb, Ta, and rare-earth elements (REE) that were liberated and redeposited as rare Be-As-Y-REE minerals, including the As-rich fergusonite-beta-(Y). The latter mineral crystallizes with monoclinic symmetry [a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, V = 293.87(14) Å 3 , space group I2/a] and has the empirical formula (Y 0.70 Dy 0.07 Er 0.05 Ca 0.05 Gd 0.02 U 0.02 Yb 0.01 Tb 0.01 Th 0.01 Nd 0.01 ) Σ0.95 (Nb 0.68 As 5+ 0.27 W 0.06 Ta 0.01 Si 0.01 ) Σ1.03 O 4 . The crystal structure of fergusonite-beta-(Y) has been refined using a thermally untreated single crystal to R 1 = 6.6% for 441 observed reflections with F o /σF o > 4. The incorporation of As in the structure of monoclinic fergusonite-type phases is discussed in the context of the data available for synthetic analogs.

The crystal structure, crystal chemistry, and low-temperature structural evolution of natural thomsonite from Terzigno, Somma-Vesuvius volcanic complex, Naples Province, Italy, have been investigated by means of in situ single-crystal X-ray diffraction, electron microprobe analysis in the wavelength dispersive mode, and Raman spectroscopy. Six structure refinements have been obtained at different temperatures: 295.5, 248.0, 198.0, 148.0, 98.0, and 296.0 K (after the low-T experiments). The reflection conditions and the structure refinements prove that the crystal of thomsonite here investigated is orthorhombic with a = 13.0809(3), b = 13.0597(3), c = 6.6051(1) Å, V = 1128.37(14) Å 3 , and space group Pbmn, which differs from thomsonite from different localities reported in previous studies (with a ~ 13.1, b ~ 13.06, c ~ 13.2 Å, and space group Pncn). The refined bond distances suggest that the Si/Al-distribution in the tetrahedral framework is fully “disordered,” giving rise to the halving of the c axis relative to that found in “ordered” thomsonites. The extra-framework population consists of: (1) one site about 50% occupied by Ca (labeled as “Ca”); (2) one site occupied by Na (~70%) and Ca (~30%) (labeled as “Na”); and (3) three water molecule sites (“W1,” “W2,” “W3”). The structure refinements allowed the location of all the proton sites, and the hydrogen-bonding scheme in the structure is provided. The low-temperature refinements show no significant change in the structure within the T-range investigated. The evolution of the unit-cell volume with T exhibits a continuous and linear trend, without any evident thermo-elastic anomaly, with thermal expansion coefficients α V = V -1 ⋅∂V/∂T= 20(2)·10 -6 K -1 (between 98.0 and 295.5 K). A list with the principal Raman active modes is provided and a comparison with the vibrational modes previously found for “ordered” thomsonite is carried out.

Microlites in pumice fragments can record the rate of magma decompression and ascent, but only if none grow while those fragments cool in the atmosphere. For highly viscous silicate melts, such crystallization is unlikely, but more basic melts are known to crystallize rapidly, and thus could partially crystallize during cooling and overprint decompression textures. To examine whether postfragmentation crystallization can occur, we examined two basaltic pumice clasts from the sub-Plinian April 1999 eruption of Shishaldin volcano, Alaska. Radial sectioning shows that microlite content doubles from rim to core in both, mainly from growth of plagioclase. Dendrite magnetite also increases greatly in content, but only within the larger pumice clast. Such radial textures demonstrate that crystallization occurred after fragmentation and before deposition (no welding occurred). Using a conductive cooling model coupled with a model for temperature in the eruption column, we estimate that rims of the pumice clasts cool to their glass-transition temperature in ~100-200 s, but their cores take ~500-2000 s to cool, which translates into cooling rates of ~0.2 to 2.5 °C/s. Using a conservative assumption that all plagioclase nucleated before cooling began, we estimate that both short and long axes grew at ~4.8 (±2.7) × 10 -7 cm/s. Such rates match those determined experimentally for basaltic melts at similar cooling rates. Magnetite grew only at the slowest cooling rates, and the rate of bulk magnetite crystallization equals that of plagioclase. Our results demonstrate that groundmass crystallization can occur in basic melts on the timescale of explosive eruptions, and so pumice clasts from such eruptions must be viewed with caution before being used to infer eruption dynamics.

