Volume 92, Issue 4

Detailed electron- and ion-microprobe analysis of diopsides extracted from a block of marble collected in the Mt. Marcy anorthosite massif (Adirondack Highlands, New York) shows that single crystals have preserved Al-Si zoning features from early contact metamorphism later modified by regional metamorphism. Diopsides show Al and Si complementary zoning from core to rim; average Al concentration in rims is constant (0.11 pfu), but values in the cores vary with crystals from 0.06 to 0.14 pfu. A δ 18 O profile measured with the ion microprobe shows no zoning. Modeling of the Al-diffusion profiles affected by regional metamorphism of the Grenville orogeny shows that original Al zoning was sharp and corresponds to a rapid change of crystal-growth conditions during early contact metamorphism: (1) diopside cores crystallized with variable Al contents due to restricted fluid circulation or differing sedimentary composition, (2) then pervasive fluid infiltration crystallized the homogeneous rim of diopside, and (3) core to rim Al zoning was later smoothed by granulite-facies metamorphism. Homogeneity of δ 18 O in core and mantle (near rim) of diopsides suggests that the crystals were isotopically homogeneous prior to and during regional metamorphism. Values of Al-Si diffusion coefficient deduced from modeling are in agreement with low-temperature extrapolation of experimental data.

The Hammadah al Hamra 193 winonaite was found in the Libyan desert in 1996. Unlike most winonaites with fine- to medium-grained equigranular textures, it consists predominantly of very large (up to 5 mm), optically continuous orthopyroxene grains enclosing smaller grains of olivine and plagioclase. It also contains large (up to 2 mm) poikilitic grains of amphibole enclosing clinopyroxene, plagioclase, olivine, and occasionally orthopyroxene, which occur interstitial to the large orthopyroxene grains. The amphibole is identified as fluoro-edenite, and textures indicate that it replaces clinopyroxene via a reaction in which diopside, olivine, and plagioclase form fluoro-edenite. Trace-element data are consistent with the formation of fluoro-edenite from clinopyroxene and plagioclase. Fluoro-edenite has a REE pattern similar to that of clinopyroxene, but has elevated abundances of Na, K, and Ba, elements typically enriched in plagioclase. The source of the F is uncertain, but may be apatite, which is fluor-apatite in this meteorite. The presence of fluoro-edenite in HaH 193, a meteorite that experienced extensive thermal metamorphism, indicates a significant stability field for this rare mineral.

This work provides a crystal-chemical description of trioctahedral micas from volcanic rocks (lavas, tuffs, ignimbrites, and xenoliths) outcropping at Mt. Sassetto (Tolfa district, Tuscan Province, central Italy). Mica crystals vary in composition from ferroan phlogopite to magnesian annite. Heterovalent octahedral substitutions are mainly related to Al 3+ , Ti 4+ , and, only in a few samples, to Fe 3+ . The two main mechanisms regulating Ti inlet into the mica structure are the Ti-oxy [ VI Ti 4+VI (Mg,Fe) 2+ -1 (OH) - -2 O 2- 2 ] and Ti-vacancy [ VI Ti 4+VI □ VI (Mg,Fe) 2+ -2 ] substitutions. In these micas, Ti content is the predominant crystal-chemical parameter and significantly affects octahedral and interlayer topology as well. Micas with the highest Ti contents deviate from the expected fractional crystallization trend in the Ti vs. Mg/(Mg + Fe tot ) diagram, possibly as a consequence of a variation in intensive parameters (T, P, f H₂ , f O₂ , f H₂O ) during crystallization in the magmatic chamber. In micas with significant Fe 3+ contents, the layer charge balance is accomplished by the following mechanisms: VI Fe 2+ -3 VI Fe 2 3+VI □, VI Fe 2+ -1 VI Fe 3+ (OH) - -1 O 2- , and VI Fe 2+ -1 VI Fe 3+IV Si -1 4+IV Al 3+ . These micas show mean electron counts greater for the octahedral M2 site than for M1, and unit-cell parameters signifcantly smaller than usual for other micas. Atmospheric weathering is unlikely to produce the observed Fe oxidation. The Fe 3+ -vacancy and Fe-oxy substitutions could represent secondary, re-equilibration effects related to post-eruptive water-rock hydrothermal processes (acid-sulfate alteration).

We have used inelastic neutron-scattering measurements to study the magnetic excitations in the antiferromagnetic and paramagnetic phases of polycrystalline fayalite, Fe 2 SiO 4- Sharp, nondispersing excitations are found in the ordered state, at 3.3, 5.4, 5.9, and 11.4 meV, and are interpreted as arising from the spin-orbit manifold of the Fe 2+ ions. These excitations are increasingly damped with increasing temperature, merging into a quasielastic continuum near the 65 K Neel temperature, although their energy does not vary with temperature. We have calculated the contribution of the heat capacity arising from these magnetic excitations and found that it compares favorably with the magnetic heat capacity deduced experimentally. Our analysis indicates that the M1 and M2 sites behave distinctly. The M1 site behaves quasi-locally and appears in the heat capacity as a Schottky anomaly that explains the shoulder in the heat capacity curve near 20 K, while the M2 site contributes predominantly to the critical lambda anomaly. The behavior of fayalite illuminates the nature of magnetic states in several related minerals, including others that also show shoulders and lambda anomalies in the heat capacity (tephroite), those that show only lambda anomalies (cobalt olivine and liebenbergite), and those that show only non-lambda anomalies (bronzite, anthophyllite, and almandine). We find no evidence to support the recent claim that some transition metal silicate and germanate olivines exhibit strong geometric frustration.

