Volume 87, Issue 10

Franckeite, approximately Pb 4.6 Ag 0.2 Sn 2.5 Fe 0.8 Sb 2 S 12.6 , consists of alternating pseudohexagonal (H) and pseudotetragonal (Q) layers. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) of freshly cleaved franckeite, from San José, Bolivia, revealed the atomic structure of the pseudohexagonal component. On AFM images, the expected pattern with b = 3.2 Å was observed. STM revealed a √3 × √3 superstructure, with b' = 6.3 Å, interpreted to be caused by tunneling effects. The pseudotetragonal layer was not identified in any images. Layer modulation, which results from the non-commensurate fit of the alternating H and Q layers, was observed with both AFM and STM. Modulation waves are sinusoidal and regular and they are always parallel. The calibrated modulation wavelengths averaged to 3.77, 4.10, 4.45, and 4.74 nm (with uncertainty ≤0.10 nm) corresponding to pseudotetragonal/pseudohexagonal (Q/H) matches of 13/12, 14/13, 15/14, and 16/15, respectively. These correspond with observations made using bulk analytical methods on individual members from the franckeite-cylindrite family but scanning probe microscopy (SPM) was able to show that Q/H match varies on a local scale, with sharp domain boundaries. Domains can be on the order of 150 nm in width.

The reaction of the (101̄4) calcite surface during dissolution was studied as a function of pH (HCl) by liquid-cell Atomic Force Microscopy (AFM) under ambient conditions of both temperature (22 °C) and partial pressure of carbon dioxide (P CO2 ). A flow through AFM liquid reaction cell was used, and solutions were renewed at a controlled flow rate and sampled for chemical analysis. In the pH range 7.5-4.3, surface dissolution proceeds via the formation of etch pits delimited by steps of height 3.3 ± 0.3 Å or a multiple of this elementary distance. Rhombohedral pits form and grow at the surface by anisotropic step retreat, at a velocity of 4.2 ± 0.2 nm/s for the steps oriented along the two equivalent directions [4̄41] + and [481̄] + , and 1.1 ± 0.2 nm/s for the steps oriented along the other two equivalent directions [4̄41] - and [481̄] - . At the pH value of 2.7, the slower rate slightly increases and is equal to 1.6 ± 0.2 nm/s. At the pH value of 1.7, microtopography measurements indicate that micrometric steps have a meandering shape, and move as a family at a velocity higher than 10 nm/s. These meandering steps are made of nanometric segments oriented along [4̄41] + , [481̄] + , [4̄41] - , and [481̄] - , and they terminate their run against sloped rough etch pits with sides made of narrow terraces delimited by steps oriented along the [481̄] + . Finally, the present study indicates that reactive area (i.e., the total amount of reactive atomic sites) of a calcite surface increases in response to higher solution acidity firstly by an increase in the step density, and secondly by production of additional roughness at step edges. The change in step edge-velocity observed below pH = 2.7 was interpreted as the response of the dissolving surface to the loss of buffering capacity at the Stern layer.

The theoretical infrared spectrum of lizardite [Mg 3 Si 2 O 5 (OH) 4 ] was computed using first-principles quantum mechanical calculations. Density functional perturbation theory allowed us to derive the low-frequency dielectric tensor of lizardite as a function of the light frequency. The infrared spectrum was then calculated using a model that takes into account the platy shape of particles. A very good agreement was obtained between theory and experiment. This agreement allows us to make an unambiguous assignment of the major absorption bands observed in the IR spectrum of lizardite, including the stretching bands of OH groups.

Powder infrared spectra of perovskites across the CaTiO 3 -SrTiO 3 solid solution have been collected at room temperature for the range 20-700 cm -1 . Frequency, intensity, and line broadening parameters vary with a pattern that appears to follow the development of the orthorhombic shear strain for the Pnma structure. The data are at least consistent with the Pm3m ↔ I4/mcm transition at Sr-rich compositions being continuous in character and the tetragonal ↔ orthorhombic transition at ~62% SrTiO 3 content being discontinuous. In spite of the fact that crystals with intermediate composition have recently been shown to have Pnma rather than Cmcm symmetry, autocorrelation analysis of their IR spectra shows that they have features which distinguish them from crystals with Pnma symmetry for CaTiO 3 rich compositions. In particular, crystals of intermediate composition have the highest values of the line broadening parameter, △corr, for the solid solution, suggesting that they are characterized by relatively high degrees of local structural heterogeneity.

Mössbauer spectra (MS) at room temperature (RT) and Fourier-transform infrared (FTIR) spectra in the OH stretching region were acquired for natural spessartine-almandine garnet samples from different Brazilian pegmatites, including the complex, zoned Alto Mirador pegmatite, the simple, zoned Escondido pegmatite, and the simple Poaiá pegmatite. From MS, it is obvious that in the samples from the Alto Mirador pegmatite, ferrous iron is present at the dodecahedral site and ferric iron at the octahedral site of the garnet structure. The Fe 2+ /Fe 3+ ratio is comparable for all samples from this pegmatite, i.e., ~10%, implying the same geological history, namely similar oxygen fugacities at the moment of garnet formation. In the simple pegmatites, however, almost no ferric iron was detected. On the basis of the multi-band FTIR spectra the nature of the point defects involved in the process of hydrogen uptake in spessartine-almandine garnet has been elucidated, if not completely clarified. The behavior of the absorption bands provides no evidence for multiple incorporation mechanisms, hence the hydrogarnet substitution is proposed to be the only mechanism for the incorporation of hydrogen in the samples studied. As geological setting is difficult to separate from chemical composition, FTIR spectra have the potential to provide useful information on some aspects of the geological history of the samples.