Fe-rich “oxydravite” and dravite from the Late Proterozoic ophiolitic mélange of the Arabo-Nubian Shield, located in Egypt and Saudi Arabia, were structurally and chemically characterized by using crystal structure refinement based on single-crystal X-ray diffraction data, electron microprobe analysis, and Mössbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and Fe 2+ over the Y and Z sites, and an ordering of Fe 3+ at Y. The disordering can be explained by the substitution mechanisms 2 Y Mg+ Z Al+ W OH = 2 Y Al+ Z Mg+ W O 2 - and 2 Y Fe 2+ + Z Fe 3+ + W OH = 2 Y Fe 3+ + Z Fe 2+ +WO 2- , which are consistent with reducing the mismatch in dimensions between YO 6 and ZO 6 octahedra. To explain the Mg-Al disordering process, as well as the occurrence of B at the T site in tourmaline, analogies have been drawn between the crystal structure of tourmaline and that of lizardite. A critical constraint in both structures is the geometrical fit of the six-membered tetrahedral ring with the attached group of three YO 6 octahedra. In tourmaline, the disordering of Mg and Al over Y and Z relieves the strain due to the misfit in dimensions of the larger triads of edge-sharing MgO 6 octahedra and the smaller Si 6 O 18 tetrahedral rings. In Al-rich tourmaline, where the octahedral cluster is smaller, the strain can be relieved by incorporating B in the tetrahedra. An opposite effect is observed by substitution of Al for Si at the tetrahedral site in Mg-rich tourmaline. Because the Al radius is intermediate between those of Mg and Si, Al plays an important structural role in accommodating the potential misfit between YO 6 , ZO 6 , and TO 4 polyhedra. The amount of Al and its distribution in the structure strongly affects the values of the unit-cell parameters of tourmaline and yields volume variations according to a quadratic model. This results from the effect of Z Al combined with the occurrence of B at T in Al-rich tourmaline. Z Al has a greater effect than Y Al as long as Al does not fully occupy the Z site.

Structural analysis, using neutron powder diffraction (NPD) data on small quantities (<300 mg) and in combination with single-crystal X-ray diffraction (SXD) data, has been employed to determine accurately the position of hydrogen and other light atoms in three rare beryllate minerals, namely bavenite, leifite/IMA 2007-017, and nabesite. For bavenite, leifite/IMA 2007-017, and nabesite, significant differences in the distribution of H, as compared to the literature using SXD analysis alone, have been found. The benefits of NPD data, even with small quantities of H-containing materials, and, more generally, in applying a combined SXD-NPD method to structure analysis of minerals are discussed, with reference to the quality of the crystallographic information obtained.

Olivine is the dominant component in coherent kimberlite rocks and related pyroclastic rocks. Quantitative characterization of olivine crystals in kimberlite rocks may be used to better understand kimberlite emplacement and eruption. Here, we construct the first complete olivine crystal size distribution (CSD) for magmatic or coherent kimberlite using two-dimensional image analysis techniques with a new method for scale-integration. Crystal size and frequency data are collected from polished slabs and thin sections, normalized to the largest scale of observation, and stereologically corrected to create a complete CSD covering the full range of observed olivine crystal sizes. Resulting CSDs show the following properties for olivine in crystal-rich interiors of intrusive coherent kimberlite in the A154N kimberlite pipe, Diavik, Northwest Territories: (1) 0.03-10.0 mm in diameter size range; (2) 46.7-51.2% modal crystal content; (3) modal% dominance by crystals >1 mm; (4) frequency dominance by crystals <1 mm; and (5) equant-axial crystal shapes that cannot be easily described by a single external morphology for any size fraction. Measured size ranges and modal crystal content are consistent with previous reported data. Relative frequency and area ratios between crystals of different sizes and shape properties differ from previous assessments. Olivine CSDs are consistent with a power-law distribution over at least two orders of magnitude, and statistical parameters (power-law exponents, Inman parameters) suggest that apparent characteristic ranges are independent of crystal frequencies, modal abundance (area%), or sampling locale. These properties suggest olivine crystals may be a single population with a shared paragenesis, and not the end-product of a combination of two populations of olivine. On this basis, olivine CSDs and population parameters have the following uses: (1) to compare and discriminate among kimberlite facies and deposit types, and (2) to test models for olivine paragenesis.

