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1.
Si K-, L- and O K-edge ELNES spectra of natural α-quartz and synthetic coesite on one side and synthetic stishovite on the other show characteristic differences that can be related, by comparison with multiple-scattering (MS) calculations, to the fourfold vs. sixfold coordination of Si in these polymorphs of SiO2. It is shown by MS calculations on large clusters that the outer shells contribute relatively little to the overall topologies of the spectra. Therefore, distinction between fourfold- and sixfold-coordinated Si is possible even on a nm scale and probably also in amorphous substances.  相似文献   

2.
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   

3.
Quartz‐rich veins in metapelitic schists of the Sanandaj‐Sirjan belt, Hamadan region, Iran, commonly contain two Al2SiO5 polymorphs, and, more rarely, three coexisting Al2SiO5 polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al2SiO5 polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe2O3 relative to host rock andalusite. Low‐grade rocks contain andalusite + quartz veins, medium‐grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high‐grade rocks contain sillimanite + quartz + plagioclase veins/leucosomes. Although most andalusite and sillimanite‐bearing veins occur in host rocks that also contain Al2SiO5, kyanite‐quartz veins crosscut rocks that lack Al2SiO5 (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite‐quartz veins post‐date andalusite–sillimanite veins and at least one intrusive phase of a granite that produced a low‐pressure–high‐temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightly below) the Al2SiO5 triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen.  相似文献   

4.
Preferential leakage of H2O from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic H2O-CO2-rich inclusions in natural quartz. Knowledge of the physical and chemical characteristics of the leakage mechanism, which may be very complex, increases the value of natural fluid inclusions to metamorphic geology. It is proposed that crystal defects play a major role in nondecrepitative preferential H2O leakage through quartz, and remain effective during metamorphism. Inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects (like Dauphiné twin boundaries). These defects allow preferential loss of H2O from H2O-CO2-rich inclusions at supercritical conditions. The transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures. The nucleation of these bubbles seems to be a recovery process in strained crystals. Solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement, caused by the growth of small bubbles at the expense of the larger original fluid inclusion. Natural fluid inclusions from Naxos (Greece) are always associated with numerous interlinked dislocations. These dislocations may have been produced by plastic derormation or by crystal growth related processes (e.g. crack healing). The presence of small bubbles on these dislocations indicates that a similar leakage mechanism for H2O must have occurred in these rocks.  相似文献   

5.
We have carried out a Raman Spectroscopic study of single crystalline quartz samples shocked to peak pressures up to 31.4GPa. Samples shocked to above 22 GPa show shifts in peak positions consistent with the quartz being under tensile stress, and new broad bands associated with the formation of high density SiO2 glass appear in the spectra. These changes are accompanied by an increase in the lattice parameters of the quartz. Formation of the diaplectic glass could be due to a metastable melting event, or spinodal lattice collapse on attainment of a mechanical stability limit of crystalline quartz, as suggested by previous studies of pressure-induced amorphization in static pressurization experiments on SiO2 and GeO2 polymorphs.  相似文献   

6.
The modified niccolite structure (Fe2N-type) of SiO2, previously found in diamond anvil experiments at 35 to 40 GPa, was formed in a porous mixture of crystalline α-quartz and copper powder at shock pressures estimated at 12 to 27 GPa. It is suggested that quartz can invert during shock compression not only to coesite, stishovite and an amorphous or glass phase of silica, but also to Fe2N-type SiO2, depending upon the shock history.  相似文献   