Disordering of Si and Al in natrolite, scolecite, mesolite, and gonnardite was investigated with 29 Si and 27 Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The results indicate that with the exception of one sample of natrolite (from San Benito County, California), the natrolite, mesolite, and scolecite samples studied all exhibit small degrees (<10% Al occupancy of Si sites) of Si-Al disorder. The spectra for these samples are consistent with Al avoidance. Gonnardite is confirmed to have extensive Si-Al disorder, with only slight preferential Si occupation of the T1 site. Fits of 29 Si MAS NMR spectra and mathematical relations based on Al avoidance were used to calculate Si and Al occupancies across the tetrahedral sites in these minerals. Configurational entropies arising from Si-Al disorder in natrolite, mesolite, and scolecite can add an addition 1-2% [up to 11 J/(mol·K)] to the total entropies of these phases at 298.15 K, whereas it may add as much as 7% to that of gonnardite [up to 27.7 J/(mol·K)]. These results also concur with previous observations of a gap in Si-Al disordering between orthorhombic and tetragonal natrolite samples and suggest that the state of disorder in natrolite is a function of temperature. The 29 Si MAS NMR spectrum of gonnardite is consistent with a disordered natrolite framework structure, and not an intergrowth of thomsonite and natrolite structural domains.

Synchrotron radiation S K- and L-edge X-ray absorption near-edge structure (XANES) spectra are reported for the cubic, rocksalt (B1) structure sulfides niningerite (MgS), alabandite (MnS), and oldhamite (CaS), and for their solid solutions (Mn,Fe)S and (Mg,Mn)S, and S L-edge XANES spectra are reported also for (Mg,Fe)S solid solutions. Pre-edge features at the S K-edge are attributed to transition of S 1s electrons to the lowest available unoccupied S 3p σ* antibonding states hybridized with metal 3d(e g ) states, and at the S L-edge to transition of S 2p electrons to unoccupied S 3s σ*, 4s σ*, and 3d antibonding states hybridized with metal 3d(e g ) states, and to a lesser extent 3d(t 2g ) states. The S K-edge XANES spectra for the solid solutions show a progressive participation of 3d orbitals in metal-S bonding with increase in substitution by Fe in (Mn,Fe)S and (Mg,Fe)S and Mn in (Mg,Mn)S through progressive increase in the area of the pre-edge feature. However, the pre-peak area does not increase linearly in each solid solution series showing that a real change in bulk electronic properties has occurred. Increase in pre-peak area reflects an increase in overall attainability of metal 3d states for hybridization with S 3p σ* antibonding states as proportionally more metal 3d orbitals become available. The S L-edge XANES spectra show progressive evolution of pre-edge features at the L 3 - and L 2 -edges (a 1 and a 2 , respectively). Only a 2 is present in the S L-edge XANES spectrum of FeS (troilite), and with progressive decrease in Fe content in (Mn,Fe)S and (Mg,Fe)S solid solutions, a 1 first appears, then becomes dominant. Since a 1 is attributed to transition of S 2p 3/2 electrons to S 3s σ* states hybridized with metal 3d(e g ) and 3d(t 2g ) states, this appears to represent an increased contribution from metal-S π-bonding. The results show that the size and position of the pre-edge features to the S K- and L-edges are controlled more by the DOS of hybridized 3d(e g β ) and 3d(t 2g β ) states and nearest-neighbor coordination of the metal atoms than by the precise coordination of S and the extended structure of the sulfide. The full multiple scattering approach has been applied to the calculation of the S K-edge XANES spectra of MgS, MnS, and CaS. Results are consistent with experimental XANES spectra, especially for the pre-edge features, which are often neglected in such calculations.

Room-temperature, single-crystal W-band (~94 GHz) electron paramagnetic resonance (EPR) spectra of a flux-grown fluorapatite (AP30-0) containing 0.8(1) ppm Gd and 0.9 ppm Mn disclose the presence of two even-isotope Gd 3+ centers (electron spin number S = 7/2 and nuclear spin number I = 0) and a 55 Mn 2+ center (S = I = 5/2). The relative abundance of the two Gd 3+ centers (corresponding to recently established centers “a” and “b” containing Gd 3+ ions at the Ca2 and Ca1 sites, respectively) in AP30-0 has been estimated to be 0.20, indicating that “a” in this sample arises from the presence of ~0.2 ppm Gd at the Ca2 site. In addition, high-resolution W-band spectra of this sample at ~120 and ~77 K disclose the 155 Gd and 157 Gd spectra of “a,” in which these isotopes (I = 3/ 2) are only ~0.02 ppm in abundance. To the best of our knowledge, this is the first-ever demonstration of structural characterization of sub-ppm-level trace elements in minerals and their synthetic analogs. Moreover, the fact that the Gd 3+ and 55 Mn 2+ centers in AP30-0 are detected despite the multiplicity of lines arising from their complex fine structures, hyperfine structures, and magneticsite splittings, suggests that the W-band EPR technique is potentially capable of characterizing trace elements with a single unpaired electron (S = 1/2) and zero nuclear spin (I = 0) at even lower concentrations. The spin-Hamiltonian parameters of the 55 Mn 2+ center, including matrices g, D, A, and P, and high-spin term of type S4, have been determined by optimization using the single-crystal W-band EPR spectra of a Gd-doped fluorapatite containing 3.0(4) ppm Mn. The principal-axis directions of D and the pseudo-symmetry axes calculated from the S4 parameters confirm that this center corresponds to occupancy of 55 Mn 2+ ions at the Ca1 site. Also, the optimized parameters suggest that the Mn 2+ -substituted Ca1 site in the flux-grown fluorapatite has rhombic (i.e., triclinic) local symmetry [e.g., D/g e β e = 436.2(6) G, E/g e β e = 1.1(1) G], slightly different from the trigonal symmetry of the ideal Ca1 site.