Solubility experiments of MnNb 2 O 6 and MnTa 2 O 6 were conducted in two nominally dry to watersaturated pegmatitic melts with different amounts of Li, F, P, and B at 700 to 1000 °C and 200 MPa to determine the maximum concentrations of Nb and Ta in pegmatitic melts. The Li 2 O, F, B 2 O 3 , and P 2 O 5 contents in the melts were 1.16, 2.99, 1.78, and 1.55 wt% for melt composition PEG1 and 1.68, 5.46, 2.75, and 2.75 wt% for melt composition PEG2 and the resulting Al/(Na+K+Li) ratio for both melts is 0.92. The experimental data show that the solubility product of manganocolumbite increases by a factor of three upon increasing the water concentration from 0 to 4 wt%. Considering that pegmatitic melts at pressures above 50 to 100 MPa are hydrous (>4 wt% H 2 O), the increase in solubility by this magnitude, over the stated range of water concentration, is not significant for pegmatites. The data also point out that the solubility of MnNb 2 O 6 and MnTa 2 O 6 is strongly temperature dependent, increasing by a factor of 50 for manganocolumbite and 15-20 for manganotantalite from 700 to 1000 °C under water-saturated conditions. The solubility also increases with increasing content of fluxing elements like Li, F, B, and P. In the pegmatite melt containing the highest amount of fluxing elements, the maximum concentrations of Ta and Nb are higher by nearly one order of magnitude when compared to a subaluminous rhyolitic melt.

We conducted a Mössbauer study of synthetic glasses with anorthite-diopside eutectic composition modified with either 5 or 1 wt% Fe 2 O 3 and variable amounts of silica. Glasses were synthesized in air in the temperature range 1400-1570 °C. Room-temperature Mössbauer spectra showed the presence of magnetically ordered Fe 3+ , for which a systematic fitting procedure was developed to correctly estimate Fe 3+ /Fe 2+ . At constant T-f O₂ we observed a minimum in the variation of Fe 3+ /Fe 2+ ratio with silica that varied from 56 to 63 wt% SiO 2 , depending on temperature. It was also demonstrated that the temperature dependence of the ferric/ferrous ratio [log(Fe 3+ /Fe 2+ ) vs. 1/T] is essentially similar for basic and silicic melts.

We characterized manganese dendrites on three different substrate rocks (rhyolite, clayey siltstone, and limestone) using high-resolution transmission electron microscopy. Manganese dendrites are mainly composed of nanometer scale Mn-(hydr)oxides, Fe-(oxy)hydroxide, sulfate, and clay minerals including hollandite, todorokite, birnessite, goethite, amorphous Fe-hydroxide, smectite, illite, and gypsum. Each sample has a different main manganese phase. Two dendrite samples have more than one manganese phase. Todorokite crystals on a limestone sample from Altmühl Valley, Germany, showed trilling intergrowths. Chain-width disorder and chain termination occur in todorokite. The sum of the numbers of octahedral wall layers and octahedral chains were equal at both sides beyond the termination. The chain termination rule was explained by the geometry of octahedral chains and the octahedral wall layer. The trilling intergrowths of todorokite might be formed by transforming the structure from birnessite to todorokite, or by multiple, epitaxial todorokite nucleations on octahedral wall layer of todorokite or birnessite. Based on chain-width disorder in the todorokite, we suggest that the todorokite might have been transformed from birnessite.

