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1.
Synthetic polycrystals of α-Mg2GeO4 (with the olivine structure) and γ-Mg2GeO4 (with the spinel structure) deformed at high temperature and pressure in their respective stability fields were investigated by analytical transmission electron microscopy. Specimens with a mean grain size of 20–30 µm deform by dislocation glide and/or climb. The predominance of glide versus climb depends on stress and grain orientation. The defect microstructures of both polymorphs are very similar to those observed in their respective silicate analogues, α- and γ-(Mg,Fe)2SiO4, and, in the case of the spinel phase, very similar to those observed in magnesium aluminate spinels. These observations suggest that Mg2GeO4 is a good rheological analogue for the Earth’s upper mantle. A spinel specimen deformed under the same conditions of temperature and strain rate as an olivine specimen was approximately three times stronger than olivine. In specimens of both phases deformed at or above 1400 K, a thin amorphous film composed of Mg, Ge, and O was detected along some grain boundaries. Grains ≤10 µm diameter surrounded by a film of amorphous phase (>10 nm thick) exhibited low dislocation densities, and deformation appeared to have occurred by grain boundary sliding.  相似文献   

2.
Infrared (IR) absorption spectra are presented for olivine (α) and spinel (γ) phases of A2SiO4 (A=Fe, Ni, Co) and Mg2GeO4. IR spectra of β phase (“modified spinel”) Co2SiO4 and of α Mg2SiO4 are also included. These results provide reference spectra for the identification of olivine high-pressure polymorphs. Isostructural and isochemical correlations are used to support a general interpretation of the spectra and to predict the spectrum of γ Mg2SiO4. A γ Mg2GeO4 sample equilibrated at 1,000° C shows evidence of partial inversion, but one equilibrated at 730° C does not. This suggests that partial inversion could occur in silicate spinels at elevated temperatures and pressures, however no evidence of inversion is seen in the ir spectra of the silicates in this study.  相似文献   

3.
An experiment conducted in a 2000-ton uniaxial split-sphere apparatus (USSA-2000) utilizes large sample volume and a substantial temperature gradient to synthesize intergrowths of the olivine and spinel polymorphs of Co2SiO4. The olivine starting material consists of a finegrained fraction (<20μm) which records the stable polymorphs along the length of the sample plus large olivine grains (100–500 μm) which help decipher the mechanism of the phase transformation. At conditions near equilibrium, the olivine-spinel transformation in the large grains occurs by inward growth of a few large single crystals of spinel nucleated on the surfaces of the olivine. The overall rate of transformation is governed by the mobility of the interphase boundary, whose morphology is crystallographically controlled by the spinel. No renucleation of spinel is observed in the host olivine crystal, even in the region immediately adjacent to the olivine/spinel interface; analysis of this region with transmission electron microscopy reveals an extremely high density of dislocations induced by plastic flow accommodating the volume change associated with the phase transformation.  相似文献   

4.
In this paper we present a theoretical investigation of the structures and relative stability of the olivine and spinel phases of Mg2SiO4. We use both a purely ionic model, based on the Modified Electron Gas (MEG) model of intermolecular forces, and a bond polarization model, developed for low pressure silica phases, to investigate the role of covalency in these compounds. The standard MEG ionic model gives adequate structural results for the two phases but incorrectly predicts the spinel phase to be more stable at zero pressure. This is mainly because the ionic modeling of Mg2SiO4 only accounts for 95 percent of the lattice energy. The remainder can be attributed to covalency and many-body effects. An extension of the MEG ionic model using “many-body” pair potentials corrects the phase stability error, but predicts structures which are in poorer agreement with experiment than the standard ionic approach. In addition, calculations using these many-body pair potentials can only account for 10 percent of the missing lattice energy. This model predicts an olivine-spinel phase transition of 8 GPa, below the experimental value of 20 GPa. Therefore, in order to understand more fully the stability of these structures we must consider polarization. A two-shell bond polarization model enhances the stability of both structures, with the olivine structure being stabilized more. This model predicts a phase transition at about 80 GPa, well above the observed value. Also, the olivine and spinel structures calculated with this approach are in poorer agreement with experiment than the ionic model. Therefore, based on our investigations, to properly model covalency in Mg2SiO4, a treatment more sophisticated than the two-shell model is needed.  相似文献   