7.
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   

8.
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   

9.
A two-body interatomic potential model for GeO2 polymorphs has been determined to simulate the structure change of them by semi-empirical procedure, total lattice energy minimization of GeO2 polymorphs. Based on this potential, two polymorphs of GeO2; α-quartz-type and rutile-type, have been reproduced using the molecular dynamics (MD) simulation techniques. Crystal structures, bulk moduli, volume thermal expansion coefficients and enthalpies of these polymorphs of GeO2 were simulated. In spite of the simple form of the potential, these simulated structural values, bulk moduli and thermal expansivities are in excellent agreement with the reliable experimental data in respect to both polymorphs. Using this potential, MD simulation was further used to study the structural changes of GeO2 under high pressure. We have investigated the pressure-induced amorphization. As reported in previous experimental studies, quartz-type GeO2 undergoes pressure-induced crystalline-to-amorphous transformation at room temperature, the same as other quartz compounds; SiO2, AlPO4. Under hydrostatic compression, in this study, α-quartz-type GeO2 transformed to a denser amorphous state at 7.4 GPa with change of the packing of oxygen ions and increase of germanium coordination. At higher pressure still, rutile-type GeO2 transformed to a new phase of CaCl2-type structure as a post-rutile candidate. Received: 29 July 1996 / Revised, accepted: 30 April 1997  相似文献   

10.
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11.
Liquidus phase relationships determined on the join CaAl2Si2O8 (anorthite)-Mg2SiO4 (forsterite)-SiO2 (quartz) at 10 kbar show that increasing pressure causes the forsterite and anorthite primary phase fields to shrink and the spinel, enstatite and silica fields to expand. The boundary line between the enstatite and forsterite fields and that between the enstatite and quartz fields both move away from the SiO2 apex as pressure increases. Therefore, simplified source peridotite would yield simplified basaltic partial melts with decreasing silica as pressure increases, as has been found in other studies. Also, increasing pressure decreases the amount of silica enrichment in residual liquids produced by fractional crystallization. Although anorthite is unstable in simplified peridotite above 9 kbar in the system CaO-MgO-Al2O3-SiO2, it is an important phase in the fractional crystallization of simplified basalts at 10 kbar and probably also in natural basalts.Contribution no. 419, Department of Geosciences, University of Texas at Dallas  相似文献   

12.
ABSTRACT The decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure H2O synthetic inclusions formed by healing fractures in natural quartz. Three different modes of non-elastic deformation, referred to as stretching, leakage or partial decrepitation, and total decrepitation have been observed. The internal pressure required to initiate non-elastic deformation is inversely related to inclusion size according to the equation: internal pressure (kbar) = 4.26 D-0.423 where D is the inclusion diameter in microns. Regularly shaped inclusions require a higher internal pressure to initiate non-elastic deformation than do irregularly shaped inclusions of similar size. Heating inclusions through the α/β quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion. Long-term heating experiments (∼2 years) suggest that the internal pressure required to initiate non-elastic deformation does not decrease significantly with time and indicates that short-lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature. Inclusions that do exhibit decreased density (higher homogenization temperature) are, however, always accompanied by a change in shape from irregular to that of a negative crystal. Observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals. These results indicate that an inclusion will not be influenced by a neighbouring inclusion, or other defect in the host phase, as long as the distance between the two is >2–4 diameters of the larger of the two inclusions.  相似文献   

13.
We report relative enthalpy measurements on quartz, cristobalite and amorphous SiO2 between 1000 and 1800 K. We have observed a glass transition around 1480 K for amorphous SiO2. From our results and available Cp, relative enthalpy, and enthalpy of solution data we have derived a consistent set of thermodynamic data for these phases. Our calculated enthalpies of fusion are 8.9 ± 1.0 kJ mole?1 for cristobalite at 1999 K and 9.4 ± 1.0 kJ mole?1 at 1700 K for quartz.  相似文献   

14.
Equilibria for several reactions in the system CaO-Al2O3-SiO2-CO2-H2O have been calculated from the reactions calcite+quartz=wollastonite+CO2 (5) and calcite+Al2SiO5+quartz=anorthite+CO2 (19) and other published experimental studies of equilibria in the systems Al2O3-SiO2-H2O and CaO-Al2O3-SiO2-H2O.The calculations indicate that the reactions laumontite+CO2=calcite+kaolinite+2 quartz+2H2O (1) and laumontite+calcite=prehnite+quartz+3H2O+CO2 (3) in the system CaO-Al2O3-SiO2-CO2-H2O, are in equilibrium with an H2O-CO2 fluid phase having -0.0075 for P fluid=P total=2000 bars.These calculations limit the stability of zeolite assemblages to low p CO2.Using the above reactions as model equilibria, several probelms of p CO2 in low grade metamorphism are discussed. (a) the problem of producing zeolitic minerals from metasedimentary assemblages of carbonate, clay mineral, quartz. (b) the significance of calcite (or aragonite) associated with zeolite (or lawsonite) in low grade metamorphism and hydrothermal alteration. (c) the reaction of zeolites (or lawsonite) with calcite (or aragonite) to produce dense Ca-Al-hydrosilicates (eg. prehnite, zoisite, grossular).  相似文献   

