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1.
With increasing pressure, MnSiO3 rhodonite stable at atmospheric pressure transforms to pyroxmangite, then to clinopyroxene and further to tetragonal garnet, which finally decomposes into MnO (rocksalt) plus SiO2 (stishovite). High temperature solution calorimetry of synthetic rhodonite, clinopyroxene and garnet forms of MnSiO3 was used to measure the enthalpies of these transitions. ΔH 974 0 for the rhodonite-clinopyroxene and ΔH 298 0 for the clinopyroxene-garnet transition are 520±490 and 8,270±590 cal/mol, respectively. The published data on the enthalpy of the rhodonite-pyroxmangite transition, phase equilibrium boundaries, compressibility and thermal expansion data are used to calculate entropy changes for the transitions. The enthalpy, entropy and volume changes are very small for all the transitions among rhodonite, pyroxmangite and clinopyroxene. The calculated boundary for the clinopyroxene-garnet transition is consistent with the published experimental results. The pyroxene-garnet transition in several materials, including MnSiO3, is characterized by a relatively small negative entropy change and large volume decrease, resulting in a small positiveP – T slope. The disproportionation of MnSiO3 garnet to MnO plus stishovite and of Mn2SiO4 olivine to garnet plus MnO are calculated to occur at about 17–18 and 14–15 GPa, respectively, at 1,000–1,500 K. 相似文献
2.
High-pressure and high-temperature experiments using a laser-heated diamond anvil cell (LHDAC) and synchrotron X-ray diffraction have revealed a phase transition in MgAl2O4. CaTi2O4-type MgAl2O4 was found to be stable at pressures between 45 and at least 117 GPa. The transition pressure of CaTi2O4-type phase in MgAl2O4 is much lower than that in the natural N-type mid-oceanic ridge basalt composition. The Birch–Murnaghan equation of state for CaTi2O4-type MgAl2O4 was determined from the experimental unit cell parameters with K
0=219(±6) GPa, K
0′=4(constrained value), and V
0=238.9(±9) Å3. The observed compressibility was in agreement with the theoretical compressibility calculated in a previous study. ε-MgAl2O4 was observed at pressures between 40 and 45 GPa, which has not been reported in natural rock compositions. The gradient (dP/dT slope) of the transition from the ε-type to CaTi2O4-type MgAl2O4 had a positive value. These results should resolve the dispute regarding the stable high-pressure phase of MgAl2O4, which has been reported in earlier studies using both the multi-anvil press and the diamond anvil cell. 相似文献
3.
Kurt Leinenweber Wataru Utsumi Yoshihiko Tsuchida Takehiko Yagi Kei Kurita 《Physics and Chemistry of Minerals》1991,18(4):244-250
New high-pressure orthorhombic (GdFeO3-type) perovskite polymorphs of MnSnO3 and FeTiO3 have been observed using in situ powder X-ray diffraction in a diamond-anvil cell with synchrotron radiation. The materials are produced by the compression of the lithium niobate polymorphs of MnSnO3 and FeTiO3 at room temperature. The lithium niobate to perovskite transition occurs reversibly at 7 GPa in MnSnO3, with a volume change of -1.5%, and at 16 GPa in FeTiO3, with a volume change of -2.8%. Both transitions show hysteresis at room temperature. For MnSnO3 perovskite at 7.35 (8) GPa, the orthorhombic cell parameters are a=5.301 (2) A, b=5.445 (2) Å, c=7.690 (8) Å and V= 221.99 (15) Å3. Volume compression data were collected between 7 and 20 GPa. The bulk modulus calculated from the compression data is 257 (18) GPa in this pressure region. For FeTiO3 perovskite at 18.0 (5) GPa, cell parameters are a=5.022 (6) Å, b=5.169 (5) Å, c=7.239 (9) Å and V= 187.94 (36) Å3. Based on published data on the quench phases, the FeTiO3 perovskite breaks down to a rocksalt + baddelyite mixture of FeO and TiO2 at 23 GPa. This is the first experimental verification of the pressure-induced breakdown of a perovskite to simple oxides. 相似文献
4.