Using the hybrid B3LYP density functional method, we computed the ab-initio thermo-chemical and -physical properties of the mineral anhydrous B (Anh-B), which has been recently suggested as a potential phase responsible for the X-discontinuity in the Earth’s mantle at ~300 km depth through the reaction forsterite + periclase = Anh-B, and also to likely split the 410 km discontinuity within the interior of a cold slab through the reaction wadsleyite/ringwoodite = Anh-B + stishovite. We first conducted an investigation of the static properties through a symmetry-preserving relaxation procedure and then computed, on the equilibrium structure, harmonic vibrational modes at the longwavelength limit corresponding to the center of the Brillouin zone (k → 0). While optic modes are the eigenvectors of the Hessian matrix at Γ point, acoustic modes were obtained by solving the non-zero components of the strain matrix. Following the Kieffer model, acoustic branches were assumed to follow sine wave dispersion when traveling within the Brillouin zone. All thermodynamic properties that depend on vibrational frequencies namely, heat capacities, thermal expansion, thermal derivative of the bulk modulus, thermal correction to internal energy, enthalpy, Gibbs free energy, thermal pressure and Debye temperature, were computed on the basis of quasi-harmonic mode-gamma analysis of the volume effects on vibrational frequencies. Moreover, the strain tensor was used to calculate several thermo-physical properties of geophysical interest (transverse and longitudinal wave velocities, shear modulus, Young’s modulus, and Poisson’s ratio). The ab-initio results derived in this study and the available data on molar volumes were used to calculate the univariant equilibrium forsterite + periclase = Anh-B. The results are in satisfactory agreement with the reversed experimental data of Ganguly and Frost (2006).

The incommensurately modulated structure of the mineral natrite has been refined for the first time. Two single-crystal grains, Lv and Kh, from two different occurrences [Mt. Karnasurt, Lovozero massif (Lv), Kola peninsula, Russia, and the pegmatite of Mt. Koashva, Khibiny massif (Kh)], have been investigated at 293 and 120 K using synchrotron X-ray data. The average structures of both minerals are identical and the basic features of the structural modulations are similar to the synthetic γ-Na 2 CO 3 phase previously published. The γ (incommensurate) → δ (lock-in) phase transition reported at low temperature for the synthetic compound was not observed down to 120 K in natural natrite. Crystalchemical aspects, especially about the second coordination sphere for the carbon atoms, are examined to explain the different structural behaviors observed at low temperature. The possible role played by the minor isomorphous substitutions in natural natrite specimens is also discussed.

Oxidative dissolution of arsenopyrite and pyrite from two former mining sites located in the French Massif Central has been studied to determine some of the critical parameters controlling the formation of secondary phases. Micro-Raman spectroscopy (μRS) and micro-scanning X-ray diffraction (μSXRD) were used for mineralogical identification and for mapping the distribution of these alteration products. The two mining sites are characterized by different pH conditions and sedimentary environments. Enguialès exhibits acidic conditions (pH ~ 3) and consists of coarse-grained tailings on a steep slope, which have been leached by meteoric waters and represent an oxidizing environment with a high solid/solution ratio. In contrast, the Cheni site shows near neutral conditions (pH ~ 6.3) and consists of muddy tailings deposited in a settling basin, which exemplify an oxidizing environment with a low solid/solution ratio. Amorphous or poorly crystalline iron arsenate (e.g., amorphous scorodite or parasymplesite-like phase) are among the first products precipitated from oxidized arsenopyrites. These iron arsenates are highly mobile and are frequently observed in association with non-arsenian pyrites, where in some instances they have matured into more crystalline forms. Arsenic may also be trapped by amorphous or poorly crystalline iron (oxy)hydroxides, as has been observed for goethite or jarosite in the latter stages of sulfide oxidation. This study also shows that rings of elemental sulfur are formed around altered sulfides, but only when conditions are near neutral.

