Volume 94, Issue 8-9

Experimental model electron density distributions obtained for the silica polymorphs coesite and stishovite are comparable with electron density distributions calculated for various silicates and siloxane molecules. The Si-O bond lengths and Si-O-Si angles calculated with first-principles density functional theory methods as a function of pressure are also comparable with the bond lengths and angles observed for coesite and quartz within the experimental error. The similarity of the topological properties of the Si-O bonded interactions and the experimental and the geometry-optimized structures for the silica polymorphs provide a basis for understanding the properties and crystal chemistry of silica. The agreement supports the argument that the bulk of the structural and physical properties of the silica polymorphs are intrinsic properties of molecular-like coordination polyhedra such that the silica polymorphs can be pictured as “supermolecules” of silica bound by virtually the same forces that bind the Si and O atoms in simple siloxane molecules. The topology of the electron density distribution is consistent with the assertion that the Si-O bonded interaction arises from the net electrostatic attraction exerted on the nuclei by the electron density accumulated between the Si and O atoms. The correlation between the Si-O bond length and Si-O-Si angle is ascribed to the progressive local concentration of the electron density in the nonbonded lone pair region of the O atom rather than to a bonded interaction that involves the d-orbitals on Si. The accumulation of deformation electron density, Δρ( r ), in the bonded and nonbonded regions of the Si-O bond, the close proximity of the bond critical point, r c , of the bond with the nodal surface of the Laplacian and the negative value of the total energy density are taken as evidence that the bond has a nontrivial component of shared character. For M-O bonded interactions for first and second row metal atoms bonded to O, ∇ 2 ρ( r c ) is positive and increases linearly as ρ( r c ) and G( r c )/ρ( r c ) both increase and as the value of H( r c ) decreases; the greater the shared character of the interaction, the larger the values of both ∇ 2 ρ( r c ) and G( r c )/ρ( r c ). In addition, a mapping of ∇ 2 ρ( r ) serves to highlight those Lewis base domains that are susceptible to electrophilic attack by H, like the O atoms in coesite involved in bent Si-O-Si angles; the narrower the angle, the greater the affinity for H. On the basis of the net charges conferred on the Si and O atoms and the bonded radii of the two atoms, the Si-O bond for stishovite, with six-coordinated Si and three-coordinated O, is indicated to be more ionic in character than that in quartz with four-coordinated Si and two-coordinated O. Unlike the conclusion reached for ionic and crystal radii, it is the bonded radius of the O atom that increases with the increasing coordination number of Si, not the radius of the Si atom. The modeling of the electron density distributions for quartz, coesite, and beryl as a function of pressure suggests that the shared character of the bonded interactions in these minerals increases slightly with increasing pressure. The insight provided by the calculations and the modeling of the electron density distributions and the structures of the silica polymorphs bodes well for future Earth materials studies that are expected to improve and clarify our understanding of the connection between properties and structure within the framework of quantum mechanical observables, to find new and improved uses for the materials and to enhance our understanding of crystal chemistry and chemical reactions of materials in their natural environment at the atomic level.

The 16 km thick early to middle Eocene Crescent Formation exposed on the Olympic Peninsula represents one of the thickest stacks of basalt on Earth. It is variably metamorphosed at low grade, with evidence of both medium- and high-P/T conditions. Metasomatism is localized within a small number of layers, with variation relative to average values of as much as five standard deviations. The upper section tends to be lower grade but few systematic trends in mineralogy or mineral composition correlate with stratigraphic position. The boundary region between the lower and upper members contains the most dramatic metasomatic alteration as well as the best examples of barroisitic amphibole. Observations are consistent with a model in which extrusion of the basalt stack led to medium-P/T burial recrystallization in the zeolite and prehnite-pumpellyite facies, followed by localized, possibly fluid-facilitated lower-blueschist facies metamorphism; this high-P/T metamorphism may reflect the lowering of isotherms caused by subduction-zone refrigeration to the west of these rocks.