Raman spectra of the two high-pressure polymorphs of SiO2 (coesite and stishovite) were investigated in the temperature range 105–875 K at atmospheric pressure. Coesite remained intact after the highest temperature run, but stishovite became amorphous at temperatures above about 842~872 K. Most Raman modes exhibit a negative frequency shift with temperature for these polymorphs, but positive trends were also observed for some modes. Except for some weak modes, nonlinear temperature variation were established for these polymorphs within the experimental uncertainty and temperature range spanned. The slopes of the variation (δvi/δT)P for these polymorphs were compared with the published values. When compared with quartz and stishovite, the four-membered rings of SiO4-tetrahedra in coesite exhibit very little change with both temperature and pressure. It is also suggested that temperature and pressure should have opposite effects on the Raman shift of each vibrational mode. 相似文献
5.
The oxygen fugacity(fO2) may affect the ionic conductivity of olivine under upper mantle conditions because Mg vacancies can be produced in the crystal structure by the oxidization of iron from Fe2+ to Fe3+. Here we investigated olivine ionic conductivity at 4 GPa, as a function of temperature, crystallographic orientation, and oxygen fugacity, corresponding to the topmost asthenospheric conditions. The results demonstrate that the ionic conductivity is insensitive to fO2 under relatively reduced conditions(fO2 below Re-ReO2 buffer), whereas it has a clear fO2-dependence under relatively oxidized conditions(fO2 around the magnetite-hematite buffer). The ionic conduction in olivine may contribute significantly to the conductivity anomaly in the topmost asthenosphere especially at relatively oxidized conditions. 相似文献
6.
Schuster Roman Habler Gerlinde Schafler Erhard Abart Rainer 《Mineralogy and Petrology》2020,114(2):105-118
Mineralogy and Petrology - Polycrystalline calcite was deformed to high strain at room-temperature and confining pressures of 1–4 GPa using high-pressure torsion. The high confining... 相似文献
7.
Shanqi Liu Yongbing Li Junli Yang Huiquan Tian Bojing Zhu Yaolin Shi 《Physics and Chemistry of Minerals》2014,41(3):189-196
The pressure-dependent elastic properties of the Fe–S system are important to understand the dynamic properties of the Earth’s interior. We have therefore undertaken a first-principles study of the structural and elastic properties of FeS2 polymorphs under high pressure using a method based on plane-wave pseudopotential density function theory. The lattice constants, elastic constants, zero-pressure bulk modulus, and its pressure derivative of pyrite are in good agreement with the previous experiments and theoretical approaches; the lattice constants of marcasite are also consistent with the available experimental data. Calculations of the elastic constants of pyrite and marcasite have been determined from 0 to 200 GPa. Based on the relationship between the calculated elastic constants and the pressure, which can provide the stability of mineral, it would appear that pyrite is stable, whereas marcasite is unstable when the pressure rises above 130 GPa. Static lattice energy calculations predict the marcasite-to-pyrite phase transition to occur at 5.4 GPa at 0 K. 相似文献
8.
Calcite and aragonite have been modeled using rigid-ion, two-body Born-type potentials, supplemented by O-C-O angular terms inside the CO3 groups. A shell model has also been developed for calcite. Atomic charges, repulsive parameters and force constants have been optimized to reproduce the equilibrium crystal structures, the elastic constants and the Raman and infrared vibrational frequencies. The rigid-ion potential RIM (atomic charges:z
O= -0.995e,z
C = 0.985e,z
Ca = 2.0e) fitted to calcite properties is able to account for those of aragonite as well. Experimental unit-cell edges, elastic constants, internal and lattice frequencies are reproduced with average relative errors of 2.1, 5.5, 2.4, 15.1% for calcite and of 0.2, 19.4, 2.5, 11.8% for aragonite, respectively. The RIM potential is suitable for thermodynamic and phase diagram simulations in the CaCO3 system, and is discussed and compared to other potentials. 相似文献
9.