Numerous petrologic studies have attempted to determine crystal growth rates in volcanic systems through several methods, including analyses of crystal size distributions in natural samples and decompression experiments on hydrous magmas. Experiments have revealed that rim growth on existing crystals will occur under a wide range of decompression conditions and is favored under conditions of low to moderate undercoolings over microlite nucleation, which has typically been the focus of decompression-induced crystallization studies. For this study, samples of eruptive clasts were collected from a Vulcanian explosion that occurred following the July 12-13, 2003 dome collapse of Soufrière Hills volcano, Montserrat. Plagioclase phenocrysts were extracted and examined with secondary ion mass spectrometry (SIMS) depth profiling. Lithium inflection depths within the profiles, along with the observed time interval between the peak in the collapse decompression and the explosion that ejected the examined samples, were used to calculate the growth rates as a result of magma devolatilization, with an average of 8.3 × 10 -8 mm/s. Anorthite content of the plagioclase rim growth indicates an average decompression magnitude of 40 MPa, inducing an undercooling of ~45 °C that favors crystal growth over microlite nucleation. However, variability in the final anorthite contents suggests that not all phenocrysts recorded an equilibrium composition reflecting accurate pressure conditions. In such events occurring over short timescales (<10 h), lithium is a more reliable indicator of decompressioninduced growth than changing anorthite content due to lithium’s rapid diffusivity.

Single crystals of spinel and hausmannite having seven different compositions in the MgAl 2 O 4 - MgMn 2 O 4 -MnMn 2 O 4 system were synthesized and structurally and chemically characterized by X-ray diffraction and electron microprobe techniques. As predicted, tetrahedral and octahedral bond lengths increase with increasing substitutions of Mn 2+ for Mg and Mn 3+ for Al, respectively. A transition from cubic to tetragonal symmetry occurs at a critical concentration of Mn 3+ > 1.4 atoms per formula unit as a result of the Jahn-Teller distortion around octahedrally coordinated Mn 3+ . The present data in conjunction with data from the literature provide a basis for quantitative analyses of the cation polyhedral-distortion parameters and their variations in spinel- and hausmannite-type structures (Fd3̄m and I4 1 /amd, respectively). In contrast to the linear correlation between <λ M > (octahedral quadratic elongation) and σ 2 M (octahedral bond-angle variance) observed for many silicates and isomorphic structures, these two distortion parameters are not correlated in multiple oxides with spinel- and hausmannite-type structures. By using a model of multiple linear regression, it is demonstrated that <λ M > varies as a function of both σ 2 M and Δ M (octahedral bond-length distortion). The degree of octahedral distortion is significant in the spinel structures and is in fact comparable with that calculated for the hausmannite-type structures. The degree of octahedral distortion is related to steric effects in both spinel- and hausmannite-type structures, whereas the electronic effects caused by Mn 3+ account for the transition from cubic to tetragonal symmetry.

The equation of state of four clinopyroxenes was measured at room temperature by single-crystal X-ray diffraction in a diamond anvil cell up to 10 GPa. The bulk modulus of pure diopside is K 0 = 106(1) GPa, K′ = 6.1(5). The bulk modulus of a pure diopside containing 600 ppm by weight of water is K 0 = 108(1) GPa, K′ = 6.3(4). The effect of water on the elastic properties of pure diopside is therefore small. In addition, we studied two hydrous samples containing 9.6 wt% Al 2 O 3 that were synthesized on the join between diopside and Ca-Tschermak component (CaAlAlSiO 6 ). The sample containing 976 ppm of water yielded K 0 = 120(1) GPa and K′ = 4.4(4), while the sample containing 2510 ppm of water yielded K0 = 114(1) GPa and K′ = 5.8(5). Aluminum therefore clearly makes the clinopyroxene stiffer. A direct comparison of the two aluminous samples, however, is difficult, since the sample containing the lower amount of water appears also to contain some M2 vacancies, indicating some solid solution with Ca-Eskola component. This is consistent with a different substitution mechanism for water in the two samples, as indicated by differences in the infrared spectra. Our results suggest that detecting upper mantle water contents from variations in P wave velocities is probably impossible. Increasing water fugacity will increase the Al content in pyroxenes and hence their stiffness. The resulting increase in bulk modulus is opposite to the effect of water on olivine. The different response of olivine and pyroxenes to water is due to a different dissolution mechanism. Under conditions of low silica activity, water in clinopyroxenes is mostly dissolved by the coupled substitution of Al 3+ + H + for Si 4+ , which does not create any vacancies. This dissolution mechanism is consistent with polarized infrared data (one single band at 3650 cm -1 ), with an excess of tetrahedral Al in the microprobe analyses of hydrous samples and with the results of structure refinements. Infrared bands at 3450 and 3500 cm -1 , which are seen in the Al-rich diopside with lower water content of this study, probably indicate Al on M1 charge compensated by a M2 vacancy and a proton.