The build up of scale inside autoclaves during high-pressure acid leaching of nickel laterite ore currently represents a significant industrial problem in processing plants. In this study, the crystallographic microstructure of natrojarosite scale developed on stirring rods inside a high-pressure acid leach autoclave are investigated using electron backscatter diffraction (EBSD). Backscatter electron imaging shows compositional layering of natrojarosite with dispersed minor iron oxide crystals and regularly spaced, parallel layer micro-porosity. Analysis of the scale by EBSD reveals that natrojarosite forms elongate grains with strong (0001) crystallographic preferred orientation with c axes aligned within the plane of the agitator substrate, consistent with unitaxial precipitation from a fluid. The natrojarosite grains contain a complex internal substructure of low-angle (<15°) boundaries that define elongate subgrains and accommodate up to 40° of systematic misorientation across single grains. Low-angle subgrain boundaries do not have simple twin relationships or low-index misorientation axes and are interpreted to have formed as a result of the propagation of crystal defects during growth rather than by syn-growth deformation due to shear stresses between viscous leachate slurry and the agitator inside the autoclave. The presence of extended defects in the scale microstructure has implications for the solubility and reactive stability of mineral processing waste as well as for the development of scale growth inhibitors.

When suspended in solution, clay platelets coated with nanometer-scale magnetite particles behave as magnetorheologic fluids that are important to a variety of industrial applications. Such dual-phase assemblages are also similar to natural aggregates that record the direction and intensity of the Earth’s magnetic field in lake and marine depositional environments. This study characterizes the mineralogical structure and magnetic behavior of montmorillonite platelets coated with aggregates of nanometerscale magnetite crystals. The distribution of magnetite crystal sizes in three different clay-magnetite assemblages was directly measured using conventional transmission electron microscopy and agrees within error with estimates derived from magnetic hysteresis measurements. Magnetic hysteresis and low field susceptibility measurements combined with electron holography experiments indicate that all three samples behave superparamagnetically at room temperature, and show increasing levels of single domain behavior as the samples are cooled to liquid nitrogen temperatures. At such low temperatures, magnetostatic interactions are observed to partially stabilize otherwise superparamagnetic grains in flux closure structures.

We have synthesized large (0.7-1.0 mm) crystals of anhydrous, water-bearing, and Fe-bearing wadsleyite by means of growth from solution in the thermal gradient field. Nearly anhydrous (<68±4 wt ppm H 2 O) Mg 2 SiO 4 crystals were grown using K 2 Mg(CO 3 ) 2 as a solvent at 16.5 GPa and 1700 °C. (Mg 0.92 Fe 0.08 ) 2 SiO 4 crystals containing 84±17 wt ppm H 2 O were grown using 92K 2 Mg(CO 3 ) 2 -8FeCl 2 as a solvent. Crystals of Fe-free wadsleyite with 1496±117 wt ppm H 2 O were synthesized at 1400 °C and 15.5 GPa by using 2KHCO 3 -Mg(OH) 2 as a solvent.

The elastic behavior and the phase stability of natural pollucite, (Cs,Na) 16 Al 16 Si 32 O 96 ⋅nH 2 O, were investigated at hydrostatic pressure by in situ single-crystal X-ray diffraction with a diamond-anvil cell. Pollucite experiences a P-induced phase transition, not previously reported in the literature, at P = 0.66 ± 0.12 GPa from cubic (Ia3̅d) to triclinic symmetry (P1̅). The phase transition is completely reversible and without any appreciable hysteresis effect. No further phase transition has been observed up to 9 GPa. Fitting the pressure-volume data of the low-pressure cubic polymorph with a second-order Birch-Murnaghan Equation-of-State (BM-EoS), we obtain V 0 = 2558.3(4) Å 3 , K T0 = 41(2) GPa, and K′ T = 4 (fixed). For the high-pressure triclinic polymorph, a third-order BM-EoS fit gives V 0 = 2577.5(40) Å 3 , K T0 = 25.1(9) GPa, and K′ T = 6.5(4). The axial bulk moduli of the high-pressure triclinic polymorph were calculated with a third-order “linearized” BM-EoS. The EoS parameters are a 0 = 13.699(12) Å, K T0 (a) = 25.5(17) GPa, and K′ T (a) = 6.8(6) for the a axis; b 0 = 13.728(12) Å, K T0 (b) = 23.2(15) GPa, and K′ T (b) = 7.7(7) for the b axis; c 0 = 13.710(7) Å, K T0 (c) = 25.2(10) GPa, and K′ T (c) = 6.8(4) for the c axis [K T0 (a):K T0 (b):K T0 (c) = 1.10:1:1.09]. Brillouin light-scattering was used to investigate the single-crystal elastic properties of pollucite at ambient conditions. The aggregate adiabatic bulk modulus (Ks) and shear modulus (G), calculated using the Voigt-Reuss-Hill averaging procedures, are Ks = 52.1(10) GPa and G = 31.5(6) GPa. The elastic response of pollucite and other isotypic materials (e.g., analcime, leucite, and wairakite) is compared. The high thermo-elastic stability of pollucite, reflected by the preservation of crystallinity at least up to 9 GPa (at room T) and 1470 K (at room P) in elastic regime, the large amount of Cs hosted in this material (Cs 2 O ~ 30 wt%), the immobility of Cs at high-temperature and high-pressure conditions, and the extremely low leaching rate of Cs, make of this open-framework silicate a functional material with potential use for fixation and deposition of Cs radioisotopes in high-level nuclear waste.