An interatomic interaction potential for silica polymorphs is derived based on the SCD model (cfr. Tijskens et al. 1994). This interaction potential incorporates all classical electrostatic interactions arising from the spherical part of the spatial extent of the atoms including many body interactions. The potential is derived from Hartree-Fock energies and electron densities for a set 72 [SiO4]4-- and [Si2O7]6--clusters with variable configuration. The long range impact of the surroundings on these clusters in the infinite system has been successfully mimicked by embedding the clusters in a finite three-dimensional array of point charges. This three-dimensional array of point charges is optimized as to reproduce the average site potential and its gradient occurring in II–IV-coordinated silica polymorphs at the central atoms of the clusters. The resulting interaction potential consists of two functions of the configurational coordinates, ?, describing spherical “atomic” electron densities, σa(x, ?) for A=Si, O. All classical electrostatic interactions are derived from these densities. A Born-Mayer type correction term ΔE qm(?) models the quantum mechanical interactions and the electrostatic interactions arising from the non-spherosymmetrical component of the electron density. The new interaction potential model shows a slightly improved reproduction of the potential surface with respect to the classical Born-Mayer ionic model and demonstrates the importance of many body interactions as charge transfer and expansion/contraction of the atomic electron densities in these systems. Also the dependence of the quantum mechanical correction term ΔE qm(?) on the Si-O-Si-bond angle proves covalent effects to be larger than suggested by the classical Born-Mayer ionic model thereby clarifying the controversy in literature on the importance of covalent effects in silica polymorphs and polymerised silicates in general. 相似文献
10.
Thomas Reinecke Heinz-Jürgen Bernhardt Richard Wirth 《Contributions to Mineralogy and Petrology》2000,139(5):584-606
Calcite in former aragonite–dolomite-bearing calc-schists from the ultrahigh-pressure metamorphic (UHPM) oceanic complex
at Lago di Cignana, Valtournanche, Italy, preserved different kinds of zoning patterns at calcite grain and phase boundaries.
These patterns are interpreted in terms of lattice diffusion and interfacial mass transport linked with a heterogeneous distribution
of fluid and its response to a changing state of stress. The succession of events that occurred during exhumation is as follows:
As the rocks entered the calcite stability field at T=530–550 °C, P ca. 1.2 GPa, aragonite occurring in the matrix and as inclusions in poikilitic garnet was completely transformed to calcite.
Combined evidence from microstructures and digital element distribution maps (Mn-, Mg-, Fe- and Ca–Kα radiation intensity
patterns) indicates that transformation rates have been much higher than rates of compositional equilibration of calcite (involving
resorption of dolomite and grain boundary transport of Mg, Fe and Ca). This rendered the phase transformation an isochemical
process. During subsequent cooling to T ca. 490 °C (where lattice diffusion effectively closed), grains of matrix calcite have developed diffusion-zoned rims, a
few hundred micrometres thick, with Mg and Fe increasing and Ca decreasing towards the phase boundary. Composition profiles
across concentrically zoned, large grains in geometrically simple surroundings can be successfully modelled with an error
function describing diffusion into a semi-infinite medium from a source of constant composition. The diffusion rims in matrix
calcite are continuous with quartz, phengite, paragonite and dolomite in the matrix. This points to an effective mass transport
on phase boundaries over a distance of several hundred micrometres, if matrix dolomite has supplied the Mg and Fe needed for
incorporation in calcite. In contrast, diffusion rims are lacking at calcite–calcite and most calcite–garnet boundaries, implying
that only very minor mass transport has occurred on these interfaces over the same T–t interval. From available grain boundary diffusion data and experimentally determined fluid–solid grain boundary structures,
inferred large differences in transport rates can be best explained by the discontinuous distribution of aqueous fluid along
grain/phase boundaries. Observed patterns of diffusion zoning indicate that fluid was distributed not only along grain-edge
channels, but spread out along most calcite–white mica and calcite–quartz two-grain junctions. On the other hand, the inferred
non-wetting of calcite grain boundaries in carbonate-rich domains is compatible with fluid–calcite–calcite dihedral angles
>60° determined by Holness and Graham (1995) for a wide range of fluid compositions under the P–T conditions of interest. Whereas differential stress has been very low at the stage of diffusion zoning (T > 490 °C), it increased as the rocks were cooling below 440 °C (at 0.3–0.5 GPa). Dislocation creep and the concomitant increase
of strain energy in matrix calcite induced migration recrystallisation of high-angle grain boundaries. For that stage, the
compositional microstructure of recrystallised calcite grain boundary domains indicates significant mass transport along calcite
two-grain junctions, which at the established low temperatures is likely to have been accomplished by ionic diffusion within
a hydrous grain boundary fluid film (“dynamic wetting” of migrating grain boundaries).