Garnet is notably absent in mafic to felsic lithologies, in low-P metamorphic terranes, and in Mg-rich bulk compositions in high-P regionally metamorphosed terranes. In the absence of garnet, existing mineral barometers cannot be applied for estimating metamorphic pressure. A new garnet-free, hornblende- plagioclase barometer has been formulated based on the reaction tremolite + tschermakite + 2 albite = 2 pargasite + 8 quartz. The intra- and cross-site mixing parameters for hornblende were retrieved exclusively from experimental data in the P-T window 1-15 kbar and 650-950 °C, after screening the available data for the presence of quartz. A linear least-squares fit to the coexisting hornblende and plagioclase composition in the experiments yields the following barometric expressions: P1(kbar) = [-9.326 + 0.01462T (K) + RTlnK ideal - 98.698X A Na -33.213X A K -20.338X M4 Na -39.101X M13 Fe ²⁺+ 100.392X M2 Al + 131.03X M2 Fe³⁺ + 82.479X M2 Fe³⁺ -118.653X T1 Al -2RTlnγ Ab ]/(-ΔV) and P2(kbar) = [-1.869 + 0.0076T (K) + RTlnK ideal -102.692X A Na- 35.251X A K -15.969X M4 Na - 40.499X M13 Fe²⁺ + 93.069XM 2 Al + 130.750X M2 Fe²⁺ + 74.226XM 2 Fe³⁺ -104.402X T1 Al -2RTlnγ Ab ]/(-ΔV) Where denotes the mole fraction of cation i in site j of amphibole, X Ab and γ Ab are the mole fraction and activity coefficient of albite in plagioclase, respectively, and ΔV is the volume change for the reaction. In the expression for P 2 , the first two terms in the numerator on the right-hand side corresponding to ΔH° and ΔS°, respectively, were adopted from an internally consistent thermodynamic database. In the expression for P 1 , the first term on the right-hand side of the numerator, ΔĤ (.9.326 kJ) = ΔH° + a 9 , where a 9 is a mixing parameter, and the second term ΔS° = 0.01462 kJ/K, were retrieved by a linear least-squares fit to the experimental data. The formulations reproduced experimental P values within 2 kbar for 83% of the experimental runs. The cumulative precision (1σ) of pressure estimated using the formulations varied between 800 and 2000 bars for errors propagated due to uncertainties in P-T, volume, and thermometric estimate, and inaccuracies in estimated composition of phases. In natural assemblages spanning a pressure range of 2.5 to 12.5 kbar, the quartz-hornblende-plagioclase barometers yielded pressure within 2 kbar of the P-value recommended by the authors, and is consistent with the stable Al 2 SiO 5 polymorph in associated metapelites.

In-situ measurements of anorthite dissolution in Na-Cl-OH solutions at an ionic strength (IS) of 0.5 mol/L (M) and in artificial seawater (IS = 0.7 M) were conducted at 22 °C using white-light, phase-shift interference microscopy (PSI-M). Nanometer-scale surface topography by PSI-M revealed three-dimensionally inhomogeneous surface dissolution, which is commonly observed as retreating steps on anorthite surfaces. Continuous dissolution of the anorthite cleavage surface (010) was successfully measured within a day. The vertical dissolution velocity was 4.3 × 10 -5 to 1.4 × 10 -3 nm/s. The obtained dissolution rates showed a typical dependency on pH with a reaction order of 0.191, and could be consistently extended to the previous data obtained under acidic conditions (Luttge et al. 1999). In-homogeneities in the vertical dissolution velocities at each pH condition could be interpreted by the step dynamics explained by the Burton-Cablera-Frank (BCF) theory (Burton et al. 1951). These results emphasize that the velocity of step retreat is a strong function of the step density, which has to be taken into account when describing the global dissolution phenomena on mineral surfaces