5.
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6.
Crystal field stabilization (CFS) plays a significant role in determining equilibrium phase boundaries in olivine→spinel transformations involving transition-metal cations, including Fe2+ which is a major constituent of the upper mantle. Previous calculations for Fe2SiO4 ignored pressure and temperature dependencies of crystal field stabilization enthalpies (CFSE) and the electronic configurational entropy (S CFS). We have calculated free energy changes (ΔG CFS) due to differences of crystal field splittings between Fe2SiO4 spinel and fayalite from: ΔG CFS=?ΔCFSE?TΔS CFS, as functions of P and T, for different energy splittings of t 2g orbital levels of Fe2+ in spinel. The results indicate that ΔG CFS is always negative, suggesting that CFS always promotes the olivine→spinel transition in Fe2SiO4, and expands the stability field of spinel at the expense of olivine. Because of crystal field effects, transition pressures for olivine→spinel transformations in compositions (Mg1?x Fe x )2SiO4 are lowered by approximately 50x kbar, which is equivalent to having raised the olivine→spinel boundary in the upper mantle by about 15 km.  相似文献   

7.
Three new crystalline phases differing in Si/Al ratio have been synthesized from compositions along the join NiAl2O4-Ni2SiO4. Four reversible univariant equilibria involving these new phases plus Ni2SiO4 (olivine) have been located within the P-T region studied (1 atm–40 kb, 1000–1700° C); an invariant point occurs near 22 kb, 1150°C.All three new phases are orthorhombic. Precession photographs and electron microprobe analyses yield the following information:Phase I: 5NiO·3Al2O3·SiO2 = 3NiAl2O4·Ni2SiO4, Pmma, a=5.67, b=11.51, c=8.10 (Å)Phase II: 7NiO·3Al2O3·2SiO2 = 3NiAl2O4· 2Ni2SiO4, Imma, a=5.66, b=17.32, c=8.11Phase III: 3NiO· Al2O3· SiO2 = NiAl2O4·Ni2SiO4, Imma, a=5.68, b=11.49, c=8.12Comparison with known structures suggests that these three phases plus NiAl2O4 spinel and high pressure Ni2SiO4 spinel belong to a homologous series based on a cubic close oxygen packing of the formula: M2n O n}-1 (T n O3n+1) where M and T are octahedrally and tetrahedrally coordinated cations, respectively. When n=1 the formula for spinel is obtained; n = 2 for phase I and phase III, both similar to the beta-phase of orthosilicates; and n = 3 for phase II which is related to the manganostibite structure.Similar phase equilibria and structural relations may occur on other joins of the aluminateorthosilicate type. Furthermore, the occurrence of such structural modifications between the spinel (aluminate) and olivine (orthosilicate) compositions suggests that there could be a corresponding polymorphic series between the olivine and spinel forms of orthosilicates.  相似文献   

8.
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9.
Raman microprobe (RMP) spectra were produced for each of the olivine and spinel structured phases of Mg2GeO4 and (Mg, Fe)2SiO4. The assembled data show that bands due to the tetrahedra in silicate and germanate olivines shift in a way that indicates a dominant mass effect. This correspondence is difficult to make in spinels due to differences in structural type. Differences in Fe/Mg content of olivine shift the tetrahedral vibration bands only slightly, but their linear shifts could be used to indicate the composition of the phase.  相似文献   

10.
The thermodynamical stability of a newly observed wadsleyite II phase in the Mg2SiO4 system is studied by the density functional theory. The wadsleyite II equation of state has been derived. The phase boundaries of Mg2SiO4 polymorphs: wadsleyite, wadsleyite II and ringwoodite are studied using the quasi-harmonic approximation at high external pressures. Clapeyron slopes determined for wadsleyite II–ringwoodite and wadsleyite–wadsleyite II boundaries are 0.0047 and 0.0058 GPa/K, respectively. It is shown that the wadsleyite II phase is not thermodynamically preferred in the pure Mg2SiO4 system and will probably not occur between wadsleyite and ringwoodite phases.  相似文献   

11.
The crystal structures and energies of SiO2 stishovite, MgO periclase, Mg2SiO4 spinel, and MgSiO3 perovskite were calculated as a function of pressure with the polarization-included electron gas (PEG) model. The calculated pressures of the spinel to perovskite phase transitions in the Mg2SiO4 and MgSiO3 systems are 26.0 GPa and 27.0 GPa, respectively, compared to the experimental zero temperature extrapolations of 27.4 GPa and 27.7 GPa. The two oxide phases are found to be the most stable form in the pressure range 24.5 GPa to 31.5 GPa, compared to the experimental zero temperature extrapolation of 26.7 GPa to 28.0 GPa. The volume changes associated with the phase transitions are in good agreement with experiment. The transition pressures calculated with the PEG model, which allows the ions to distort from spherical symmetry, are in much better agreement with experiment than those calculated with the modified electron gas (MEG) model, which constrains the ions to be spherical.  相似文献   