Received: 10 January 2000 / Accepted: 10 April 2000 相似文献
11.
Tori Z. Forbes 《Geochimica et cosmochimica acta》2011,75(24):7893-7905
Calcium carbonate (CaCO3) is an important component of the near-surface environment. Understanding the nature of its precipitation is important for a variety of environmental processes, as well as for the geologic sequestration of anthropogenic carbon dioxide. Calcite is the most thermodynamically stable bulk polymorph, but energy crossovers may exist that could favor the precipitation of vaterite or aragonite with decreasing particle size. The purpose of this study is to determine the surface energy of calcite, which is the first step towards understanding the effect of particle size on thermodynamic stability in the calcium carbonate system. The enthalpies of five well-characterized calcite samples (four nanophase and one bulk) were measured by acid solution isothermal and water adsorption calorimetric techniques. From the calorimetric data, the surface energies of calcite were determined to be 1.48 ± 0.21 and 1.87 ± 0.16 J/m2 for hydrous and anhydrous surfaces. These values are similar to those measured for many oxides but larger than predicted from computational models for idealized calcite surfaces. The surfaces of synthetic CaCO3 particles contain a range of planes and defect structures, which may give rise to the difference between the experimental and modeled values. 相似文献
12.
Peter W. Mirwald 《Physics and Chemistry of Minerals》1979,4(4):291-297
The electrical conductivity of calcite cleavage fragments was measured using direct current. Five temperature intervals, characterized by different activation energies of conduction, were established — T 1: (300°)–500° C, T 2: 500–725° C, T 3: 725°–800° C, T 4: 800°–985° C and T 5: 985°–(1,200)° C. The data above 600° C indicate various intrinsic conduction mechanisms which are likely to be related to disorder in the CO3-sublattice of calcite. The transitions between the temperature intervals T 2-T 3-T 4 and T 4-T 5 are probably correlated with high-temperature transitions of calcite at 800° and 985° C. Measurement of the thermoelectric voltage indicates a conductivity dominated by negatively charged carriers. 相似文献
13.
GUILLAUME CAILLEAU ERIC P. VERRECCHIA OLIVIER BRAISSANT† LAURENT EMMANUEL‡ 《Sedimentology》2009,56(6):1858-1875
Needle fibre calcite is one of the most ubiquitous habits of calcite in vadose environments (caves deposits, soil pores, etc.). Its origin, either through inorganic, indirect or direct biological processes, has long been debated. In this study, investigations at 11 sites in Europe, Africa and Central America support arguments for its biogenic origin. The wide range of needle morphologies is the result of a gradual evolution of the simplest type, a rod. This rod is the elementary brick which, by aggregation and welding, builds more complex needles. The absence of cross‐welded needles implies that they are welded in a mould, or under a longitudinal and unidirectional constraint, before being released inside the soil pores. The difference between the lengthening of the needles and the c axis can be explained by the existence of needles observed under a scanning electron microscope in organic sleeves, which can act as a mould during rod growth. Complex morphologies with epitaxial outgrowths on straight rods cannot have grown entirely inside organic microtubes; they must result from soil diagenesis after the release of straight rods in a soil‐free medium. Whisker crystals are interpreted as the result of growth and coalescence of euhedral crystals on a rod. Rhomb chains are considered to be the consequence of successive epitaxial growth steps on a needle during variations in growth conditions. Isotopic signatures for needle fibre calcite vary from ?16·63‰ to +1·10‰ and from ?8·63‰ to ?2·25‰ for δ13C and δ18O, respectively. The absence of high δ18O values for needle fibre calcite precludes a purely physicochemical origin (evaporative) for this particular habit of calcite. As epitaxial growth cannot precipitate in the same conditions as initial needles, needle fibre calcite stable isotopic signatures should be used with caution as a proxy for palaeoenvironmental reconstructions. In addition, it is suggested that the term needle fibre calcite should be kept for the original biogenic form. The other habit should be referred to as epitaxial forms of needle fibre calcite. 相似文献
14.