The P-T conditions of both equilibria were determined precisely by reversal experiments in a piston-cylinder apparatus. On the basis of 8 experiments, brackets for the rutile-magnesite-geikielite equilibrium are 7.0-7.1 kbar at 800 °C, 8.6-8.7 kbar at 850 °C, and 10.5-10.7 kbar at 900 °C. On the basis of 9 experiments, brackets for the zircon-magnesite-baddeleyite-forsterite equilibrium are 7.1- 7.7 kbar at 800 °C, 9.2-9.4 kbar at 850 °C, and 10.7-10.9 kbar at 900 °C. Considering experimental uncertainties in P (±300 bars) and T (±3 °C), equilibrium curves calculated from both the Berman and the Holland and Powell databases pass through all brackets. Molar Gibbs free energy of formation from the elements at 1 bar and 298 K for geikielite and zircon, derived from the experiments and consistent with the Berman database, are -1481.94 ± 0.67 kJ and -1917.54 ± 1.25 kJ, respectively. Corresponding values consistent with the Holland and Powell database are -1479.30 ± 0.74 kJ and -1918.47 ± 1.49 kJ. Application of the two equilibria indicate that: (1) the mole fraction of CO 2 in fluid was 0.54-1.00 when geikielite and baddeleyite formed during contact metamorphism of siliceous dolomites in the Ballachulish aureole, Scotland; (2) the activity of CO 2 could have been as low as 2.10 -5 during ultra-high pressure metamorphism of magnesite-bearing ecologites; and (3) the activity of CO 2 was <0.18 during one instance of mantle metasomatism.

The martian basalts Los Angeles, QUE 94201, Shergotty, and Zagami contain several late-stage mineral types including oxides, sulfides, phosphates, and associated silicate assemblages. Symplectites consisting of two- and three-phase assemblages are present in Los Angeles, QUE 94201, and Shergotty. The three-phase symplectite is composed of hedenbergitic pyroxene, fayalitic olivine, and an SiO 2 polymorph, and the two-phase symplectite consists of fayalitic olivine and an SiO 2 polymorph. These symplectites are commonly found in close association with the Ca-phosphate merrillite [general formula Ca 9 (Mg,Fe 2+ )(PO 4 ) 7 ]. Scanning Electron Microscopy was used to examine the distribution and occurrence of the symplectites, and point-count and electron-microprobe data were used to recast symplectites in terms of pyroxene stoichiometry. The reconstructed three-phase symplectite compositions plot close to lunar (metastable) pyroxferroite on the pyroxene quadrilateral diagram and indicate that pyroxene compositions ranged into the “forbidden region” and eventually crystallized pyroxferroite, which subsequently broke down to the three-phase symplectite upon cooling. Twophase symplectites, which occur directly adjacent to merrillite, may be the result of breakdown of metastable ferrosilite. The crystal chemistry of merrillite and its close association with the symplectites indicates that during crystallization of pyroxene, co-crystallizing merrillite contributes to depletion of Ca and Mg and the formation of metastable pyroxferroite and ferrosilite.

X-ray fluorescence elemental maps and Cu K-edge X-ray absorption near-edge structure (XANES) spectra were recorded for individual vapor and brine phase fluid inclusions from the Mole Granite, NSW, Australia. The maps indicate that Cu is concentrated in the vapor inclusions and, at room temperature, distributed uniformly in the condensed liquid. Opaque precipitates in these inclusions do not contain Cu. The absorption spectra identify the stable complexes as [Cu(H 2 O) 6 ] 2+ at 25 °C, [CuCl 2 ] 1- at 200 °C, and either [CuCl 2 ] 1- or [CuCl(H 2 O)] at the homogenization temperature of around 400 °C. These changes in Cu coordination and oxidation state are fully reversible. We suggest that the vapor phase partitioning of Cu as a chloride complex from a high-density brine may occur under acidic conditions. Estmates of fluid acidity at the time of boiling may potentially predict the metal distribution between epithermal and porphyry-type environments in hydrothermal systems.