12.
Ca2SiO4 dicalcium silicate has been transformed at high pressure in a diamond-anvil cell (DAC) coupled with a YAG laser heater, in order to study the high-pressure modifications of this compound. Starting material was the olivine form of Ca2SiO4 (γ-polymorph). Several samples have been synthesized at loading pressures of 4.5, 10 and 15 GPa respectively, at room temperature. Other samples have been obtained at pressures ranging between 4.5 and 45 GPa and temperatures estimated to be about 2500 °C. The study of the quenched high pressure and/or high temperature phases has been performed using analytical transmission electron microscopy (ATEM) and X-ray diffraction (XRD). All the polymorphs of Ca2SiO4 usually produced with high temperatures, including α-Ca2SiO4, have been observed in the samples recovered from the high-pressure experiments. The α′-Ca2SiO4 and α-Ca2SiO4 polymorphs have been obtained at ambient conditions for the first time without stabilizing impurities. A new modification of α′-Ca2SiO4 has also been synthesized. Finally, the breakdown at high-pressure and temperature of Ca2SiO4 into CaSiO3 and CaO is reported.  相似文献   

13.
The finding of ilmenite rods in olivine from orogenic peridotites has sparked a discussion about the processes of incorporation and exsolution of titanium in olivine. We have experimentally investigated the solubility of Ti in olivine as a function of composition, temperature and pressure in the synthetic TiO2–MgO–SiO2 system. Experiments at atmospheric pressure in the temperature range 1,200–1,500°C showed that the highest concentration of TiO2 is obtained when olivine coexists with spinel (Mg2TiO4). The amount of TiO2 in olivine in the assemblages olivine + spinel + periclase and olivine + spinel + ilmenite at 1,500°C was 1.25 wt.%. Changes in the coexisting phases and decreasing temperature result in a significant reduction of the Ti solubility. Olivine coexisting with pseudobrookite (MgTi2O5) and a Ti–Si-rich melt at 1,500°C displays a fourfold lower TiO2 content than when buffered with spinel. A similar decrease in solubility is obtained by a decrease in temperature to 1,200°C. There is a negative correlation between Ti and Si and no correlation between Ti and Mg in Ti-bearing olivine. Together with the established phase relations this suggests that there is a direct substitution of Ti for Si at these temperatures, such that the substituting component has the stoichiometry Mg2TiO4. The unit cell volume of olivine increases systematically with increasing TiO2 content demonstrating that the measured TiO2 contents in olivine are not caused by micro-inclusions but by incorporation of Ti in the olivine structure. Least squares fitting of 20 olivine unit cell volumes against the Ti content yield the relation: V (Å3)=290.12(1) + 23.67(85) NTi. The partial molar volume of end-member Mg2TiO4 olivine (NTi=1) is thus 47.24±0.13 cm3. The change of the Ti solubilty in olivine coexistent with rutile and orthopyroxene with pressure was investigated by piston cylinder experiments at 1,400°C from 15 to 55 kbar. There is no increase in TiO2 contents with pressure and in all the experiments olivine contains ~0.2 wt.% TiO2. Moreover, a thermodynamic analysis indicates that Ti contents of olivine coexisting with rutile and orthopyroxene should decrease rather than increase with increasing pressure. These data indicate that the ilmenite exsolution observed in some natural olivine does not signify an ultra-deep origin of peridotite massifs.  相似文献   

14.
A thermochemical data base for phases in the system Fe-Mg-Si-O at high pressures up to 300 kbar is established by supplementing the available calorimetric data with data calculated from experimental high pressure synthesis studies. Phases included in the data base are the SiO2 polymorphs, rock salt solid solutions (Fe-Mg-O), Fe2O3, Fe3O4, (Mg, Fe)2SiO4 olivine, spinel, modified spinel and (Mg, Fe)SiO3 perovskite and pyroxene. Phases not included are the MgSiO3-ilmenite and -garnet. Fe-Mg solution properties of olivine, spinel, perovskite and wustite (rock salt) are estimated. The wüstite solid solution has been modeled as a nonideal solution of three end members; FeO, FeO1.5 and MgO. The new data base is made consistent with most of the available information on high pressure phase studies. The data base is useful in generating phase diagrams of various different compositions for the purpose of planning new experiments and analysing existing phase synthesis data.  相似文献   