The crystal chemistry of 13 spinel samples from the Somma-Vesuvius volcanic complex (Italy) was studied by crystal structural refinement and electron microprobe analysis, processing the data with a tested optimization model to obtain site populations. Five samples come from the volcanic xenoliths found in Pollena Quarry and belong to the deposits produced by the Pollena eruption of A.D. 472. All crystals are characterized by Fe 2+ ↔ Mg substitution and pertain to the Mg-rich portion of the spinel sensu stricto-hercynite join. The crystals show a partially inverted cation configuration with inversion degree ranging from 0.20-0.25. Aluminum contents characterize the crystal-chemistry: the unit-cell parameter decreases with increasing total Al and the M-O bond length decreases with increasing M Al. Using a well-tested geothermometer based on the M Al+ T Mg = T Al+ M Mg intracrystalline exchange, closure temperatures were calculated for the Pollena Quarry spinels. Results range from 694 to 809 °C. The latter value is assumed to be representative of the temperature of the Pollena eruption and is in excellent agreement with that reported in the literature (800 °C) and estimated by experimental phase equilibria.

This paper reports a complete set of TG, DTG, and DTA data, coupled with emitted gas analysis, for well-constrained, almost pure serpentine samples. Serpentine dehydroxylation takes place between 550 and 800 °C, with DTG and DTA peak temperatures progressively decreasing from antigorite (720 and 715 °C, respectively) to lizardite (708 and 714 °C), polygonal serpentine (685 and 691 °C), and chrysotile (650 and 654 °C). Antigorite has an additional diagnostic signal at ~740-760 °C, always absent in the other serpentines, and dependent on antigorite superperiodicity (T shift of ~20 °C from 36 to 49 Å modulation wavelength). A sharp exothermic peak occurs at extremely constant temperatures (~820 °C), independently from the starting serpentine structure. The high-T mineral assemblage is always represented by forsterite and enstatite. Based on the observed relationships between serpentine structures and DTG/DTA dehydroxylation temperatures, thermal analysis may represent a useful tool for serpentine identification, particularly in the case of natural massive samples where different varieties are mutually intermixed. The accurate definition of serpentine mineralogy would have obvious implications in both geological-petrological and health-related issues.

Anthoinite, AlWO 3 (OH) 3 , from the Mt. Misobo Mine, Democratic Republic of the Congo, has triclinic symmetry with cell parameters a = 8.196(1) Å, b = 9.187(1) Å, c = 11.316(1) Å, α = 92.82(1)°, β = 94.08(1)°, γ = 90.23(1)°, space group I1̄, Z = 8. The structure was solved by applying ab initio structure solution methods (Reverse Monte Carlo/Simulated Annealing) to both X-ray and neutron powder diffraction data and was refined using the Rietveld method. The structure is built up of two types of M 4 (O,OH) 16 planar tetrameric clusters of edge-sharing octahedra, one containing predominantly Al and the other predominantly W. The Al-rich and W-rich clusters interconnect via corner sharing to form stepped layers parallel to (001). The layers are held together by strong hydrogen bonding. The structure can be described as a rocksalt derivative structure, with the close-packed anion layers parallel to (012), and with Al and W atoms ordered into one third of the octahedral sites within the cubic close-packed anion lattice. The structure is complicated by partial disorder between Al and W in the tetrameric clusters and associated disorder in the H atom sites. Infrared and 27 Al MAS NMR results are also presented for anthoinite.