V. Paul Wright 《Geological Journal》1984,19(1):23-32
Needle-fibre calcite occurs in association with root structures in a Lower Carboniferous calcrete crust. The origin of this form of calcite is discussed and the scattered literature from soil science, microbiology and geology is reviewed. It is concluded that while needle-fibre calcite is not diagnostic of any environment its origin is related to the activities of various micro-organisms, especially fungi. The use of needle-fibre calcite as a climatic indicator is reviewed and it is used, with other criteria, as evidence of fluctuating climates in the Lower Carboniferous. 相似文献
15.
We have used density functional theory to investigate the stability of MgAl2O4 polymorphs under pressure. Our results can reasonably explain the transition sequence of MgAl2O4 polymorphs observed in previous experiments. The spinel phase (stable at ambient conditions) dissociates into periclase and
corundum at 14 GPa. With increasing pressure, a phase change from the two oxides to a calcium-ferrite phase occurs, and finally
transforms to a calcium-titanate phase at 68 GPa. The calcium-titanate phase is stable up to at least 150 GPa, and we did
not observe a stability field for a hexagonal phase or periclase + Rh2O3(II)-type Al2O3. The bulk moduli of the phases calculated in this study are in good agreement with those measured in high-pressure experiments.
Our results differ from those of a previous study using similar methods. We attribute this inconsistency to an incomplete
optimization of a cell shape and ionic positions at high pressures in the previous calculations. 相似文献
16.
I. B. M. Kosamu M.Sc. M. Obst Ph.D. 《International Journal of Environmental Science and Technology》2009,6(4):557-562
This study assessed the role of picocyanobacterial photosynthesis in the induction of calcite precipitation. It aimed at establishing whether photosynthetic uptake of bicarbonate by Synechoccoccus cells leads to calcite nucleation. The precipitation of calcite was initiated by addition of previously washed cyanobacterial cells of Synechococcus strain PCC 7942 to solutions of calcium carbonate at different saturation levels with respect to calcite. Precipitation experiments were performed under controlled laboratory conditions in two set-ups: one in which photosynthesis was inhibited using a herbicide called Diuron and the other one in which photosynthesis was taking place. During the experiments, a pH meter monitored the pH and ion selective electrodes monitored concentrations of carbonate and calcium ions. The morphology of the precipitated crystals was analysed using Scanning Electron Microscopy. When the kinetics of calcium carbonate nucleation by the Synechococcus cells were compared for the two sets of experiments, there were very little differences. In fact, the induction times for precipitation reactions with photosynthesis were shorter due to the uptake of carbon dioxide. It is therefore, concluded that photosynthesis does not directly influence the nucleation of calcite at the surface of Synechococcus cells with sufficient supply of carbon dioxide, i.e. cells took up carbon dioxide and not bicarbonate. The microscopic observations, however, provided some evidence that picocyanobacterial cell walls act as a template for calcite nucleation. 相似文献
17.
At high pressures, CdGeO3 pyroxenoid transforms to garnet, then to ilmenite, and finally to perovskite. Enthalpies of transition among the four phases were measured by high temperature calorimetry. The entropies of transition and slopes of the boundaries were calculated using the measured enthalpies and free energies calculated from the phase equilibrium data. Pyroxenoid and garnet are very similar energetically. However garnet is a high pressure phase because of its lower entropy and smaller volume. The pyroxenoid-garnet transition has a small positiveP-T slope. Ilmenite is intermediate in enthalpy between garnet and perovskite, but is lower in entropy than both phases. Therefore the garnet-ilmenite transition has a positivedP/dT, while a negativedP/dT is calculated for the ilmenite-perovskite transition. The thermochemical data for the CdGeO3 phases are generally consistent with the observed high pressure phase relations. The high entropy of perovskite relative to ilmenite, observed in several ABO3 comounds including CdGeO3, is related to the structural features of perovskite, in which relatively small divalent cations occupy the large sites of 8–12 fold coordination. The thermochemistry of the CdGeO3 polymorphs shows several similarities to that of the CaGeO3 system. 相似文献
18.