15.
High-resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of olivine [(Mg0.87Fe0.13)2SiO4], bronzite [(Mg0.8Fe0.2)2Si2O6] and diopside [Ca(Mg0.8Fe0.2)Si2O6] were collected before and after leaching in pH ∼2 solutions with the Kratos magnetic confinement charge compensation system which minimizes differential charge broadening. The leached samples yield Si 2p, Mg 2p, Ca 2p and O 1s XPS spectral linewidths and lineshapes similar to those collected from the respective pristine samples prior to leaching. As with previous XPS studies on crushed samples, our broadscan XPS spectra show evidence for initial, preferential leaching of cations (i.e., Ca2+ and Mg2+) from the near-surface of these minerals. The O 1s spectra of leached olivine and pyroxenes show an additional peak due to OH−, which arises from H+ exchange with near-surface cations (Ca2+ and Mg2+) via electrophilic attack of H+ on the M-O-Si moiety to produce the H2Mg(M1)SiO4(surf) complex at olivine surfaces, and two complexes, H2Mg(M1)Si2O6(surf) and H4Si2O6(surf) at diopside and enstatite surfaces. The olivine and pyroxene surface complexes H2Mg(M1)SiO4(surf) and H2Mg(M1)Si2O6(surf) have been proposed previously, but the second pyroxene surface complex H4Si2O6(surf) has not. Two electrophilic reactions occur in both olivine and pyroxene. For olivine, the more rapid attacks the M2-O-Si moiety producing H2Mg(M1)SiO4(surf); while the second attacks the M1-O-Si moiety ultimately producing H4SiO4 which is released to solution. For pyroxenes, the first electrophilic reaction produces H2Mg(M1)Si2O6(surf), while the second produces.H4Si2O6(surf). These two reactions are followed by a nucleophilic attack of H2O (or H3O+) on Si of H4Si2O6(surf). This reaction is responsible for rupture of the brigding oxygen bond of the Si-O-Si moiety and release of H4SiO4 to solution. The intensity of the OH− peak for the leached pyroxenes is about double the OH− intensity for the leached olivine, consistent with the equivalent of about a monolayer of the above surface complexes being formed in all three minerals.Valence band XPS spectra and density functional calculations demonstrate the remarkable insensitivity of the valence band to leaching of Ca2+ and Mg2+ from the surface layers. This insensitivity is due to a dearth of Ca and Mg valence electron density in the valence band: the Ca-O and Mg-O bonds are highly ionic, with metal-derived s orbital electrons taking on strong O 2p character. The valence band spectrum of leached olivine shows an additional very weak peak at about 13.5 eV, which is assigned to Si 3s valence orbitals in the surface complex H2Mg(M1)SiO4, as indicated by high quality density functional calculations on an olivine where Mg2+ in M2 is replaced by 2H+. The intensity of this new peak is consistent with formation of the equivalent of a monolayer of the surface complex.  相似文献   

16.
Partition of Fe2+ and Mg between coexisting (Mg, Fe)2SiO4 spinel and (Mg, Fe)SiO3 pyroxene was investigated at pressures 80 and 90 kbar and at temperatures 840 and 1050° C, using tetrahedral-anvil type of high pressure apparatus. Olivine-spinel solid solution equilibria in the system Mg2SiO4-Fe2SiO4 were discussed in the light of the partition reaction. Partition of Fe2+ and Mg in both olivine-spinel and pyroxene-spinel systems can not be regarded as that between ideal solid solutions. By applying the simple solution model for the partition of Fe2+ and Mg, sign of the heat of mixing was estimated to be positive for all olivine, spinel and pyroxene. Relative concentration of Fe2+ in spinel in the pyroxene-spinel system is likely to cause some change in the chemical composition of modified spinel () or spinel () in the transition zone of the mantle. A considerable change is also expected in the transition pressure of to ( + ) and ( + ) to .Presented at the symposium Recent Advances in the Studies of Rocks and Minerals at High Pressures and Temperatures held in Montreal, 1972. Jointly sponsored by the International Mineralogical Association and the Commission on Experimental Petrology.  相似文献   