The kinetics of the breakdown reaction dolomite = periclase + calcite + CO 2 were investigated using cores of dolomitic marble. Two samples of Reed Dolomite from southwestern Nevada were cut into cylinders approximately 4 × 6 mm in size. The cores were sealed in gold capsules with isotopically enriched water (H 2 18 O or HD 18 O 0.5 16 O 0.5 ). The samples were heated in a cold-seal hydrothermal apparatus to 650-750 °C at 100 MPa for durations ranging from 2.59 days. The cores were then sectioned and examined by EPMA, XRD, and SIMS techniques. All experiments showed some amount of reaction regardless of duration or temperature. Reaction products occurred mainly along grain boundaries, fractures within grains, and along sample edges. Ion images and isotope-ratio analysis indicated that reaction products exchanged with infiltrating fluids. Reaction rates were calculated from measured extents of reaction, which were determined from automated EPMA modes. At 700 °C, we measured reaction rates ranging from 3.8 × 10 -14 to 2.3 × 10 -12 mol/mm 2 ·s. The extent of reaction is proportional to the square root of time, suggesting a diffusion-controlled process. A shrinking-core model for the dolomite breakdown reaction fits the grain-size data, suggesting that diffusion of H₂O and CO₂ through the mantle of reaction products controlled the rate. Apparent activation energies for that diffusion are ~283 ± 32 kJ/mol for coarse-grained samples and ~333 ± 36 kJ/mol for fine-grained samples. Initial reaction occurred relatively fast near the surface of dolomite grains, but continued diffusion through the reaction products ultimately controlled the rate of dolomite breakdown.

The chemical evolution of the galena (001) cleavage surface dissolving in oxygen-saturated solutions was investigated by fluid-cell micro-Raman Spectroscopy (μRS) and solution chemistry. In this novel design of μRS apparatus, the solution in the fluid cell is continuously renewed. A fairly thick (several tens to hundreds of nanometers) layer forms at the galena surface in solutions with pH between 1 and 5.8. This surface layer is composed of Pb oxides, sulfates, and metastable species of sulfur. Native sulfur forms at pH 1 and 4.6, but is not a predominant surface species at pH 5.8. Dissolution rates, measured by solution chemistry, decrease with pH and reaction time. The formation of Pb oxides in these experiments at such low pH values contrasts with thermodynamic predictions based on properties at the macroscale (bulk solution). The in situ assessment of surface speciation confirms that sulfur can partially oxidize at the interface, and indicates that this process of sulfur oxidation depends on pH. We propose that sulfur oxidation may take place, at least partially, during the reaction of dissolved molecular oxygen with S atoms at the galena surface, or in the immediate vicinity. After this first step of reaction, oxygen combines with Pb ions to form Pb oxide at the interface.

The effects of pressure on the OH stretching frequencies of natural talc and two samples of synthetic 10-Å phase have been measured using a diamond-anvil cell and a synchrotron infrared source. The 10-Å phase was synthesized at 6.0-6.5 GPa, 600 °C for 46 hours (sample 10Å-46) and 160 hours (10Å-160). Spectra were collected up to 9.0 GPa (talc), 9.9 GPa (10Å-46), and 9.6 GPa (10Å-160). The OH stretching vibration of Mg 3 OH groups in talc occurs at 3677 cm -1 at ambient pressure, and increases linearly with pressure at 0.97(2) cm -1 GPa -1 . The same vibration occurs in 10-Å phase, but shows negligible pressure shift up to 2 GPa, above which the frequency increases linearly to the maximum pressure studied, at a rate of 0.96(3) cm -1 GPa -1 (10Å-46) and 0.87(3) cm -1 GPa -1 (10Å- 160). Two other prominent bands in the 10-Å phase spectrum are suggested to be due to stretching of interlayer H 2 O, hydrogen-bonded to the nearest tetrahedral sheet. These bands also show little change over the first 2 GPa of compression, as most of the compression of the structure is taken up by closing non-hydrogen bonded gaps between interlayer H 2 O and tetrahedral sheets. Between 2 and 4 GPa, changes in band intensities suggest a rearrangement of the interlayer H 2 O.

Single crystals of Cu-substituted bieberite (Co 1-x Cu x )SO 4 ·7 H 2 O have been synthesized from aqueous solution by the evaporation method. It is shown that the solubility of Cu 2+ in bieberite is limited to 0.46 atoms per formula unit (apfu). Chalcantite CuSO 4 ·5H 2 O only accommodates a very small amount of ~0.03 Co 2+ atoms. Structure analysis by X-ray diffraction of single crystals along the join CoSO 4 ·7H 2 O-CuSO 4 ·5H 2 O showed that the hepta-hydrate phases are monoclinic with P2 1 /c symmetry at room temperature, whereas the penta-hydrate phases are triclinic, P1̅. Within the Co 1-x Cu x SO 4 ·7H 2 O series (0 ≤ x ≤ 0.46), lattice parameters are distinctly altered by the Cu 2+ substitution, however, the observed changes cannot be ascribed to different ionic radii of Co 2+ and Cu 2+ but are due to an increasing distortion of the lattice by increasing Cu 2+ content. Cu 2+ is not distributed randomly over the two possible crystallographic metal sites but exclusively enters the M2 site. While all the oxygen atoms coordinating the M1 site act as donors of hydrogen bonding to the sulfate tetrahedron, one oxygen atom at the M2 site is a hydrogen bond acceptor. This may cause the slight tetragonal elongation of the M2 coordination polyhedron, which favors the preferential occupation of this site by Cu 2+ . Optical spectra show typical features of both the Co 2+ and the Cu 2+ absorption signatures. While in Co 2+ -rich hepta-hydrate phases Co 2+ absorption bands arising from both M1 and M2 sites are detected, there are only bands arising from the M1 site in Co 0.54 Cu 0.46 SO 4 ·7H 2 O. This supports the observation of selective substitution of Cu 2+ at the elongated M2 octahedral site. Cu 2+ absorption bands can be assigned on the basis of D4h symmetry with increasing splitting of the Cu 2+ , 2 T 2g , and 2 Eg energy levels with increasing Cu 2+ content.