We investigated experimentally trace-element partitioning between clinopyroxene and anhydrous silicate melt at conditions relevant to near-solidus melting of quartz eclogite in the upper mantle. Partitioning experiments were carried out at 3.0 GPa and 1335-1365 °C on a synthetic mixture similar to a low-degree melt of a quartz eclogite. Clinopyroxene is Al 2 O 3 -rich (~17 wt%), cation deficient (~3.92 cations per six O atoms), and coexists with andesitic melt (~57 wt% SiO 2 ), quartz, and traces of rutile. The clinopyroxene cation deficiency indicates a large proportion of vacant M2 sites in form of the Ca-Eskola (CaEs) component (Ca 0.5 □ 0.5 AlSi 2 O 6 ). Clinopyroxene-melt partition coefficients (D-values) were determined for Sr, Y, Zr, Nb, REE, Th, and U by SIMS, and for Sc, V, Cr, Mn, Co, Ni, K, and major elements by electron microprobe analysis. Compared to previous studies of clinopyroxene with >10 wt% Al 2 O 3 , partition coefficients in vacancy-rich aluminous clinopyroxene for elements substituting into the M2 site are elevated for monovalent cations (D Na ~ 1.0, D K = 0.027), and reduced for trivalent cations, particularly for LREE (D La = 0.029). D Th and D U range from 0.0017 to 0.0037, much lower than previously assumed for clinopyroxene pertaining to partial melting of eclogite in the upper mantle, and D U /D Th is apparently >1. Observed partitioning behavior of elements substituting into M2 is attributable to the low average charge (1.53- 1.57) of the site. This low average charge increases the number of configurations available to charge balance 1+ cations and vacancies, and decreases the number of configurations available to charge balance cations with high valences. Stablization of low-valence species suggests that these vacancyrich clinopyroxenes may be substantial hosts of noble gases in the mantle. Partition coefficients for cations substituting into the M1 site show relatively little effect from the high vacancy concentrations of M2, but partitioning systematics may be obscured by strong crystal field stabilization of some cations in the M1 site, notably Ni 2+ and Cr 3+ . Application of the measured D-values of this study to partial-melting calculations results in small increases in the Sm/Yb of liquids produced by modest extents of melting of quartz eclogite. Significant enrichments in ( 230 Th)/( 238 U) activity ratios (>1.21) are expected for decompression melting of quartz eclogite at upwelling rates below 5 cm/ year, but enrichments would be small (<1.11) above 10 cm/year.

Alkali feldspar grains found in a syenite from the Patagonian Andes, southern Chile, have bulk compositions of about Or 40 Ab 59 An 0.5 , and consist of two parts when viewed under an optical microscope: a clear part and a turbid part. Grain interiors are mixtures of the two parts, whereas the rims are mainly turbid. The microscopically clear part, which is almost free of micropores, is cryptoperthitic, whereas the turbid part is microperthitic. The microperthite is of the patch type, and the turbidity is due to abundant micropores that are polygonal and generally less than 1 mm in diameter. The patch microperthite has been formed by coarsening of primary cryptoperthite by hydrothermal reactions. An incipient stage of the microperthite formation is recorded as the segregation of the Or-rich feldspar with diagonal elongation and Ab-rich feldspar into aggregates that have a “butterfly” shape. Each butterfly aggregate of microperthite is generally less than 10 mm in length. The centers of the butterfly aggregates are usually occupied by round fluorite particles about 1 mm in diameter, which were identified by EPMA and TEM analyses. The fluorite particles may have been formed at the fluid stage. The microperthite formation may have started as butterfly aggregates along the interfaces with the fluorite particles at the hydrothermal stage. The butterfly aggregates have changed to patch microperthite with further coarsening. The timing and process of the formation of the fluorite particles are important in relation to the evolution of feldspar microtextures, and the behavior of fluorine in alkaline igneous rocks.

While characterizing the mineralogy of the kimberlite at Venkatampalle, Andhra Pradesh, India (Lat. 14° 56′ 00″ N, Long. 77° 22′ 15″ E), we found the second occurrence of zemkorite (Na,K) 2 Ca(CO 3 ) 2 , which was first reported in 1989 in the Udachnaya kimberlite in Siberia. We also report the first Fourier-transform infrared spectroscopic (FTIR), scanning electron microscopic, thermo-gravimetric, and high-temperature heat capacity measurements on this rare carbonate. Powder X-ray diffraction (XRD) patterns show 24 well-resolved diffraction lines, all of which can be indexed with a hexagonal cell with a = 10.038(5) Å and c = 12.726(5) Å. The vibrational spectrum of zemkorite at room temperature exhibits 13 distinct absorption bands in the frequency range 2000 to 400 cm -1 . The IR bands of zemkorite indicate a structural similarity with shortite but are quite distinct from other alkali carbonates such as nyerereite, butschiite, and fairchildite. The heat capacity of zemkorite has been measured up to 700 K by differential scanning calorimetric techniques. The temperature dependence of the heat capacity of zemkorite was fitted with the polynomial C p = 140.2 + 8.584 × 10 -2 T - 2.458 × 10 6 T -2 . The upper thermal stability of zemkorite is ~700 K, which is similar to shortite. Zemkorite may have formed during the late stages of kimberlite genesis, possibly as a result of metasomatism or by the breakdown of metasomatic natrocarbonatitic minerals or glass that segregated on decompression melting in the upper mantle.

Experiments carried out between 280 and 980 °C demonstrate that dihedral angles for galena, θ gn , in sphalerite-galena-sphalerite triple-junctions decrease with increasing temperature, and that the rate of change increases in the same direction. Similar behavior is evident for sphalerite, θ sp , in galena-sphalerite-galena triple-junctions, and also for pyrrhotite, θ po , in sphalerite-pyrrhotite-sphalerite triple-junctions. Triple-junction thermometry (TJT) is therefore most sensitive at high to very high temperatures where isotope thermometers are least sensitive. The method relies on the temperaturedependence of competitive surface tensions between shared surfaces of intergrown minerals. Because chemical interaction is not a prerequisite, the TJT method is potentially applicable to a variety of mineral pairs found in regional metamorphic situations. The method requires a slightly modified microscope equipped with a precision X-Y stage. The application of θ gn -T (temperature) and θ po -T calibrations to regionally metamorphosed sulfide ores yields temperatures that compare reasonably with temperatures obtained from sulfur isotopes and other geothermometers. Maximum regional metamorphic temperatures of ~470-480, ~590, and ~700 °C were obtained for the Bathurst (New Brunswick = greenschist facies), Ruttan (Manitoba = amphibolite facies) and Broken Hill (N.S.W. = granulite facies) deposits, respectively. The θ gn and θ po thermometers also reveal recrystallization effects in microfabrics.