Townendite, a new U-bearing mineral belonging to the trigonal lovozerite group has been found in the lujavrite layers of nepheline syenites in the Ilímaussaq alkaline complex in Southern Greenland. Its composition from electron microprobe analysis is Na 6.08 Ca 0.16 Mn 0.17 Fe 0.13 Zr 0.67 Y 0.13 Sn 0.04 U 0.02 Ti 0.01 Si 6 O 16.35 (OH) 1.65 . Townendite has rhombohedral symmetry, R3m, with cell parameters (hexagonal setting) a = 10.345(2) Å, c = 13.103(2) Å. The structure has been refined using single-crystal X-ray data to R 1 = 0.056 for 532 reflections with F > 4σ(F). Lovozerite group minerals are cyclosilicates in which Si 6 (O,OH) 18 rings of tetrahedra share corners with isolated MO 6 octahedra (M = Zr, Ti, Ca) to form a 3D framework, with large cations, mainly Na + , located in cavities in the framework. Townendite differs from other compositionally related lovozerite group minerals in having a high occupancy (92%) of another set of octahedrally coordinated sites that share faces with the MO 6 octahedra, giving linear face-shared trimers parallel to [001]. These sites are occupied predominantly by Na + . Full occupancy of all sites corresponds to the ideal composition Na 8 ZrSi 6 O 18 . Townendite is a significant contributor, along with steenstrupine-(Ce), to U resources in the Ilímaussaq alkaline complex.

The compressional behavior of kaolinite (space group C1) from Keokuk, Iowa, has been studied to 7.8 GPa under static compression by synchrotron X-ray powder diffraction. Two phase transformations occur at ~3.7 (kaolinite-I/II) and ~7 GPa (kaolinite-II/III) that are registered by major changes in diffraction patterns. The layer shift involved in the I/II transformation preserves hydrogen-bonded contacts across the interlayer. This transformation removes cross-interlayer Al-Si superpositions and is analogous to the low-/ high-dickite transformation, which occurs at ~2.6 GPa (Dera at al. 2003). The 7 GPa transformation from kaolinite-II to “kaolinite-III” involves considerable contraction of the a-b plane (2.5%), marked shortening normal to the polyhedral layers, and a volume contraction of about 3%. Comparison of experimental patterns of kaolinite-III with those calculated for non-standard polytypes predicted by ab initio methods to 60 GPa by Mercier and Le Page (2009) allows the identification of kaolinite-III with their high-pressure, one-layer Model 18 polytype. Kaolinite-III can be quenched to room pressure, although significant transformation to kaolinite-I was observed at 0.5 GPa on decompression. At ambient conditions kaolinite-III is 6% more dense than kaolinite-I, nacrite, and low-dickite. Elastic moduli for kaolinite-I and kaolinite-III have been obtained that indicate that kaolinite-III is more compressible than kaolinite-I, due primarily to in-plane (a-b) softening. Both high-pressure polytypes encountered experimentally were predicted ab initio to be strong candidates for high-pressure structures by Mercier and Le Page (2008, 2009). This agreement between experimental and ab initio studies is encouraging for the application of ab initio methods to complex polytypic systems.

The crystal structure of gypsum-II, a polymorph of CaSO 4 ·2H 2 O stable above 4 GPa, has been solved using single-crystal synchrotron X-ray diffraction data collected at 5.35 and 6.74 GPa. Gypsum- II is monoclinic, space group P21/n, with lattice parameters a = 5.865(12), b = 15.045(14), c = 5.478(12) Å, β = 115.3(2)°, and V = 437.0(14) Å 3 at 5.35 GPa, and a = 5.776(2), b = 15.017(2), c = 5.473(2) Å, β = 114.98(4)°, and V = 430.3(2) Å 3 at 6.74 GPa. The crystal structure has been refined to R 1 = 3.7 (5.35 GPa) and 3.9% (6.74 GPa). It closely resembles that of gypsum at room pressure with a stacking of CaO 8 and SO 4 polyhedra along the b-axis to form layers. With increasing pressure, a continuous increase in distortion of the SO 4 tetrahedron and a strong change in the bonding style of the water molecules are observed. The mechanism of phase transformations previously hypothesized in gypsum, on the basis of high-pressure spectroscopic data, is here clarified for the polymorph stable between 4-8 GPa.