Clinoenstatite exsolution lamellae have been discovered in clinopyroxene grains of garnet peridotite from the Bixiling massif of the ultrahigh-pressure metamorphic (UHPM) terrane in the Dabie Mountains, China. Using transmission electron microscopy, we show here that although the lamellae are low clinoenstatite (Lclen) with P2 1 /c space-group symmetry, they contain nanometer-scale antiphase domains indicating that the originally precipitating phase had C2/c space-group symmetry. The lamellae are oriented parallel to [010] and ~18° to [001] of the host.approximately parallel to (401). Both the very cold exhumation path of the Dabie UHPM terrane and the orientation of the lamellae preclude the possibility that the originally precipitating phase could have been high-temperature clinoenstatite (HTclen). We conclude that the lamellae precipitated as high-pressure clinoenstatite (HPclen) and subsequently inverted to Lclen. Analysis of the incomplete high-pressure crystallographic data available for compositions approximating those of the lamellae and host suggests a minimum pressure of precipitation of ~9 GPa for Dabie and about 12 GPa for similar exsolution previously observed in the Alpe Arami peridotite, Switzerland. The Bixiling complex is a crustal cumulate, hence this result extends the minimum depth of subduction of continental rocks in the Dabie/SuLu orogen to ~300 km (P > 9 GPa).

Low- and high-temperature heat capacities of five natural chlorite-group samples were measured using the heat-capacity option of the Physical Properties Measurement System (Quantum Design), which is based on the principles of heat-pulse calorimetry, and by differential scanning calorimetry. Comprehensive chemical analyses were performed on these samples by electron microprobe analysis, by inductively coupled plasma mass spectrometry, and by Karl-Fischer titration (for H 2 O). The natural chlorites span a range in X Fe from 0.052 to 0.885 with increasing Al-content due to the Tschermak substitution with increasing X Fe . The measured heat capacities were extrapolated to the end-member compositions of chamosite and clinochlore. Integration of heat-capacity data yields the calorimetric standard entropies of chamosite (Fe 5 Al)[Si 3 AlO 10 ](OH) 8 and clinochlore (Mg 5 Al)[Si 3 AlO 10 ](OH) 8 , with values of 572.0 ± 0.2 and 425.6 ± 0.4 J/(mol·K), respectively. The C P -polynomial for end-member chamosite is C P = 1151.7 - 8.4564 × 10 3 ·T -0.5 -13.206 × 10 6 ·T -2 + 15.233 × 10 8 ·T -3 [J/(mol·K)], valid in the temperature range of 298.15.900 K, and that for end-member clinochlore is C P = 1160.5 - 9.9819 × 10 3 ·T -0.5 . 5.9534 × 10 6 ·T -2 + 3.8677 × 10 8 ·T -3 [J/(mol·K)], valid in the temperature range of 298.15-1000 K. The Fe-rich chlorites exhibit an asymmetric distribution of the excess heat capacity in a plot of C ex P vs. T, with a maximum at about 52 K. By analogy to annite, we interpret this peak to represent the magnetic ordering temperature. Based on our standard entropy value for chamosite, the enthalpy of formation of berthierine (Fe 2.5 Al 0.5 )[Si 1.5 Al 0.5 O 5 ](OH) 4 was estimated as -3570.30 kJ/mol using a reported onset temperature of 70 °C at 16 MPa for the berthierine-chamosite polymorphic transition.

Synthetic NaGaSi 2 O 6 -pyroxene, space group C2/c, was investigated at low temperature between 295 to 110 K by X-ray diffraction. The evolution of the unit-cell parameters as a function of temperature does not indicate any structural phase transition down to 110 K. Calculated values for the Debye temperature, θ D , and the Grüneisen parameter, γ, are 653(39) and 0.84(8) K, respectively, close to those of other pyroxenes. Five complete intensity data collections were performed at 295, 235, 190, 145, and 110 K to investigate the temperature dependence of the crystal structure. The space group remains C2/c down to 110 K, but anomalies were found between 235 and 190 K for the temperature dependencies of the volume and strain of the M1 polyhedron. This anomalous behavior could be related to variations of the Ga-O2(C1,D1) bond length and the O1(A1,B1)-Ga-O2(C1,D1) bond angle. The C2/c-P1̅ phase transition observed in a previous work for NaTiSi 2 O 6 at about 200 K did not occur for NaGaSi 2 O 6 down to the minimum temperature investigated in this work. This is not due to the different ionic radius at M1 site or to a different tetrahedral chain extension between these two compositions but it is likely due to unfilled t 2g orbitals, which do not exist in NaGaSi 2 O 6 .

