Showing papers in "American Mineralogist in 1992"

The temperature dependence of the cation distribution in magnesioferrite (MgFe2O4) from powder XRD structural refinements and Mössbauer spectroscopy

Abstract: Magnesioferrite (MgFe204 spinel) was synthesized in equilibrium with excess MgO using a flux method. Chemical analysis shows that the MgFe204 is stoichiometric, within analytical uncertainty. Samples were annealed at intervals of 50 °C between 450 and 1250 °C and quenched in H20. The time needed to reach equilibrium at low temperatures (450650 0C) was assessed by monitoring the change of lattice parameter (ao) with time. In addition, the changes in ao at 550-650 °C were reversed, using material annealed at 400 °C for 50 d. Some samples were further characterized by measuring their Neel temperatures, using DSC. The cation distributions in the quenched samples were determined using both powder X-ray diffraction and M6ssbauer spectroscopy. To increase the resolution in the latter, the samples were studied in an applied magnetic field of 4.5 T at 12-171 K. The cation distribution changes smoothly with temperature, with the cation inversion parameter (x) decreasing from 0.90 at 450 °C to 0.72 at 1100 0c. The precision with which x can be determined is ::to.004 from XRD and -::to.O! from the M6ssbauer spectroscopy. The two sets of measurements agree well with each other, the mean difference in x being 0.0056 ::t 0.0102. Thermodynamic modeling shows that the cation distribution can be described with a nonlinear enthalpy of disordering model, with aMg-Fe}+ = 26.6 ::t 0.4 and {1= - 21. 7 ::t 0.3 kJ/g-atom. Incorporation of a small excess (nonconfigurational) entropy term into the model gives a slightly better fit, however. The disordering in MgFe204 as a function of temperature is virtually identical to that found in Fe,04 from thermopower measurements. The present results differ to a greater or lesser extent from most of the previous work on MgFe204. This is most likely attributable to differences in stoichiometry. We also present some results on nonstoichiometric "MgFe204," i.e., solutions of MgFe204 and 1'-Fe20" which tend to confirm this hypothesis, at least for some of the earlier studies.

Diffusive reequilibration of melt and fluid inclusions

Abstract: A mathematical model is presented that investigates the diffusive reequilibratrion of chemical species in melt inclusions with external melt through the host crystal. Analytic solution of the differential equations reveals that reequilibration is easier (1) for species with higher diffusivities in the host crystal, (2) for species with higher partition coefficients (k) between host crystal and melt and (3) for smaller-sized host crystals and melt inclusions

Structural chemistry of Mn, Fe, Co, and Ni in manganese hydrous oxides; Part II, information from EXAFS spectroscopy and electron and X-ray diffraction

Abstract: The shortand long-range structural order of various hydrous oxides were investigated using extended X-ray absorption fine structure (EXAFS), X-ray and electron diffraction (XRD and SAED) and in situ chemical analysis (EDS). Materials examined include synthetic sodium birnessite, natural magnesium birnessite, natural nickel copper birnessite, synthetic vernadite (o-Mn02), a series of natural iron vernadite samples, natural manganese goethite, two natural cobalt nickel asbolane samples, and natural cobalt asbolane. The structure of birnessite is similar to that of chalcophanite, but there are no cornersharing Mn4+-Mn4+ octahedra. Sodium and magnesium birnessite differ primarily in the stacking mode of their Mn octahedral sheets and their interlayer structure. Similarities and differences between these two minerals have been considered in terms of anion closepacked models. In contrast to birnessite, synthetic vernadite (o-Mn02) has edgeand corner-sharing Mn4+ octahedra and a three-dimensional anionic framework; o-Mn02 does not appear to be a c-disordered birnessite. In iron vernadite and manganese goethite, Fe3+ and Mn4+ ions are segregated in coherent scattering domains. In both minerals, Mn atoms form phyllomanganate-like domains; in iron vernadite, Fe domains have a feroxyhite-like local structure. Asbolane has been found to have a mixed-layer structure. Cobalt nickel asbolane has layers of Mn02, Ni(OR)2' and possibly CoOOR or CO(OH)3 alternating regularly along the c axis. In cobalt asbolane, Mn2+ tetrahedral layers are presumably regularly interstratified with layers of Mn02 and (CoOOH). This study provides new examples of hybrid structures among low-temperature materials and confirms their heterogeneous nature on a very fine scale.