The solubility of calcite in NaCl-H 2 O solutions was measured at 600-900 °C, 10 kbar, at NaCl concentrations ranging from dilute to near halite saturation, and at 6-14 kbar, 700 °C, in 30 mol% NaCl solutions. Solubility was determined from the weight loss of cleavage rhombs of a pure natural calcite after experiments of 1/2 to 6 days in a piston-cylinder apparatus with NaCl-graphite furnaces. CaCO 3 molality (m CaCO₃ ) increases greatly with NaCl mole fraction (X NaCl ): at 800 °C and 10 kbar, m CaCO₃ increases from ~0.1 in pure H 2 O to near 4.0 at halite saturation (X NaCl ~ 0.6). There is also a large temperature effect at 10 kbar, with m CaCO₃ increasing from 0.25 at 600 °C to 3.0 at 900 °C at X NaCl = 0.3. There is only a 20% increase with increasing pressure between 6 and 14 kbar at 700 °C and X NaCl = 0.3. Melting to a carbonate-rich liquid was inferred at 900 °C, 10 kbar, from X NaCl of 0 to 0.2. The composition, temperature, and pressure dependence of m CaCO₃ are described by: m CaCO₃ = [-0.051 + 1.65 × 10 -4 T + X 2 NaCl exp(-3.071 + 4.749 × 10 -6 T 2 )] (0.76 + 0.024P) with T in Kelvins and P in kbar. The predicted increase of calcite solubility with salinity and temperature is so great that critical mixing of NaCl-rich hydrous carbonate liquid and CaCO 3 -rich saline solution is probable at 10 kbar near 1000 °C and X NaCl ~ 0.4. The experimental results suggest a genetic mechanism for the enigmatic carbonated shear zones, such as the Attur Valley of southern India, where crustal rocks have been replaced by up to 20% by calcite and ankerite with mantle-like stable-isotope signatures. The high CaCO 3 carrying capacity of concentrated alkali-chloride solutions, together with the drastic decrease in solubility between 1000 and 700 °C, plausibly account for large-scale emplacement of mantle-derived carbonate from concentrated chloride-carbonate solutions (or hydrosaline magmas) formed as immiscible fluids in the late stages of alkalic magmatism. Such solutions may also mobilize sulfate and phosphate minerals, which would have important consequences for redistribution of incompatible and heat-producing elements in the crust.

The evolution of the unit-cell parameters of stottite [FeGe(OH) 6 ], a compound with a tetragonal octahedral framework related to the perovskite structure, has been determined to a maximum pressure of 7.8 GPa by single-crystal X-ray diffraction at room temperature. Stottite does not exhibit any phase transitions in this pressure range. A fit of a third-order Birch-Murnaghan equation of state to the pressure-volume data yields values of V 0 = 425.67(2) Å 3 , KT0 = 78.4(3) GPa and K 0 ' = 6.18(10). Analysis of the unit-cell parameter data shows that c is approximately 10% more compressible a. Compressional moduli for the axes are K a0 = 81.3(3) GPa and K' a0 = 6.4(1), K c0 = 73.3(6) GPa and K' c 0 = 5.7(2). We relate these axial compressibilities to the structure of stottite, which, unlike related cubic protonated octahedral frameworks such as burtite [CaSn(OH) 6 ], is expected to have a highly anisotropic hydrogen-bonding topology: a high degree of hydrogen-bonded connectivity parallel to (001) and very little parallel to [001]. Enhanced hydrogen bonding within the (001) plane may stiffen the structure along <100>. We also make some provisional comparisons with structural and elasticity data for perovskites and show that the absence of a central, non-framework cation in the stottite structure allows octahedral tilts in excess of 40°. The stottite structure is much softer than any known oxide perovskite. The relative importance of an empty cavity site vs. the role of hydrogen bonding is likely to be a major issue in understanding the compressional behavior of protonated octahedral frameworks.

Pressure-induced structural modifications in the zeolite bikitaite are studied by means of in situ synchrotron X-ray powder diffraction and ab initio molecular dynamics. The experimental cell parameters were refined up to 9 GPa, at which pressure we found reductions of 4.5, 4.5, 6.3, and 15% in a, b, c, and V, respectively. Minor variations were observed for the cell angles. Complete X-ray amorphization is not achieved in the investigated P range, moreover the P-induced effects on the bikitaite structure are completely reversible. Because it was possible to extract only the cell parameters from the powder patterns, the atomic coordinates at 5.7 and 9.0 GPa were obtained by means of Car-Parrinello simulations using the unit-cell parameters experimentally determined at these pressures. Analysis of the computational results for increasing pressures showed that the volume contraction is essentially due to rotations of the tetrahedra; the 8-ring channels become more circular; the pyroxene chain becomes more corrugated in the b-c plane; and the mean Li-O bond distances and coordination polyhedral volumes decrease with increasing pressure without significant distortion of the internal angles. The peculiar aspect of the bikitaite structure, i.e., the presence in the channels of a “floating” one-dimensional water chain, is only partially maintained at high pressure; the compression brings framework O atoms close enough to water hydrogen atoms to allow the formation of host-guest hydrogen bonds, without, however, destroying the one-dimensional chain.