The single-crystal elastic constants of zoisite Ca 2 Al 3 Si 3 O 12 (OH) were determined by Brillouin scattering at ambient conditions. The elastic tensor was obtained by an inversion of acoustic velocity data for three different crystal planes. The aggregate bulk modulus, shear modulus, and Poisson.s ratio are K S0 = 125.3(4) GPa, G 0 = 72.9(2) GPa, and σ 0 = 0.26(1) for the VRH (Voigt-Reuss-Hill) average, respectively. The maximum azimuthal anisotropy of zoisite is 22% for compressional velocity and 33% for shear velocity. The maximum shear splitting is 21% along the [001] direction. Our results resolve the discrepancies in bulk modulus and axial compressibilities reported from static compression studies, and provide the first experimental constraints on the shear modulus. Trends in the elastic moduli of minerals in the CaO-Al 2 O 3 -SiO 2 -H 2 O (CASH) system are evaluated.

Titanium L 3,2 and O K electron energy loss near-edge structures (ELNES) of seven Ti oxides have been measured in a transmission electron microscope to obtain information on the valence state and site geometry of Ti. The coordination of Ti in all phases studied is octahedral, whereas the valence states occurring are Ti 2+ , Ti 3+ , and Ti 4+ . Effects of polyhedra distortions are particularly observed for two oxides with mixed Ti 3+ -Ti 4+ valence state, i.e., the Magnéli phases Ti 4 O 7 and Ti 5 O 9 . A prominent pre-peak in the Ti L 3 edge is attributed to the orthorhombic polyhedra distortions in these compounds, leading to complex crystal field splitting. The effect of valence state manifests itself in a systematic chemical shift of Ti white lines by 2 eV per valence state. On the basis of collected Ti L 3,2 ELNES spectra we propose a new quantification technique for the determination of Ti 4+ /Ti 3+ ratios. Complementary O K ELNES spectra were well reproduced by Density Functional Theory calculation, revealing that the O K-edge is sensitive to the covalent bonding in all analyzed oxides.

The local structural environments in a series of natural and synthetic alunite samples [ideally AAl 3 (SO 4 ) 2 (OH) 6 , A = H 3 O + , D 3 O + , Na + , and K + ] have been probed by solid-state 1 H, 2 H, 23 Na, 27 Al, and 39 K NMR spectroscopy. The natural alunite [KAl 3 (SO 4 ) 2 (OH) 6 ] and synthetic hydronium alunite samples contain few structural defects, whereas the synthetic natroalunite and alunite samples have ca. 10% Al vacancies based on 27 Al NMR. A new 27 Al local environment (Al D ) was observed and assigned to Al with one Al vacancy in the first cation sphere. Three different proton environments, Al 2 -OH, Al - OH 2 , and H 3 O + are detected by 1 H and 2 H MAS NMR. The hydronium ion (H 3 O + ) is only observed in hydronium alunite, and is associated with the stoichiometric regions of the sample. It was not detected in 1 H and 2 H NMR spectra of alunite and natroalunite despite K (Na) occupancies of significantly less than 100%, as determined from elemental analysis. Thus, our NMR results suggest that the common assumption, namely that an A vacancy and an Al 3+ vacancy are compensated by adding an H 3 O + and 3H + (creating 3 Al-OH 2 groups), respectively, is too simplistic. Instead, a significant fraction of the Al 3+ vacancies are compensated for by 4 H + ions, resulting in 4 Al-OH 2 groups per vacancy. This substitution is accompanied by the simultaneous deprotonation of a H 3 O + ion present on the A site. The resultant H 2 O molecule is unnecessary for charge balance, accounting for the A-site deficiency often observed. The presence of Al 3+ and A + vacancies appears closely correlated based on NMR.

We present a diagram that shows the effect of pH, temperature, grain size, composition, hydrodynamics, and the laboratory/field discrepancy on the lifetime of olivine grains in weathering environments. Because the persistence of olivine grains on Mars can be used to constrain the duration of liquid water, we can use this diagram to predict a range of possible maximum contact times for olivine grains with liquid water before they dissolve away completely. Depending upon the physicochemical conditions, this contact time could range between a few thousand and several million years.

Pale-green massive serpentine veins cutting the Yugoslavian serpentinites consist of 15- and 30-sectors polygonal serpentine in random orientation but becoming stress-oriented close to the vein walls. [100] cross-sections show ordered one-layer and two-layer projected polytype sequences, as well as disordered multilayer sequences. From sector to sector, these sections have systematically alternating polytypes that obey the O-L-R and O-R-L alternation rules in 15- and 30-sectors polygonal serpentine, respectively. Furthermore, [010] longitudinal sections systematically show a unique two-layer sequence. By combining the two projections, we reconstruct the basic 3D polytype assemblage that from sector to sector consists of alternating M5-M15-M15 or M5-M16-M16 polytypes. The individual fibers may be described as cyclical pseudo-twins, based upon continuous 1:1 layers, with possible octahedral rotation from one sector to the other. Continuous curved sector boundaries join adjacent sectors, with no tetrahedral sheet inversion; the curved boundaries behave as similar folds and have variable interplanar spacings. Simulated electron diffraction patterns reproduce all the observed diffraction geometries, even for faulted stacking sequences, and explain numbers of diffraction cusps and chords. Additional XRPD reflections are indexed based on polytypes present in 15- and 30-sectors polygonal serpentines.