Anandite, a trioctahedral mica, has an ideal chemical formula of Ba(FeMg)3(SiFe 3+ )O 10 (OH)S and a microprobe-derived formula of (Ba 0.96 K 0.03 Na 0.01 ) Σ=1.00 (Fe 2+ 2.01 Mg 0.46 Fe 3+ 0.28 Al 0.10 Mn 3+ 0.04 Mn 2+ 0.04 Ti 0.01 ) Σ=2.93 (Si 2.60 Fe 3+ 1.40 ) Σ=4.00 O 10 [(OH) 0.96 S 0.84 Cl 0.16 F 0.04 ] Σ=2.00 , where S 2- primarily substitutes for (OH) - . A single-crystal, X-ray structure determination of anandite-2M was refined to R 1 = 0.0443 and wR 2 = 0.1232 on F 2 in space group Am. Cell parameters are: a = 5.4431(3), b = 9.4719(6), c = 20.042(1) Å, and β = 95.046(1)°. Layer stacking is analogous to a 1M stacking pattern (parallel unidirectional a/3 shifts within layers and octahedral set I only occupied), however cation order and (S,OH) positional disorder produce a two-layer repeat with β equaling ~95°. Subgroup symmetry of Am results from tetrahedral sheets within layers that are non-centrosymmetric with unequal compositions (Si 0.61 Fe 3+ 0.39 vs. Si 0.79 Fe 3+ 0.21 ) and thicknesses (difference of 0.209 Å), and there is positional and site-occupancy disorder (four sites with S of 0, 30, 52, and 58%) of (S,OH). Characteristics of anandite-2M, which are similar to those of anandite-2O, include (1) alternation of smaller tetrahedral rings containing four Si-rich tetrahedra (T1a: 1.643 Å, T2b: 1.657 Å) and two Fe 3+ -rich tetrahedra (T2a: 1.733 Å, T1b: 1.760 Å) and larger rings containing four Fe3 + -rich tetrahedra and two Si-rich tetrahedra within each layer; (2) nearly in-phase wave forms of basal O atoms across the interlayer (Δz = -0.110 and 0.011 Å and across the interlayer Δz = -0.121 and 0.007 Å); and (3) attraction that results in Ba being shifted toward S (0.070 Å) and S being shifted toward Ba (0.117 Å average) along the c axis, relative to the ideal. Bond-valence calculations show that Ba is shifted toward the undersaturated, bridging-basal O atoms of the Fe 3+ -rich tetrahedra and toward S-rich sites to achieve charge balance. Comparison of anandite-2M and anandite-2O shows that they possess unit cells (2M setting) that have nearly equal a axes, unequal b and c axes, and β (anandite-2M is smaller by 0.0371 Å, larger by 0.08 Å, and smaller by 0.089°, respectively). Moreover, anandite-2O exhibits larger Fe 3+ -rich tetrahedral rings than anandite-2M, which allow for a greater shift in Ba (difference of 0.03 Å). The ordering and consequent absence of a twofold axis in anandite-2M allows the in-phase wave structure of basal O atoms, which was previously thought only possible in the orthorhombic P cell.

Low- and high-temperature heat capacities for a series of synthetic high structural state plagioclase crystals (Ab-An) were measured using both a relaxation calorimeter and a differential scanning calorimeter. The measurements were performed at temperatures between 5 and 800 K on milligram-sized polycrystalline samples that had been characterized in a previous study. The data show positive excess heat capacities of mixing at temperatures below 300 K with a maximum value of ~2 J/(mol·K). Below ~70 K, the excess heat capacities exceed two standard deviations and are thus significant. Above 300 K, the measurements indicate negative excess heat capacities with a maximum of ca. -1.5 J/(mol·K) at about 400 K, and do not exceed two standard deviations. The excess vibrational entropies of mixing are positive with an asymmetric variation. At T = 298.15 K, the largest deviation from ideal behavior occurs at Ab 20 An 80 amounting to ΔS ex vib = 2.8 ± 2.4 J/(mol·K). An asymmetric Margules mixing model was found to adequately describe the vibrational entropy-composition behavior, yielding W Svib AbAn = 16.4 J/(mol·K) and = W Svib AnAb 4.7 J/(mol·K).