The zeolite bikitaite, Li 2 [Al 2 Si 4 O 12 ]·2H 2 O, has structural channels containing infinite chains of H 2 O molecules running parallel to [010]. One hydrogen atom of an H 2 O molecule is weakly hydrogen bonded to an O atom of a neighboring molecule, while the other hydrogen atom is unbonded. The molecules are ordered and the chains they form have been called “one-dimensional ice.” Polarized Raman spectra of single crystals in the wavenumber range 40-4000 cm -1 were measured from 5 to 625 K. At low temperatures, four different O-H stretching vibrations can be observed between 3330 and 3600 cm -1 , as well as H 2 O bending vibrations at about 1640-1650 cm -1 . The two lower wavenumber hydrogen-bonded O-H stretching modes increase in wavenumber with increasing temperature, while the higher wavenumber non-hydrogen-bonded OH modes decrease in wavenumber. The temperature dependence of the linewidths of the O-H stretching modes and the degree of hydrogen bonding between neighboring H 2 O molecules show that the main cause of line broadening is modulation of the OH potential from low-energy thermal O···O vibrations in the H 2 O chains. At elevated temperatures, the different O-H stretching modes become similar in energy and only a single symmetric H 2 O stretching band is observed above 520 K. At these temperatures the H 2 O molecules lose their hydrogen bonding and are only bonded to Li cations at the walls of the channels.

We report major element, boron, lithium, and water (present as structurally bound hydroxyl) contents for a suite of ten mantle-derived olivine crystals, measured by electron microprobe, secondary ion mass spectrometry, and Fourier-transform infrared spectroscopy. Water measurements are based on re-analysis and/or re-processing of data previously reported in the literature. Analyzed olivines have lithium, boron, and water concentrations of 0.9-7.8 and 0.01-67, and 0.8-61 ppm (by weight), respectively. One olivine from Kingiti, Tanzania (possibly derived from metasomatized peridotite) is anomalous, with boron and lithium contents of 67 and 7.8 ppm, respectively. The remaining olivine samples have lithium and boron contents below 3 and 1 ppm, respectively. Although lithium, boron, and water contents vary substantially, their cation proportions are not strongly correlated, arguing against simple coupled substitutions involving these elements. Importantly, the incorporation of boron and water in olivine by the B(F,OH)Si -1 O -1 substitution does not appear to be a universal feature of mantle olivine, although it may be significant in those with the highest boron contents. Our data also support suggestions that olivine may be an important reservoir for hydrogen, lithium, and boron in the lithospheric and upper asthenospheric mantle, and may thus play a key role in the geochemical cycling of these elements within the mantle, and between the mantle and crust.

The crystal structures of three tourmaline crystals: (Na 0.49 K 0.01 Ca 0.48 ) (Mg 1.35 Fe 2+ 0.94 Fe 3+ 0.49 Ti 0.20 ) (Al 4.58 Fe 3+ 0.62 Mg 0.80 ) (Si 5.99 Al 0.01 ) O 18 (BO 3 ) 3.03 (OH) 3.18 F 0.18 O 0.64 , a = 16.017(2), c = 7.256(2) Å, V = 1612.2(4) Å 3 , R3m, Z = 3; (Na 0.64 K 0.01 Ca 0.03 ) (Mn 0.18 Fe 2+ 1.71 Al 0.88 Li 0.11 Zn 0.03 Ti 0.07 ) (Al 5.67 Fe 3+ 0.28 Mg 0.05 ) (Si 5.76 Al 0.24 ) O 18 (BO 3 ) 2.99 (OH) 3.96 F 0.17 , a = 15.983(2), c = 7.152(2) Å, V = 1582.1(4) Å 3 ; (Na 0.81 K 0.01 Ca 0.01 ) (Mn 0.02 Mg 0.61 Fe 2+ 0.90 Al 0.80 Li 0.70 Zn 0.01 Ti 0.06 ) Al 6.00 (Si 5.97 Al 0.03 ) O 18 (BO 3 ) 2.93 (OH) 3.42 F 0.55 O 0..03 , a = 15.921(3), c = 7.137(2) Å, V = 1566.7(6) Å 3 , have been refined to R-indices of 1.3- 2.2% using X-ray intensity data collected with a four-circle diffractometer using MoKα X-radiation. The crystals were analyzed by electron- and ion-microprobe techniques for all major and minor elements in the crystals. Unit formulae were calculated on the basis of 31 anions (O, OH, F) and the Fe 3+ /(Fe 2+ + Fe 3+ ) ratio was calculated for electroneutrality. The refined site-scattering values and the observed <Y-O> and <Z-O> distances were used to assign site populations that are in accord with the unit formulae calculated from the electron- and ion-microprobe compositions. The B contents are equal to 3.0 apfu (atoms per formula unit) within experimental error. In two of the crystals, (OH + F) = 4.0 apfu. However, the third crystal has (OH + F) = 3.36 apfu and O 2- is dominant at the W(O1) site, and is an “oxy” tourmaline as defined by Hawthorne and Henry (1999). Non-spherical electron-density was observed at the X site, suggesting that there is some positional disorder at this site associated with occupancy of X by Ca and Na, possibly coupled with variable anion occupancy of the O1 site.