The crystal structure of a new light-induced alteration product obtained from a natural non-stoichiometric arsenic sulfide (original chemical formula As 4 S 4.35 ) was solved in the space group Pccn, and refined to a final R index of 9.89%. Unit-cell parameters are: a = 19.352(7), b = 10.166(3), c = 8.697(4) Å, V = 1711(1) Å 3 ; Z = 8. The structural refinement results yielded a chemical formula close to As 4 S 5 . The structure consists of discrete, covalently bonded As 4 S 5 molecules, which are held together by van der Waals forces. The molecular packing is similar to that of the original crystal, which, in turn, is the same as to that of β-As 4 S 4 . The phase originated from a continuous, room-temperature, lightinduced alteration process that does not require a complete rearrangement of the molecular packing and therefore does not imply the loss of coherency between crystalline domains.

A freshwater population of magnetotactic bacteria has been extracted from the Seine River (France) and studied using transmission electron microscopy. Seventeen different morphotypes were recognized using morphological criteria, which rely on the number of magnetite crystals and their organization within cells, the size and shape of the cells and their statistical distribution. This study revealed new features in some magnetotactic bacteria that have not been described in the literature. In addition X-ray energy dispersive spectroscopy and electron diffraction analyses revealed cells containing Ba-rich and CaO inclusions. Two major modes of magnetite crystals growth were derived from the distributions of the crystal shapes in this population. Numerous cases of crystals elongations along axes different from the [111] axis are related to one singular process of crystal growth. Thus, this population of magnetites collected from cells extracted from the Seine River does not meet some of the criteria for biogenicity, which have been used so far for biomagnetites, particularly those concerning the [111] elongation axis.

The silica polymorph moganite is commonly intergrown with quartz in microcrystalline silica varieties that are less than ~100 Ma in age. Synchrotron X-ray diffraction suggests that a displacive phase transition occurs when moganite is heated above ~570 K, with an increase in symmetry from I2/a to Imab. In the present study, we employed hard-mode Raman spectroscopy to confirm the existence of the α-β moganite transformation and to offer complementary insight into the transition mechanism. Our analysis of the displacement of the 501 Δcm -1 symmetric stretching-bending vibration (B 3g mode) with changing temperature strongly supports the existence of a monoclinic-to-orthorhombic phase transition between 570 and 590 K. Between 593 and 723 K, however, the mode remained fixed at 496 Δcm -1 . This behavior was repeated on cooling, but with a hysteresis of over 100 K. We offer three hypotheses that may explain this observation: (1) the intergrowth of nanoscale quartz lamellae within moganite may exert a strain that inhibits the transition; (2) the transition may exhibit a martensitic character marked by the co-existence of α- and β-moganite over a finite temperature interval; and (3) the α- and β-moganite transition may occur via an intermediate phase.

The existence of the only ternary compound, Ag 3 AuSe 2 (fischesserite), in the Ag-Au-Se system was confirmed by solid-state annealing method. The selenium analog of petrovskaite (AgAuS) was not observed under the experimental conditions (in the temperature range of 350-773 K and own vapor pressure). The solid-state reactions 2Ag(cr) + Se(s) = Ag 2 Se(cr) Ag(cr) + Ag 3 AuSe 2 (cr) = 2Ag 2 Se(cr) + Au(cr) 3Ag(cr) + 2AuSe(s) = Ag 3 AuSe 2 (cr) + Au(cr) were studied by the electromotive force (EMF) technique in all solid-state galvanic cells with Ag 4 RbI 5 as a solid electrolyte. The experiments were run in a dry argon gas flow at atmospheric pressure. The following standard thermodynamic properties of the stable phases were determined at 298.15 K and 1 bar (10 5 Pa): ΔfG°(Ag 2 Se, low naumannite) = -49470 (±130) J/mol, S°(Ag 2 Se, low naumannite) = 149.99 (±0.56) J/(K·mol); ΔfG°(Ag 2 Se, high naumannite) = -47430 (±290) J/mol, S°(Ag 2 Se, high naumannite) = 169.01 (±0.78) J/(K·mol); ΔfG°(Ag 3 AuSe 2 , fischesserite) = -86450 (±320) J/mol, S°(Ag 3 AuSe 2 , fischesserite) = 290.80 (±1.26) J/(K·mol); ΔfG°(β-AuSe) = -4110 (±1300) J/mol; S°(β-AuSe) = 75.49 (±3.55) J/(K·mol). The phase transition point for low naumannite-high naumannite was determined at T trs = 405.4 K, with the enthalpy of transition of Δ trs H = -7713 (±550) J/mol.