The diverse range of calcic-amphibole compositions in eruptive products from the ca. 330 ka Matahina eruption (ca. 160 km 3 rhyolitic magma) of the Okataina Volcanic Complex, Taupo Volcanic Zone, including crystal-rich basalt-dacite pumice from post-collapse deposits, reveals several pre- and syn-eruption magmatic processes. (1) Amphibole phenocrysts in the basaltic-andesite and andesite crystallized at the highest pressures and temperatures (P to 0.6 ± 0.06 GPa and T to 950 °C), equivalent to mid-crustal depths (13-22 km). Inter- and intra-crystalline compositions range from Timagnesiohastingsite → Ti-tschermakite → tschermakite → magnesiohornblende and some display gradual decreases in T from core to rim, both consistent with magma differentiation by cooling at depth. (2) The largest amphibole crystals from the basaltic-andesite to andesite display several core to rim increases in T (to 70 °C), indicating that new, hotter magma periodically fluxed the crystal mush. (3) The dominant population of rhyolite amphibole is small and bladed (magnesiohornblende) and crystallized at low P-T conditions (P = 0.3 GPa, T = 765 °C), equivalent to the eruptive P-T conditions. Dacitic and low-silica rhyolitic amphibole (tschermakite-magnesiohornblende) form two distinct populations, which nucleated at two different T (high: 820 °C and low: 750 °C). These compositional variations, governed primarily by differences in T conditions during crystal growth, record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive temperature. Therefore, the diversity in amphibole compositions can be reconciled as an exchange of crystals + liquid between the basaltic-andesite to dacite from the mid-crust and rhyolite from the upper crust, which quenched against one another, modifying the dacite to low-silica rhyolite compositions as the eruption progressed.

TiO 2 solubility in rutile-saturated felsic melts and coexisting minerals was determined at 1.5-3.5 GPa, 750-1250 °C, and 5-30 wt% H 2 O. TiO 2 solubility in the melt primarily increases with temperature and melt basicity; it increases slightly with water content in the melt, and it decreases with pressure. A general TiO 2 solubility model was obtained and is expressed as: ln(TiO 2 )melt = ln(TiO 2 )rutile + 1.701 - (9041/T) - 0.173P + 0.348FM + 0.016H 2 O, where TiO 2 and H 2 O are in wt%, T is in Kelvin, P in GPa, and FM is the melt composition parameter given by FM = (1/Si)·[Na + K + 2(Ca + Fe + Mn + Mg)]/ Al, in which the chemical symbols represent cation fractions. TiO 2 solubility in amphibole, garnet, and clinopyroxene also increases with temperature and empirical equations describing this temperature dependence were derived. These data were used to assess the protolith TiO 2 content required for rutile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition. The results show that only 0.8-1.0 wt% TiO 2 is required for rutile saturation during low-degree (<20%) melting. Rutile is stable up to ~1150 °C with 1.6 wt% TiO 2 in the protolith and 30-40% melting for dehydration melting and up to ~1050 °C and 50-60% melting for fluid-present melting. The data also show that 0.7-0.8 wt% TiO 2 in the protolith is needed for rutile saturation during subsolidus dehydration. Therefore, nearly all basaltic protoliths in deep-crustal settings and subduction zones will be saturated with rutile during subsolidus dehydration and low-degree melting at hydrous conditions. Archean tonalites-trondhjemites-granites (TTG) are widely accepted to be the products of lowdegree melting of metabasalts at the amphibolite to eclogite transition, with rutile being present in the residue. Comparison of natural TTG compositions with our experimental rutile solubility data indicates that the dominant TTG magmas were produced at temperatures of 750-950 °C, which requires that the partial melting occurred at hydrous conditions. Models involving melting at the base of oceanic plateaus are inadequate to explain TTG genesis because the plateau root zones are likely dominated by anhydrous cumulates. A slab-melting model satisfies the requirement of a hydrous metabasalt, which during subduction would melt to produce voluminous TTG melts under high Archean geothermal gradients. The geothermal gradients responsible are estimated to be between 10 and 19 °C/km based on a pressure range of 1.5-2.5 GPa for the amphibolite to eclogite transition.