We have revised the structure model of antigorite so that they conform to observations made using high-resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction. The new models retain the original half-wave configuration proposed by Kunze (1956). The Kunze model, and all subsequent research, assumes the occurrence of four- and eightmembered silicate rings in one of the two places where there are reversals of tetrahedron orientations in the tetrahedral sheets. However, TEM images at sufficiently high resolution show no traces of such rings and only half the number of octahedral-sheet offsets as occur in the Kunze model. Using our measurements and models, we generated atom positions for antigorite unit cells having various modulation lengths and then calculated the corresponding images, which provide good matches with our experimental HRTEM images. We also characterized and described antigorite structures with different modulation wavelengths and stacking sequences. Depending on the number of polyhedra in a unit cell and the presence or absence of b/3 shifts between adjoining tetrahedral and octahedral sheets, the antigorite crystals have monoclinic or triclinic symmetry, which we call antigorite-M and antigorite-T, respectively. By resolving the tetrahedral and octahedral positions, we were able to make a direct estimate of the compositions of specific antigorite samples.

The structure of tuperssuatsiaite from the Aris phonolite in central Namibia was refined to an R index of 0.075 for 905 observed reflections [I > 2σ(I)] obtained with a four-circle X-ray diffractometer. Tuperssuatsiaite is monoclinic, C2/m, with a = 14.034(7), b = 17.841(7), c = 5.265(2) Å, and β = 103.67(4)°. Microprobe analysis gave an average composition of Na 1.87 Fe 2.14 Mn 0.48 Ti 0.14 Al 0.03 Mg 0.02 [Si 8 O 20 ](OH) 2 ·n(H 2 O), with five octahedral sites. The structure is closely related to that of palygorskite Mg 5 [Si 8 O 20 ](OH) 2 ·H 2 O. Tuperssuatsiaite consists of ribbons of SiO 4 tetrahedra linked by bands of octahedra running parallel to c. Channels occur that could be occupied by H 2 O as in palygorskite. The octahedral band contains three edge-sharing, six-coordinated sites labeled M1, M2, and M3. This band consists of alternating M3-M1-M3 and M2-M2 octahedra along [001]. The M1 and M2 sites both contain Fe and Mn, and M3 is occupied by Na. Bondvalence calculations indicate a formal charge of 2.48 for M1 and 2.67 for M2, i.e., a charge distribution of 2R 3+ :1R 2+ . Octahedral angle variance and volume are similar for both sites, indicating disorder in the distribution of Fe and Mn atoms between the sites. Charge balance requirements agree with the presence of Fe 3+ and Mn 2+ .

To study structural changes with changing chemical composition in the octahedral sheet of trioctahedral potassium-rich 1M micas, 3 natural and 12 synthetic micas were examined by singlecrystal X-ray diffraction. Samples with Ni 2+ , Mg 2+ , Co 2+ , Fe 2+ , and Al 3+ in the octahedral sheet and with Si 4+ , Al 3+ and one sample with Ga 3+ in the tetrahedral sheet were prepared at high temperatures and pressures, which yielded crystallites ≤200 mm in size. For samples with approximately AlSi 3 composition of the tetrahedral sheet and with no octahedral Al 3+ , mean M1-O and M2-O octahedral bond lengths correlate very well with the mean ionic radius of the octahedral cations. For these samples, the M1 and M2 sites are found to be very similar or identical within experimental error in terms of mean M-O bond lengths, bond angles, and polyhedral distortion parameters. Octahedral distortion is negatively correlated with the size of the octahedral cation. Octahedra in Ni 2+ -mica show the largest deviation from ideal octahedral geometry, whereas those in annite are closest to ideal octahedral geometry, but they are still significantly flattened. Octahedral Al 3+ prefers the M2 site. This causes the mean M1-O bond length to decrease less with increasing Al 3+ content as compared to the mean M2-O distance. This Al 3+ preference for M2 also causes the M2 site to become smaller and more distorted than the M1 site. Refinement of the Mg-Fe ratios in the octahedral sites along the annite-phlogopite join shows that the two cations are statistically distributed over M1 and M2. Tetrahedra are regular and show only a small elongation along c*. Tetrahedral distortion parameters for the AlSi 3 micas show no correlation with chemistry of the octahedral layer. However, the mean T-O bond lengths increase slightly with increasing size of the octahedral cation. With decreasing size of the lateral dimension of the octahedral sheet in the (001) plane, the tetrahedral sheet shows increasing ditrigonal distortion. Largest tetrahedral rotation angles are observed for synthetic near end-member siderophyllite with α = 11.5° and for tetra-gallium-phlogopite KMg 3 GaSi 3 O 10 (OH) 2 with α = 10.8°.

Matrix effects in secondary-ion mass spectrometric (SIMS) analysis of light elements (H, Li, Be, B, and F) have been investigated in phenacite, kornerupine, danburite, axinite, spodumene, tourmaline, hambergite, and mica, all of which were epoxy-mounted in a known crystallographic orientation relative to the primary-ion beam. As reference chemical information, we used data from electron microprobe analysis (EMPA) and from single-crystal structure-refinement (SREF) on the same crystals used for SIMS. Quantification of secondary-ion intensities into concentrations was done using Si as the reference matrix element. The results indicate that matrix effects due to crystallographic orientation are <10% relative, or below analytical uncertainty for most analyzed elements. In dioctahedral mica, there is a difference in H/Si ion yield (IY) of ~25% relative when the crystal is analyzed parallel and orthogonal to the main cleavage (which is perpendicular to the c axis). The magnitude of this effect is significant and higher than our SIMS accuracy for H in micas: ±10% relative. Among the analyzed elements, Be is affected least by matrix effects, even when present as a major element. The most significant chemical effects on SIMS analysis of H, Li, F, and B in silicates seem to be related to the Fe (+Mn) content of the matrix: the light-element IY decreases as the Fe (+Mn) content increases, as previously seen in tourmaline, axinite, and kornerupine. Silicon and Al seem to have complementary and opposite effects on IY with respect to Fe and Mn. The agreement between SIMS and SREF is close for most light elements when they are present as major constituents. The results of our study also show that analytical problems are still present for B by EMPA, and this technique may not be adequate to measure B accurately in some minerals.