Rhombohedral (9R) and hexagonal (6H) perovskites in BaSiO 3 were observed by using in-situ X-ray diffraction methods in high-pressure conditions. The 9R perovskite was found after successive laser heating at 27.9 GPa, and the 6H phase was identified at 48.5 GPa after laser annealing. No intermediate phase between the 9R and 6H phases was found in the pressure range between 27.9 and 48.5 GPa. These phases could not be recovered during decompression at room temperature and became amorphous at ambient pressure. The 9R and 6H structures are characterized by the periodicity of the stacking sequence of edge- and corner-sharing SiO 6 octahedra. In terms of the length of the O-O distance, the face-sharing SiO 6 octahedron in BaSiO 3 perovskite is the most distorted among oxide hexagonal perovskites.

A single-crystal X-ray diffraction study in a diamond anvil cell up to 5.41 GPa was carried out on a clinochlore [monoclinic polytype IIb-2, S.G. C2/m, (Mg 9.09 Fe 2+ 1.01 Mn 0.02 Ti 0.01 Cr 0.02 Al 1.80 ) Σ=11.95 (Si 6.35 Al 1.65 ) Σ=8O20 (OH) 16 ] from Val Malenco, Italy. The bulk modulus of monoclinic clinochlore calculated by fitting unit-cell volumes and pressures to a third-order Birch-Murnaghan Equation of State (EoS), is K 0 = 71(9) GPa with K’ = 8(5). Axial compressibility values were β 0a EoS = 3.8(1), β 0b EoS = 3.6(1), and β 0c EoS = 5.4(5) 10.3 GPa.1, showing that axial anisotropy is much less than that found for other phyllosilicates. Compressibility data are in fair agreement with literature data, which are based on powder neutron and synchrotron diffraction methods. Results were compared with the behavior of the triclinic polytype of similar composition and coexisting in the same rock. Symmetry has little overall influence on compressibility, but compared with the triclinic polytype of similar composition and coexisting in the same hand specimen, the monoclinic polytype is slightly less rigid. Comparison of structural refinements at different pressures showed that structural deformations mainly affect the interlayer region, where hydrogen bonds are important for the structural properties of the phase. The mean decrease in OH-O distances was about 9% in the pressure range 0.5 GPa. Structural behavior was very similar to that found for the triclinic polytype. Although energy differences between polytypes are relatively small, their compressional behavior may have implications in terms of relative stability. A computation of molar volume applying an isothermal EoS shows that the triclinic polytype is lower in volume up to 0.9 GPa, above which the volume of the monoclinic phase is smaller. This fact gives information on the relative stability of the two polytypes and a possible explanation for the greater abundance of the triclinic polytype in low to medium-P environments, as is commonly observed in nature.

Uranyl oxide hydrate minerals are common in the altered zones of U deposits and are expected to form where spent nuclear fuel is altered in an oxidizing environment. Consistent with all known uranyl oxide hydrate minerals, metaschoepite [(UO 2 ) 4 O(OH) 6 ](H 2 O) 5 , contains sheets of uranyl polyhedra with H 2 O groups located in the interlayer. Several crystals along the series from metaschoepite to Na-substituted metaschoepite (Na-MS), approximate formula Na[(UO 2 ) 4 O 2 (OH) 5 ](H 2 O) 5 , have been synthesized and their structures have been determined. Each contains sheets of uranyl pentagonal bipyramids that are topologically identical to those in metaschoepite, schoepite and fourmarierite. The sheets are electroneutral in metaschoepite, but substitution of O for OH located at the equatorial vertices of the uranyl polyhedra gives a charged sheet that is balanced by incorporation of Na in the interlayer. Synthesis of crystals of Na-MS from a solution containing ~500 ppm Np 5+ , followed by analysis of the crystals using laser-ablation inductively coupled-plasma mass-spectroscopy, demonstrated that the crystals incorporate Np. This is in contrast to earlier studies that showed no incorporation of Np 5+ in synthetic metaschoepite (which has electroneutral sheets), and supports the hypothesis that Np 5+ incorporation is more likely in uranyl oxide hydrates with charged species in the interlayer.

Crystallization experiments were conducted at 200 MPa and water-undersaturated conditions to quantify the effect of small amounts of H 2 O on the crystallization temperature of olivine in basaltic melts (e.g., Mid-Oceanic Ridge Basalts, MORB).The H 2 O concentrations in the quenched glasses, determined by infrared spectroscopy and Karl-Fischer-Titration, ranged between 0.25 and 4.2 wt% H 2 O. The dry liquidus temperature was estimated from experiments at 0.1 MPa (H 2 O-free) and from the known pressure dependence of olivine crystallization temperature. The liquidus temperature depression can be predicted by the empirical equation (T DRY - T WET ) = 39.69·C H ₂ O 0.73 (wt%). The comparison of the experimental results with available crystallization models and empirical methods shows that most of the predicted liquidus temperature depressions differ significantly from that observed in this study.