17.
Subsolidus phase relations have been determined in the systems SiO2-Cr-O and MgO-SiO2-Cr-O in equilibrium with metallic Cr, at 1100 to 1500℃ and 0 to 2.88 GPa. The results show that there are no ternary phases in the SiO2-Cr-O system at these conditions, i.e., only the assemblage eskolaite-Cr-metal-quartz (or tridymite) is found. In the MgO-containing system, however, extensive substitution of Cr2+ for Mg is observed in (Mg, Cr2+)2SiO4 olivine, (Mg, Cr2+)2Si2O6 pyroxene, and (Mg, Cr2+)Cr2O4 spinel. Cr3+ levels in olivine and pyroxene are below detection limits. The pyroxene is orthohombic at XCrPx2+ < 0.2, monoclinic at higher XCrPx2+ . Thestructure of the spinels becomes tetragonally distorted at XCr2+Sp >0.2. The experimental datahave been fitted to a thermodynamic model, and the authors obtained the mixing parameter (W) of Mg-Cr2+ in olivine, pyroxene and spinel, and the relation between temperatures and free energies of formation for the end-members: Cr2+-olivine (Cr2SiO4), Cr2+-pyroxene (Cr2Si2O6)  相似文献   

18.
Chromian valleriite occurs as a replacement product of chrome spinel in the Nepean and Perseverance nickel-iron sulphide deposits of Western Australia. Compositions determined for chromian valleriites are variable due to intergrowth with spinel and to substitution of Fe and Ni for Cu. At Perseverance, valleriite, with minor admixed chrome spinel, has a composition expressed by the formula Fe1.18Cu0.81Ni0.01S2.00·2.03[Mg0.50Cr0.37Mn0.08Fe0.03Ti0.01Al0.01(OH)2.00]. Nepean valleriites are similarly intergrown with spinel, with the purest chromian valleriites varying in composition from a nickel-poor Fe1.21Cu0.78Ni0.01S2.00·1-46.[Mg0.43Cr0.18Mn0.05Fe0.30(OH)2] to Fe1.58Cu0.28Ni0.15S2.00·1.79[Mg0.51Cr0.30Fe0.16Ti0.03(OH)2]. Conversion of chrome spinel to valleriite is attributed to addition of Cu and S from the breakdown of chalcopyrite, addition of Mg from serpentinising olivine and limited addition of Ni from the breakdown of pentlandite to mackinawite. These processes are thought to occur as post-metamorphic hydrothermal reactions in nickel sulphide ores that have been subjected to high grade regional metamorphism.  相似文献   

19.
The solubility and incorporation mechanisms of water in synthetic and natural MgAl2O4 spinel have been investigated in a series of high-pressure/temperature annealing experiments. In contrast to most other nominally anhydrous minerals, natural spinel appears to be completely anhydrous. On the other hand, non-stoichiometric Al-rich synthetic (defect) spinel can accommodate several hundred ppm water in the form of structurally-incorporated hydrogen. Infrared (IR) spectra of hydrated defect spinel contain one main O-H stretching band at 3343-3352 cm−1 and a doublet consisting of two distinct O-H bands at 3505-3517 cm−1 and 3557-3566 cm−1. IR spectra and structural refinements based on single-crystal X-ray data are consistent with hydrogen incorporation in defect spinel onto both octahedral and tetrahedral O-O edges. Fine structure of O-H bands in IR spectra can be explained by partial coupling of interstitial hydrogen with cation vacancies, or by the effects of Mg-Al disorder on the tetrahedral site. The concentration of cation vacancies in defect spinel is a major control on hydrogen affinity. The commercial availability of large single crystals of defect spinel coupled with high water solubility and similarities in water incorporation mechanisms between hydrous defect spinel and hydrous ringwoodite (Mg2SiO4) suggests that synthetic defect spinel may be a useful low-pressure analogue material for investigating the causes and consequences of water incorporation in the lower part of Earth’s mantle transition zone.  相似文献   

20.
We use an approach based upon the Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the phonon dispersion of forsterite and the lattice dynamical behaviour of the beta-phase and spinel polymorphs of Mg2SiO4. The potential used (THB1) was derived largely empirically using data from simple binary oxides, and has previously been successfully used to model the infrared and Raman behaviour of forsterite. It includes ‘bond bending’ terms, that model the directionality of the Si-O bond, in addition to the pair-wise additive Coulombic and short range terms. The phonon dispersion relationships of the Mg2SiO4 polymorphs predicted by THB1 were used to calculate the heat capacities, entropies, thermal expansion coefficients and Gruneisen parameters of these phases. The predicted heat capacities and entropies are in outstandingly good agreement with those determined experimentally. The predicted thermodynamic data of these phases were used to construct a phase diagram for this system, which has Clausius-Clapeyron slopes in very close agreement with those found by experiment, but which has predicted transformation pressures that show less close agreement with those inferred from experiment. The overall success, however, that we have in predicting the lattice dynamical and thermodynamic properties of the Mg2SiO4 polymorphs shows that our potential THB1 represents a significant step towards finding the elusive quantitative link between the microscopic or atomistic behaviour of minerals and their macroscopic properties.  相似文献   

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