Mineralogical changes in a set of phyllosilicates, differing in their layer nature, chemical composition, octahedral character, and Al content of the tetrahedral sheet, were analyzed after hydrothermal reaction in an alkaline solution. The composition of the alkaline solution was selected to simulate the first stage of cement degradation [NaOH-KOH-Ca(OH) 2 ]. The reaction products have been analyzed by XRD, 29 Si and 27 Al MAS NMR spectroscopy, SEM/EDX, and TEM. The results indicate that the main factor influencing the stability of the clays is the occupation of the octahedral sheet such that all trioctahedral members withstand the alkaline attack, whereas most of the dioctahedral clays suffer a complete dissolution and crystallization of new phases. Second, clays with Al in the tetrahedral sheet of their layers are shown to be less stable than those with a pure Si tetrahedral sheet.

The new mineral plumbophyllite, Pb 2 Si 4 O 10 ·H 2 O, is orthorhombic with space group Pbcn and cell parameters a = 13.2083(4), b = 9.7832(3), c = 8.6545(2) Å, V = 1118.33(5) Å 3 , and Z = 4. It occurs as colorless to pale blue prismatic crystals to 3 mm, with wedge-shaped terminations at the Blue Bell claims, about 11 km west of Baker, San Bernardino County, California. It is found in narrow veins in a highly siliceous hornfels in association with cerussite, chrysocolla, fluorite, goethite, gypsum, mimetite, opal, plumbotsumite, quartz, sepiolite, and wulfenite. The streak is white, the luster is vitreous, the Mohs hardness is about 5, and there is one perfect cleavage, {100}. The measured density is 3.96(5) g/cm 3 and the calculated density is 3.940 g/cm 3 . Optical properties (589 nm): biaxial (+), α = 1.674(2), β = 1.684(2), γ = 1.708(2), 2V = 66(2)°, dispersion r > v (strong); X = b, Y = c, Z = a. Electron microprobe analysis provided PbO 60.25, CuO 0.23, SiO 2 36.22 wt%, and CHN analysis provided H 2 O 3.29 wt% for a total of 99.99 wt%. Powder IR spectroscopy confirmed the presence of H 2 O and single-crystal IR spectroscopy indicated the H 2 O to be oriented perpendicular to the b axis. Raman spectra were also obtained. The strongest powder X-ray diffraction lines are [d (hkl) I]: 7.88(110)97, 6.63(200)35, 4.90(020)38, 3.623(202)100, 3.166(130)45, 2.938(312/411/222)57, 2.555(132/213)51, and 2.243(521/332)50. The atomic structure (R 1 = 2.04%) consists of undulating sheets of silicate tetrahedra between which are located Pb atoms and channels containing H 2 O (and Pb 2+ lone-pair electrons). The silicate sheets can be described as consisting of zigzag pyroxene-like (SiO 3 ) n chains joined laterally into sheets with the unshared tetrahedral apices in successive chains pointed alternately up and down, a configuration also found in pentagonite.

We present the first direct observation of high-coordinated Si and Al occurring together in a series of high-pressure sodium aluminosilicate glasses, quenched from melts at 6 GPa. Using 29 Si MAS NMR, we observe that a small amount of Al does not have a significant effect on the amount of V Si or VI Si generated, but that larger Al concentrations lead to a gradual decrease in both these species. 27 Al MAS NMR spectra show that samples with small amounts of Al have extremely high mean Al coordination values of up to 5.49, but that larger Al concentrations cause a gradual decrease in both V Al and VI Al. Although mean Al and Si coordination numbers both decrease with increasing Al contents, the weighted combined (Al+Si) coordination number increases. Silicon and Al resonances shift in frequency with increasing pressure or changing Al concentration, indicating additional structural changes, including compression of network bond angles. Increases in the 23 Na isotropic chemical shifts indicate decreases in the mean Na-O bond lengths with increasing pressure, which are more dramatic at higher Al contents. Recovered glass densities are about 10 to 15% greater than those of similar ambient pressure samples. However, the density increases due to the combined coordination changes of Al and Si are estimated to total only about 1 to 2%, and are roughly constant with composition despite the large effects of Al content on the individual coordinations of the two cations. Thus, effects of other structural changes must be significant to the overall densification. Apparent equilibrium constants for reactions involving the generation of high-coordinated species show systematic behavior, which suggests an internal consistency to the observed Si and Al coordination number shifts.