Optical anomalies and crystal structures have been studied for an elbaite-liddicoatite tourmaline specimen from the Malkhan pegmatite field (Transbaikalia region, Russia). The specimen is characterized by a complicated zoning and sector zoning distribution of anomalous biaxiality, with the axial angle 2V increasing from 3 up to 23° going from the first to the last growth zones of the most developed growth sector, o{022̅1}. The crystal structures of three samples cut out from different growth zones of that sector and characterized by the axial angles 11, 16, and 23° were refined in the trigonal space group R3m as well as in its monoclinic and triclinic subgroups Cm, R1, and P1, respectively (R = 0.019-0.039). The final choice of the low symmetry space group R1 is based on the analysis of the diffraction patterns and on the results of crystal structures refinements, which revealed ordered Al/Li distributions over the Y octahedral sites. The degree of order at Y sites correlates with the axial angle 2V, which implies a causal relationship. The Al/Li ordering reflects the geometrical differences of the octahedral sites with respect to the growth front orientation. The results obtained suggest that a growth dissymmetrization mechanism is the main reason for the observed cation ordering and optical anomalies.

We describe the shape and internal structure of polyhedral spheroids found in serpentinized peridotites. Serpentine spheroids resemble geodesic domes made of ~160 to 180 triangular facets. At facet edges, the nested layers bend by ~14° along their three <010> crystallographic directions, resulting in an onion-like structure with lateral continuity of the layers. The stacking of the serpentine layers within sectors is controlled by interlayer bonding. These polyhedral onions correspond to a novel type of spherical nanostructure for layered materials.

Ion microprobe U-Pa measurements in zircon crystals from a Holocene rhyolite (Salton Buttes, California) are utilized to assess the partitioning of Pa and U between zircon and melt. The relative sensitivity factor (RSF) for ion microprobe U-Pa zircon analysis was calibrated on a natural high-U secular equilibrium zircon from Buff Peak (Nevada). Zircon crystals from Salton Buttes rhyolite show evidence for excess 231 Pa with a weighted average ( 231 Pa)/( 235 U) activity ratio of 1.9. From this ratio, model zircon-melt partition coefficient ratios D Pa /D U = 0.9.2.2 are obtained, assuming plausible limits for ( 231 Pa)/( 235 U)melt and correcting for the age of zircon crystallization (~15 ka). These values roughly fit lattice strain models for tetravalent cations, but are more than one order of magnitude lower than predictions for pentavalent Pa. Based on this appraisal of Pa partitioning, initial 231 Pa disequilibrium caused by zircon-melt fractionation alone can produce minor discordance or excess 207 Pb in Late Cenozoic zircon, but is insignificant for older U-Pb zircon ages.

Structural information on divalent cations such as Mg 2+ should have important implications for magmatic liquids because of their abundance in the Earth.s interior; nevertheless, little is confirmed about their coordination environments. We here apply a 25 Mg triple-quantum magic-angle spinning (3QMAS) NMR technique at an ultra-high magnetic field (21.8 T) and successfully show the occurrence of multiple Mg sites in MgSiO 3 glass. We find that these sites are distinguished by the degree of polyhedral distortion, not by the coordination number. The present study concludes that the highly distorted MgO 6 species occur in MgSiO 3 glass, in strong contrast with a recent radial distribution study.

The heat capacities of a series of synthetic forsterite (Fo).fayalite (Fa), Mg 2 SiO 4 .Fe 2 SiO 4 , olivines have been measured between 5 and 300 K on milligram-sized samples with the Physical Properties Measurement System (Quantum Design). The heat capacities for fayalite and fayalite-rich olivine are marked by a sharp lambda-type anomaly defining a transition from the paramagnetic to an antiferromagentic state, which in the case of fayalite occurs at T N = 64.5 K. In forsterite-rich compositions a feature in the C P data around 25 K is observable and it could possibly be linked to a magnetic transition. Additionally, all Fe-bearing olivines show a Schottky-type anomaly. Excess heat capacities of mixing, ΔC xs P , for the various Fe-Mg olivine solid-solution compositions were calculated applying the equation ΔC xs P = C ss P -[(1 - XFa) C P o + X Fa C P Fa ] using fitted C P polynomials for each composition. The calorimetric entropies at 298.15 K, Scal, were determined by solving the C P integral . If a symmetric Margules mixing model ΔS xs = Ws·X Fa (1 -XFa) is taken to describe the entropy of mixing behavior for the Fo-Fa binary, it yields an interaction parameter of W s = -1.6 ± 1.7 J/(mol·K) on a onecation basis. The calorimetric data thus indicate ideal entropy of mixing behavior. Adopting, however, a value of W Ol S,Mg-Fe = -1.6 J/(mol·K) one can calculate a value for the excess Gibbs free energy of mixing of W Ol G,Mg-Fe = 6.9 kJ/mol at 1000 K using the most recent solution calorimetric study of Kojitani and Akaogi (1994) on Fo-Fa olivine with W Ol H,Mg-Fe = 5.3 kJ/mol. This WOl G,Mg-Fe value should be considered a maximum upper limit for thermodynamic nonideality. Using solely calorimetric data, the T-X phase diagram for the Fo-Fa binary is calculated at 1 bar and 50 kbar and compared to that obtained from a model-dependent thermodynamic analysis. The results suggest that exsolution in Fe-Mg olivine should only be possible in low-temperature environments depending on kinetic behavior.