Dielectric constants of silicate garnets and the oxide additivity rule

Abstract: The dielectric constants and dielectric loss values of a series of garnets were measured at l MHz using a two-terminal method and empirically determined edge corrections. The results are five intermediate pyrope-almandine samples, k'=11.96-12.35; spessartine, k'=11.65; two andradite samples, k'=10.53-10.59; and three grossular samples, k'=8.53-8.8l. The deviations of measured dielectric polarizabilities as determined using the Clausius-Mosotti equation from those calculated using the sum of oxide polarizabilities according to ɑ_D(M_2M'X_4) = 2ɑ_D(MX) + ɑ_D(M'X_2) is +5.0 to 6.5% for the pyrope-almandine samples,+ 1.9% for spessartine, -2.3% of or the andradite samples, and -5.5 to -7.0% for grossular. These deviations from additivity are believed to result from garnet structural constraints leading to "rattling" Mg ions and "compressed" Ca ions.

Single-crystal X-ray structure study of synthetic pyrope almandine garnets at 100 and 293 K

Abstract: The crystal structures of synthetic pyrope (Mg3AI2Si3012),almandine (FeAI2Si3012), and the solid-solution garnet compositions PY8o-Alm2o, PY6o-Alm4o, and PY2o-Alm8o have been refined in space group Ia1d from high-precision X-ray diffraction data with sin 0/>..> 0.4 A-I measured at 100 and 293 K. There is no indication of lower symmetry for pyrope, almandine, or solid-solution members. Experimentally determined atomic coordinates and displacement parameters for the solid-solution compositions are in good agreement with those linearly interpolated from the end-members. Thus there are no apparent structural features that could account for substantial nonideal enthalpies of mixing in the system pyrope-almandine. The tetrahedral rotation angle is inversely correlated with the X-O distance. Fe2t substitution on the eight-coordinated X site of pyrope, or increasing temperature, decreases the rigid tetrahedral rotation in garnet. The large and anisotropic displacement parameters for the X-site cations in garnet are mainly a result of anisotropic thermal vibrations along the longer X-O bonds, which produce nonrigid polyhedral behavior for the dodecahedral site. The tetrahedra and octahedra behave as rigid bodies. These strong vibrations of the former give rise to the relatively large heat capacities and third-law entropies in garnet. Previous proposals concerning subsite dodecahedral ordering in pyrope must be revised.

Conversion of perovskite to anatase and TiO2 (B): A TEM study and the use of fundamental building blocks for understanding relationships among the TiO2 minerals

Abstract: TiO 2 crystallizes in at least seven modifications encompassing several major structure types including those of rutile, hollandite and PbO 2 . We have adopted a fundamental building block (FBB) approach in order to understand better the relationships between these structures. The identification of common structural elements provides insights into possible phase transformation mechanisms as well as the weathering reactions that convert perovskite (CaTiO 3 ) to TiO 2 anatase and TiO 2 (B)

High-pressure structural study of dolomite and ankerite

Abstract: Structural parameters have been refined using X-ray intensity data for a stoichiometric dolomite single crystal at room pressure and at 1.50, 2.90,3.70, and 4.69 GPa; and for a single crystal of a ferroan dolomite with approximately 70 molo/o CaFe(COr)' at room pressure and at 1.90,2.97, and 4.0 GPa. The principal structural change with increasing pressure is compression of the CaOu and (Mg,Fe)Ou octahedra. The CO, group remains essentially invariant throughout the pressure range studied. The effect of the polyhedral compression is reflected in the anisotropic compression of the unit-cell parameters. In both dolomite and ankerite, c is approximately three times as compressible as a. In addition to displaying similar axial compressibilities, dolomite and ankerite display similar bond compressibilities and bulk moduli. The isothermal bulk modulus of dolomite determined from cell volume compression data, assuming the pressure derivative is 4, is 94 GPa, and that of ankerite is 9l GPa. The polyhedral bulk moduli of CaO. and (Mg,Fe)Ou are also similar, with CaOu being slightly more compressible than (Mg,Fe)Ou' The latter has a greater compressibility than generally observed in other oxides and silicates. The distortion of the octahedra, though already small, decreases slightly with pressure. No phase change was observed in either compound throughout the pressure range studied.