In this study, the mineral chemistry and alteration of bastnäsite-(Ce), parisite-(Ce), and synchysite- (Ce) was investigated by EMPA, TG, DTG, CHNS, and SCXRD techniques. Relevant chemical data on light and volatile elements and X-ray diffraction data useful to evaluate the effect of OH content on the unit-cell volume of such carbonates are provided. Significant amounts of OH in both the parisite-(Ce) and synchysite-(Ce) were found, and these data are the first direct evidence of hydration for these carbonates. Single-crystal X-ray diffraction data showed that hydration causes an increase in the unit-cell volume for parisite and synchysite. Parisite-(Ce) and synchysite-(Ce) OH end-members have not been described in nature so far, and the amount of water must be taken into account for evaluating changes in stability field conditions. Textural relationships and replacement processes are discussed.

In this paper, we present an experimental study of the interaction between gypsum (010) surfaces and aqueous solutions of Na 2 CO 3 with different concentrations. This interaction leads to the carbonatation (i.e., the transformation into carbonate minerals) of gypsum crystals, which under ambient conditions shows the characteristics of a mineral replacement and leads to the formation of pseudomorphs consisting of an aggregate of calcite crystals. Carbonatation progress was monitored by scanning electron microscopy (SEM) and glancing incidence X-ray diffraction (GIXRD). The carbonatation advances from outside to inside the gypsum crystal and occurs through a sequence of reactions, which involves the dissolution of gypsum and the simultaneous crystallization of different polymorphs of CaCO 3 [amorphous calcium carbonate (ACC), vaterite, aragonite, and calcite], as well as several solvent-mediated transformations between these polymorphs. The sequence in which CaCO 3 phases form is interpreted taking into consideration nucleation kinetics and the qualitative evolution of several chemical parameters in the aqueous solution. The textural characteristics of the transformed regions are described. The degree of faithfulness of the pseudomorphs obtained is related to the kinetics of the carbonatation process, which in turn depends on the initial concentration of carbonate in the aqueous solutions. Finally, changes in the rate at which the transformation front advances are discussed on the basis of both textural and physicochemical considerations.

The local electronic structure and stereochemistry of calcite, aragonite, dolomite, ferroan dolomite, manganoan calcite, synthetic carbonate hydroxylapatite (CHAP), and CaS (synthetic oldhamite) have been studied using Ca L 2,3 -edge X-ray absorption near-edge structure (XANES) spectroscopy. The XANES spectra of the calcite- and dolomite-structure carbonates are identical within error of measurement, confirming the local nature of X-ray absorption at the L 2,3 edge of Ca 2+ . The Ca L 2,3 -edge XANES spectrum of aragonite is distinct and indicates a weak crystal-field splitting of positive 10Dq. Separate Ca1 and Ca2 sites are resolved in the XANES of hydroxylapatite and CHAP: Ca1 appears to have a very weak crystal field of negative 10Dq, and Ca2 has a weak crystal field of positive 10Dq. The Ca L 2,3 -edge XANES spectrum of CaS reflects both Ca and S unoccupied 3d states, and is used to show progressive oxidation of the sulfide on exposure to air. The L 2,3 X-ray absorption edge of 3d 0 cations is associated with the 2p 5 3d 1 excited electronic state. It is, therefore, a novel technique for studying the crystal field of K + , Ca 2+ , Sc 3+ , and Ti 4+ , which do not have populated 3d orbitals in their ground state.