When pure crystalline calcite seeds are added to supersaturated seawater, precipitation results in a coating which with time equilibrates at atmospheric pressure with seawater and corresponds to a calcite containing probably only 2 or 3% of MgCO3 (mole fraction).If synthetic crystalline magnesian calcite is added, the surface layer equilibrates not only with respect to seawater but also in relation with the crystalline sites initiating precipitation. Adding Mg0.03Ca0.97CO3 results in a coating with a solubility close to that of calcite. This confirms that the surface coating on pure calcite seeds contains about 2 or 3% MgCO3 (K'sp = 10?6.30).The surface layer precipitated on a synthetic Mg0.08Ca0.92CO3 equilibrates finally with a carbonate more soluble than calcite (K'sp = 10?5.94) corresponding to the seeds composition.Experiments at 1000 kg cm t-2 imply that when magnesian calcites are precipitated at the surface of calcite or magnesian calcite seeds, the precipitate must be hydrated, otherwise pressure accelerated recrystallization or rearrangement with loss of Mg would thermodynamically be impossible.By changing the pressure of a seawater sample originally saturated with a solid carbonate phase, changes in pH result from the effect of pressure on the dissociation constants of carbonic acid and boric acid causing either undersaturation or supersaturation with respect to the solid. By changing pressure we can show whether precipitation, dissolution and recrystallization are reversible processes if pH is taken as criteria of reversibility. 相似文献
19.
S.K. Mishra 《International Journal of Mineral Processing》1978,5(1):69-83
Microelectrophoresis technique was used to study the electrokinetic properties of some apatites and calcite. The results indicate that similar to oxide minerals, H+ and OH? ions function as potential-determining ions for apatite and calcite. However, because of their variable compositions, the apatites from different deposits showed different i.e.p. values ranging from pH 3.5 to 6.7, while the i.e.p. of calcite occurred at pH 8.2.Study of the effect of lattice-forming ions on the zeta potential indicates that calcium ions produce specific ionic adsorption effects on apatites, whereas they are potential-determining ions for calcite. On the other hand, as expected, phosphate and carbonate ions function as potential-determining ions for apatite and calcite, respectively.The experimental data suggest that the zeta potential of apatite is a function of mineral—water contact time. The degree of variation in zeta-potential values appears to be influenced by the electrolytic environment of the apatite suspension. 相似文献
20.
Cobalt, like Mg, may cause the precipitation of aragonite rather than calcite in aqueous solutions due to the adsorption and crystal poisoning of calcite by a hydrated ion. Solutions containing NaCl and CaCl2, having the ionic strength and Ca content of seawater (35‰ salinity), were spiked with known amounts of CoCl2. Calcium carbonate was precipitated by the addition of 0.7 ml of 1 M Na2CO3. All experimental runs were made at 25°C, and all products were examined by X-ray diffraction. At low concentrations of Co (< 5·?4M) calcite and vaterite formed. At concentrations from 5·10?4 M to 2·10?3M, the products consisted of combinations of calcite and vaterite; aragonite and calcite; aragonite and vaterite; calcite, vaterite and aragonite. In solutions of 3·10?3M CoCl2, most precipitates were aragonite with only one sample containing a small amount of calcite. All precipitates from 5·10?3M CoCl2 solutions either contained aragonite or were amorphous. Solutions with concentrations of 1 · 10?2M CoCl2 produced only amorphous precipitates. All precipitates contained an amorphous violet phase, assumed to be basic cobaltous carbonate (2CoCO3·Co(OH)2·H2O). 相似文献