Water, CO2, Cl, and F in melt inclusions in phenocrysts from three Holocene explosive eruptions, Crater Lake, Oregon

Abstract: Rare melt inclusions ~ 100 µm in diameter trapped near the boundaries of corroded patchy zones in plagioclase phenocrysts from Plinian pumice of three Holocene eruptions were analyzed by IR spectroscopy for molecular H_2O, OH groups, and CO_2 and by electron microprobe for Cl and F. The three rhyodacitic eruptions, each of which began with aPlinian phase, occurred over ~200 yr. The Llao Rock and Cleetwood eruptions ended with degassed lava flows and the subsequent climactic eruption with voluminous ignimbrite. Groundmass glass and melt inclusion compositions (anhydrous) are similar. Inclusions with total H_2O concentrations of 1.2-3.4 wt% are adjacent to fractures or are hourglass inclusions, suggesting partial degassing caused by depressurization. Melt inclusions in phenocrysts in climactic ignimbrite may have vesiculated for the same reason. Intact inclusions in Plinian pumices have total H_2O concentrations believed to represent magmatic H_2O contents (wt%): Llao Rock, 5.3, 5.3; Cleetwood, 3.8, 4.7; climactic 3.9 ± 0.2 (lσ, n = 6). Ratios of OR/molecular H_2O indicate closure temperatures of 200-500oC that reflect syn- or posteruptive cooling. CO_2 concentrations are ≤ 25 ppm. H_2O and CO_2 concentrations indicate saturation pressures of 1.0-1.8 kbar or depths ≥ 5 km. Six inclusions from climactic pumice average 400 ± 40 (lσ) ppm F, 1880 ± 70 ppm Cl. F and Cl concentrations of the other samples are similar and not well correlated with each other or with total H_2O. Location of melt inclusions near boundaries of patchy zones, which are mantled by oscillatory-zoned overgrowths, suggests that their H_2O concentrations represent magmatic values significantly before eruption. Although mean H_2O concentrations of analyzed melt inclusions decrease for the three successive eruptions, it is not certain that this indicates a corresponding change in magmatic H_2O content during the interval between eruptions.

Raman and infrared study of pressure-induced structural changes in MgSiO3, CaMgSi2O6, and CaSiO3 glasses

Abstract: The principal pressure-induced changes in spectra observed are reversible, so that the characterization of the high density states of these materials requires in situ measurements at high pressure. The dominant change in all three glasses is a marked shift of the midfrequency Raman bands to higher frequencies at higher pressure

Temperature dependence of cation disorder in MgAl2O4 spinel using 27Al and 17O magic-angle spinning NMR

Abstract: Determination of the disorder of Mg 2+ and Al 3+ cations between the tetrahedral and octahedral sites in MgAl 2 O 4 spinel. for synthetic powders quenched from temperatures between 700 and 1400°C. Disorder, measured by the fraction of Al in the tetrahedral site and described by the inversion parameter x, increased with temperature from x=0.22 at 700°C to x=0.29 at 1000°C

A new interatomic potential model for calcite; applications to lattice dynamics studies, phase transition, and isotope fractionation

Abstract: A microscopic model for calcite and aragonite has been developed that includes short range repulsive atom-atom interactions; two-, three-, and four-body potentials; and longrange Coulomb interactions. The structures and elastic constants of both phases are reproduced satisfactorily, as are the phonon frequencies for calcite. The observed softening of a transverse acoustic phonon in the [1,0,4] direction in calcite is reproduced by our model. Applications of the model include the calculation of reduced fractionation coefficients for 160/180 of calcite and isotope exchange coefficients of calcite and H20 that are in reasonably good agreement with experimental data. The lattice parameters of the hightemperature disordered phase of calcite have been calculated and are in good agreement with experimental data. It is inferred that the temperature dependence of the spontaneous strain accompanying the phase transition in calcite is due to short range order of the C03 groups, which also explains the anomalous negative thermal expansion along [100].

The biodurability of chrysotile asbestos

Abstract: Lung cancer, asbestosis, and, to a lesser extent, pleural mesothelioma have been linked to inhalation of chrysotile [MgrSirOr(OH)o] asbestos. The duration and intensity of exposure, along with fiber size and shape and mineral type (surface and chemical properties) appear to play important roles in the development of these diseases. The fluids in lung tissue contain very low concentrations ofMg and Si, and therefore they are undersaturated with respect to chrysotile. Therefore, chrysotile's persistence in lung tissue is simply a result of its dissolution kinetics in the various biological environments of the lung. The dissolution reaction for chrysotile for pH < 9 is MgrSirOs(OH)4 * 6H* : 3Mg'* * 2H4SiOo + HrO. This reaction proceeds in two steps. First, the magnesium hydroxide layer of the chrysotile dissolves, leaving behind silica that dissolves at a slower rate. Therefore, the fiber lifetime (i.e., biodurability) depends upon the rate of silica release. For the range of undersaturation found in lung tissue, the rate of silica release, 5.9 (+ 3.9; x l0-'0 moV m2s, is independent of pH. A shrinking fiber model predicts that a fiber of chrysotile I pm in diameter will completely dissolve in 9 (+ 4.5) months.