An understanding of the thermodynamics of triple-chain silicate phases can offer insight into their formation in nature. Because the more common Mg triple-chain silicates have not yet been synthesized, this study focused on the Na-analogue to clinojimthompsonite (Na-cjt), which forms readily in a laboratory setting. Synthetic Na-cjt was characterized using electron microscopy and microanalysis, thermogravimetric analysis, infrared spectroscopy, and loss on ignition to determine its water content. The results of these studies indicated a formula for Na-cjt of Na 3.74 Mg 8.13 Si 12 O 32.00 (OH) 4 ·1.40H 2 O, although there was substantial uncertainty (±13%) associated with the total water (OH + H 2 O) content. A series of experimental reversals was performed over a temperature range of 350-650 °C and a pressure range of 0.1-1 GPa for the reaction between Na-cjt and a double-chain silicate. In the pressure range of 0.2-0.5 GPa, this reaction has the stoichiometry Na 3.74 Mg 8.13 Si 12 O 32.00 (OH) 4 ·1.40 H 2 O (Na-cjt) = 1.5 Na 2.49 Mg 5.42 Si 8 O 21.34 (OH) 2.66 (Na-Mg amph) + 1.40 H 2 O. The equilibrium boundary reaches a maximum temperature near 550 °C at 0.4 GPa, continuing to lower temperatures above this pressure. The third-law entropy and enthalpy of formation of both Na-cjt and the associated Na-Mg amphibole were measured using adiabatic and oxide-melt solution calorimetry, respectively. Calculating the equilibrium boundary for the reaction using the calorimetrically determined values does not produce the same boundary found through the experimental reversals. The discrepancy was attributed to water variability in Na-cjt with pressure. This variability and its effect on Na-cjt’s thermodynamic values can be expressed by the following equations that, for simplicity, indicate the total water content (OH + H 2 O) as moles of H 2 O in Na-cjt over the range of 0.1 to 1.0 GPa: ν H₂O (moles of H 2 O in Na 3.74 Mg 8.13 Si 12 O 30.60 ·ν H₂O ) = 11.06 + 28.36P - 5.25P 2 - 29.01P 0.5 (P in GPa) ΔH° f of Na 3.74 Mg 8.13 Si 12 O 30.60 ·ν H₂O (kJ/mol) = -295.5ν H₂O ,Na-cjt - 17 408.8 S° of Na 3.74 Mg 8.13 Si 12 O 30.60 ·ν H₂O [J/(K⋅mol)] = 43.4ν H₂O ,Na-cjt + 756.5 V of Na 3.74 Mg 8.13 Si 12 O 30.60 ·ν H₂O [J/(bar·mol)] = 1.46ν H₂O ,Na-cjt + 36.90 where the entropy does not include any contributions from configurational mixing. If one includes configurational entropy contributions from cation and vacancy disorder in the triple-chain silicate and amphibole, one obtains S° and ΔH° f for Na-cjt of 919.2 J/(K⋅mol) and -18 417.6 kJ/mol, respectively, and S° and ΔH° f for Na-Mg amphibole of 650.9 J/(K⋅mol) and -11 919.7 kJ/mol, respectively.

The compression behaviors of δ-AlOOH and δ-AlOOD were investigated under quasi-hydrostatic conditions at pressures up to 63.5 and 34.9 GPa, respectively, using results from synchrotron X-ray diffraction experiments conducted at ambient temperature. Because of the geometric isotope effect, at ambient pressure, the a and b axes of δ-AlOOD, which define the plane in which the hydrogen bond lies, are longer than those of δ-AOOH. Under increasing pressure, the a and b axes of δ-AlOOH stiffen at 10 GPa, although the c axis shows no marked change. Identical behavior was found in δ-AlOOD, but the change in compressibility was observed at a slightly higher pressure of 12 GPa. Axial ratios a/c and b/c first decrease rapidly with increasing pressure, then begin to increase at pressures >10 GPa in δ-AlOOH and >12 GPa in δ-AlOOD. At these pressures, the pressure dependence of a/b also changes from increasing to decreasing. The unit-cell volumes of δ-AlOOH and δ-AlOOD become slightly less compressible at high pressures. Assuming K 0 ′ = 4, the calculated bulk moduli of δ-AlOOH below and above 10 GPa are 152(2) and 219(3) GPa, respectively. Those of δ-AlOOD below and above 12 GPa are 151(1) and 207(2) GPa, respectively.

A complete crystal-chemical characterization of erionite-K from Rome, Oregon, was obtained by combining field emission scanning electron microscopy, laboratory parallel-beam transmission powder diffraction, and 57 Fe Mössbauer spectroscopy. Rietveld refinement results evidenced that the most striking difference in comparison with the structure of erionite-Ca is significant K at a K2 site (½, 0, 0), which is empty in erionite-Ca. In addition, site Ca1 shows low occupancy and Ca3 is vacant. The oxidation and coordination state of Fe, whose occurrence was revealed by chemical analysis, have been clarified by exploiting room- and low-temperature 57 Fe Mössbauer spectroscopy. The majority of Fe (95%) was attributed to Fe 3+ -bearing, superparamagnetic, oxide-like nanoparticles with dimensions between 1 and 9 nm, and the remaining 5% was attributed to hematite particles with size ≥10 nm, both located on the crystal surface.