The compression behavior of Al-bearing stishovite was investigated by powder X-ray diffraction up to 40 GPa with the BL13A beamline at the Photon Factory (KEK, Japan). A reliable equation of state for stishovite was obtained using a diamond anvil cell coupled with a yttrium-aluminum-garnet (YAG) laser-heating. A sample containing 2.1 wt% Al 2 O 3 was heated using a YAG laser at each pressure increment to relax deviatoric stress. X-ray diffraction measurements were carried out at 300 K using the angle-dispersive technique. A least squares refinement of the data yielded equation of state parameters where the bulk modulus Ko = 282 (±2) GPa when the first pressure derivative of the bulk modulus Ko' was fixed at 4. The effect of Al is to decrease slightly the bulk modulus of stishovite and increase the density of the subducted oceanic crust. The enhanced compressibility of Al-bearing stishovite certainly has geophysical and geochemical implications for the fate of the subducted slab, as this mineral is the main constituent of subducted mid-oceanic ridge basalt (MORB) in the Earth’s mantle.

The microscopic behavior of the C2/m ↔ P2 1 /m phase transition occurring in cummingtonite (36 mol% Fe) as a function of temperature has been investigated using Mössbauer spectroscopy performed in the temperature range 100-550 K. The quadrupole splitting (△E Q ) of the M4 doublet shows a change in slope at the critical temperature of the phase transition. The difference between △E Q of the low-symmetry phase and the values obtained at the same temperatures by extrapolating △E Q of the C2/m phase has been used as a measure proportional to the local order parameter associated with the phase transition. The Mössbauer data, which provide a measure of the local microscopic order parameter, scale in an identical manner to the macroscopic properties, indicating that the sub-unit scale structure of cummingtonite evolves in a parallel manner with the macroscopic average structure on a much longer length scale.

X-ray microprobes are among the most important new analytical techniques to emerge from third generation synchrotron facilities. Here we show how X-ray fluorescence, diffraction, and absorption can be used in parallel to determine the structural form of trace elements in heterogeneous matrices at the micrometer-scale of resolution. Scanning X-ray microfluorescence (μSXRF) and microdiffraction (μSXRD) first are used to identify the host solid phase by mapping the distributions of elements and solid species, respectively. Micro-extended X-ray absorption fine structure (μEXAFS) spectroscopy is then used to determine the mechanism of trace element binding by the host phase at the molecular scale. To illustrate the complementary application of these three techniques, we studied how nickel is sequestered in soil ferromanganese nodules, an overwhelmingly complex natural matrix consisting of submicrometer to nanometer sized particles with varying structures and chemical compositions. We show that nickel substitutes for Mn 3+ in the manganese layer of the MnO 2 - Al(OH) 3 mixed-layer oxide lithiophorite. The affinity of Ni for lithiophorite was characteristic of micronodules sampled from soils across the U.S.A. and Europe. Since many natural and synthetic materials are heterogeneous at nanometer to micrometer scales, the synergistic use of μSXRF, μSXRD, and μEXAFS is expected to have broad applications to earth and materials science.

A new method has been developed to orient 50-100 μm size single crystals to obtain XANES or EXAFS spectra along any crystallographic orientation relative to a polarized synchrotron X-ray beam. The method uses a modified spindle stage and an X-ray goniometer head to hold a crystal and the glass fiber on which it is mounted. The orientation of the crystallographic axes within the crystal can be determined by X-ray diffraction in all cases or with the use of the polarizing light microscope if the minerals are biaxial. Once an optical orientation direction has been located, it can be oriented parallel to the polarization direction of the synchrotron source for spectral acquisition. Mineral samples representing the three optical classes (i.e., isotropic, uniaxial, and biaxial) were oriented along their a, b, and c crystallographic axes, and Fe-XANES spectra were obtained. In all cases the interactions of polarized X-rays with the crystal structures are analogous to the interactions of visible light (e.g., as represented by refractive index). For the isotropic almandine sample the XANES spectra were similar at all orientations. For the uniaxial buergerite and scapolite samples the spectra were similar along the a and b axes and different along c. A detailed set of spectra acquired at multiple (known) optical orientations in the scapolite crystal demonstrates that spectral features vary in intensity as a function of orientation, with maxima and minima when the X-rays are polarized along optical orientations. For the biaxial fayalite sample, the XANES spectra differed along all three crystallographic axes, as expected by analogy to optical properties.

We have developed a method for mounting and polishing small crystals (<1 mm) to expose melt inclusions for analysis. Individual crystals are mounted in epoxy on one end of a 3.2 mm × 1.3 cm polycarbonate rod. The rod is inserted into a two-piece polishing tool with an adjusting screw to control the amount of sample exposed for grinding and polishing. Thus, individual crystals can be ground and polished so that features of interest (e.g., melt inclusions) are exposed on the polished surface.