The light-induced alteration of realgar to pararealgar

Abstract: Realgar from various locations, high-purity synthetic realgar, and synthetic realgar with 2 rnolo/o Sb were subjected to unfiltered sunlight, filtered sunlight, and filtered light from a quanz-rungsten-halogen lamp for various times. Both single-crystal and powder samples were used. All of the realgar transformed to pararealgar at wavelengths between about 500 and 670 nm, but no transformation occurred at wavelengths greater than about 670 nm. The reaction rate decreased at wavelengths above 560 nm and was very slow at wavelengths greater than 610 nm. The high-temperature polymorph,0, also transformed to pararealgar. The alteration to pararealgar did not proceed directly; an intermediate phase, 1, of unknown crystal structure but having the same stoichiometry as realgar, formed first. The time to form 1 increased with increasing wavelength at a constant flux of photons. There was iro measurable difference in stoichiometry between realgar and pararealgar; both were slightly hyperstoichiometric. Pararealgar forms on the surface as a thin layer or nodules and then fissures at some critical thickness, causing the well-known degradation ofrealgar exposed to light. The reaction is reversible at elevated temperatures. In all cases, even below the a-B realgar transformation temperature, l always formed initially, followed by the formation of p. The a phase then formed from the p phase at a rate dependent upon temperature. The time to transform B (formed during the reverse reaction) was about I d at 220'C and 2 d at 17 5 "C, whereas transformation of normal 0, cooled from above the transformation temperature to either l7 5 or 220'C, takes several months. It is proposed that light breaks As-As bonds, which are weaker than As-S bonds, and that the covalently bonded cage molecules form a new crystal structure in which free As is intercalated. The behavior is compared to the photo-decomposition of orpiment, which has been studied extensivelY.

Selective preservation of melt inclusions in igneous phenocrysts

Abstract: Phenocrysts that contain melt and gas inclusions experience dilatational stresses during eruptions because the bulk modulus of silicate melt is an order of magnitude less than the elastic moduli of crystals and because dissolved volatile components can exsolve in response to decompression. Vve present a physical model in which the evolution of pressure inside an inclusion is coupled with elastic deformation of the host crystal during ascent. This deformation is small, and only limited decompression of inclusions can take place, which keeps most volatile Inolecules in solution. For subaerial eruptions j)y, the difference between the pressure within the inclusion and that outside the crystal is typically within 80-100% of the pressure at which the inclusion was formed. For submarine eruptions, j)y is reduced by an amount corresponding to the weight of overlying H20. All else being equal, j)y is higher for an inclusion containing a gas bubble at the time of trapping than for a pure melt inclusion. Stresses in the host fall off rapidly over a distance similar to inclusion radius, and for virtually all common inclusion sizes, the host should behave as an effectively infinite environment. Cracks in the host are likely to nucleate on preexisting microfractures that intersect the inclusion walls, and simple arguments support the notion that the stress needed to open a crack increases with decreasing length of microfracture. This suggests that the easiest microfracture to open will be of a length approximately equal to the inclusion radius and hence that smaller inclusions may withstand greater overpressures than larger inclusions. Quantitative estimates suggest that cracking of host crystals should be common, even those formed in shallow crustal magma chambers. If rapid exsolution follows cracking, decrepitation results and the inclusion contents escape. If kinetic effects substantially retard exsolution, rapid eruption and quenching of melt in the inclusion may trap volatile:s before decrepitation occurs. The preservation of inclusions is likely to depend on the d,epth of formation, inclusion size, and the physical regime of eruption.

Variations of bond lengths and volumes of silicate tetrahedra with temperature

Abstract: AesrRAcr The apparent SiO bond lengths recorded for silicates over a range oftemperatures either typically are invariant or exhibit a contraction with increasing temperature. A rigid-body thermal analysis was completed for the tetrahedra in nine silicates whose structures have been determined over a range of temperatures from 15 to 1250 K and whose tetrahedra seem to behave as rigid units. The coordinates provided by the analysis yield bond lengths and polyhedral volumes corrected for the librational motion of each silicate tetrahedron. The bond lengths and volumes estimated for tetrahedra with four bridging O atoms seem to increase with temperature at a faster rate than those with four nonbridglng O atoms. Those for tetrahedra with two or three nonbridging O atoms tend to increase at an intermediate rate. An analysis of the rigid-body motion of coordinated polyhedra yields a simple but accurate expression for correcting bond lengths for thermal vibrations. It also indicates that the temperature factors ofthe coordinating anions ofrigid polyhedra should be larger than those ofthe coordinated cations, regardless ofthe masses ofthe constituent atoms.