We have investigated the antigorite m = 17 structure [Mg 48 Si 34 O 85 (OH) 62 ] by density functional theory (DFT), with the aim to probe the method on such a large and low symmetry (Pm) system. We found a satisfactory match with the experiments using both LDA and GGA approximations to exchangecorrelation, although the former performs slightly better here. Predicted cell constants are within 0.3% for LDA, and 0.5% for GGA, of the experimental values. Average atomic displacements after relaxation are within ~0.06 Å. All the fine structural details of antigorite are reproduced: apical Si-O bonds shorter than basal Si-O bonds; external Mg-O distances shorter than internal Mg-O distances; pronounced tetrahedral ditrigonal distortion. Where palpably biased bond distances were present in the experimental data because of disorder, theoretical methods promptly recover the structure into more reliable bond geometry. These findings let one envisage the employment of DFT calculations as a promising tool for refining or validating complex structures, whenever the experiments suffer of limitations due to the poor quality of the material being investigated. As an additional benefit, we also compare the total energies of two competing models for the m = 17 antigorite structure, the one refined by Capitani and Mellini (2004) and that proposed by Dódony et al. (2002). We found the former more stable, both as published (ΔE = 1.1 kJ/mol·atom -1 ) and after full cell relaxation at constant volume (ΔE = 0.5 kJ/mol·atom -1 ).

In light of the controversy surrounding the origin of sulfate-rich rocks analyzed by the Mars Exploration Rover Opportunity, thermodynamic and kinetic models explored the consequences of in situ alteration of basaltic sand by pristine sulfuric acid. Simulations at 273 K and current martian atmosphere show that it is possible to simultaneously account for both chemical and mineralogical observations at the Meridiani landing site, but only when the amounts of water are small (water/rock mass ratio ≤ 1), the aqueous solutions are highly acidic (pH < 3), and the lifetimes of liquid water are extremely short (on the order of tens of years). Furthermore, the best agreement between observations and models is obtained if evolved fluids are removed after alteration. If this simple self-consistent scenario is relevant to bedrock formation at Meridiani, it provides stringent constraints on the issues of where and when liquid water was present at the surface of Mars.

Several papers reporting exotic native elements have been published within the last few decades. The “native” occurrences described are rather dubious in view of the lack of solid proof of their relationships with the host-rock minerals. Consequently, the genetic models proposed ranging from bio-reduction to the influence of deep-mantle, strongly reduced fluids, are somewhat speculative. Here we present data for a unique Al 0 flake protruding from the phlogopite matrix of a rock specimen collected from a desilicated pegmatite vein. The geologic setting suggests two processes that might have played a key role in the Al 0 formation: (1) desilication of pegmatite, resulting in its Al residual enrichment; and (2) serpentinization of an ultramafic body, providing a strongly reduced front (H 2 and hydrocarbons) toward the serpentinite/pegmatite contact. These processes have presumably led to the reduction of Al to Al 0 at discrete sites of alumina-rich minerals.

Quartz solubility in H 2 O-CO 2 fluids was measured at 800 °C and 10 kbar. Mixed fluids were generated from hydrous oxalic acid, silver oxalate, silver carbonate, and liquid H 2 O; solubility was determined by weight changes of the quartz crystals. Stringent blank tests and weighing procedures were used to establish the CO 2 and H 2 O contents of experimental fluids. Using experimentally constrained models of H 2 O activity and mixing of silica monomers and dimers, a logarithmic plot of monomer activity vs. H 2 O activity yields a linear data array of constant slope n insignificantly different from 4 (n = 4.024, R 2 = 0.997), where n is the total monomer hydration number. Moreover, all high-quality quartz solubility data in H 2 O-CO 2 fluids at lower temperature and pressure fall on the same line when SiO 2 and H 2 O activities are calculated with this formulation. The same analysis for the dimer yields slope n of 7 (n = 7.049, R 2 = 0.996). Our results show that neutral silica monomers and dimers have fixed stoichiometry of Si(OH) 4 ·2H 2 O and Si 2 O(OH) 6 ·4H 2 O (i.e., two solvating H 2 O molecules per Si) in H 2 O-CO 2 fluids for a large range of crustal metasomatic processes. This provides a simple basis for modeling H 2 O-CO 2 -SiO 2 fluids over a range of pressure, temperature, CO 2 , and SiO 2 activity.