15.
Hydrothermal syntheses were made mainly in the binary system SiO2-H2O in a temperature range between 300 ° C and 500 ° C and pressures from 0.2 kbar up to 4.0 kbar with various starting materials. In this way the transformation behavior of different amorphous silicas via cristobalite and keatite to quartz were observed. This behavior depends mainly on the parameters: pressure, temperature, run duration and state of the starting material. Four reaction paths have been observed: in most experiments the complete reaction sequence “amorphous silica→cristobalite→keatite→quartz” took place. Less often the reactions: “amorphous silica→cristobalite→quartz” and: “amorphous silica→keatite→quarts” were observed. Very few samples were found with a direct transition of amorphous silica into quartz at high pressures. A kinetic model is given in form of a pressure-temperature-time diagram of the system SiO2-H2O under hydrothermal conditions.  相似文献   

16.
The rate of dissolution of Fontainebleau sand (pure quartz) into sodium hydroxide solutions (from 0.001 M to 0.5 M) has been determined at 90°C in well-stirred vessels. Dissolution leads to an equilibrium state, controlled by the solubility of quartz in pure water as undissociated silicic acid H4SiO4. As long as the initial molality of sodium hydroxide does not exceed 0.02 mol · kg−1, the dissolution leads only to the formation of the three monomeric species H4SiO4, H3SiO4− and H2SiO42−, while polymers appear in the silica-rich solutions obtained in more alkaline media. The rate of dissolution can be represented by an adaptation of Stöber's model to alkaline solutions; the basic assumption is that the quartz surface is partially covered by a layer of adsorbed silicate ions, which represent an intermediate species between solid and dissolved silica.  相似文献   

17.
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   

18.
The effect of silica concentration on the solubility of Al and Ti in diopsidic pyroxenes has been investigated at one atmosphere in the system diopside-CaTiAl2O6-SiO2 at temperatures between 1150–1420° C. The composition of pyroxene in the system diopside-CaTiAl2O6-SiO2 is influenced by the total SiO2 content. Near the join diopside-CaTiAl2O6, the pyroxene forms a solid solution with the CaTiAl2O6 molecule, and co-exists with perovskite for compositions greater than 11 weight percent CaTiAl2O6. Anorthite is an accompanying phase. With increasing total SiO2 content a series of mineralogical changes involving Ti-bearing phases occur. Sphene solid solution co-exists with diopside solid solution, anorthite, and perovskite in a small compositional range near the diopside-CaTiAl2O6 join. Additional total SiO2 results in the elimination of perovskite and a decrease of solid solution in the pyroxenes. With further increase in SiO2 content, tridymite appears and the pyroxene is approximately pure diopside. Rutile joins diopside, anorthite, sphene solid solution and tridymite over a broad compositional range in the Ti, Si-rich part of the system. These results demonstrate that increased silica concentration decreases the solubility of Al and Ti in diopsidic pyroxenes and controls the stability of co-existing Ti-bearing phases.  相似文献   

19.
Cordierite–quartz and plagioclase–quartz intergrowths in a paragneiss from northern Labrador (the Tasiuyak Gneiss) were studied using SEM, STEM and TEM. The gneiss experienced granulite facies conditions and partial melting during both regional and, subsequently, during contact metamorphism. The microstructures examined all results from the contact metamorphism. Cordierite–quartz intergrowths occur on coarse and fine scales. The former sometimes exist as a ‘geometric’ intergrowth in which the interface between cordierite and quartz appears planar at the resolution of the optical microscope and SEM. The latter exists in several microstructural variants. Plagioclase is present as a minor component of the intergrowth in some examples of both the coarse and fine intergrowth. Grain boundaries in cordierite–quartz intergrowths are occupied by amorphous material or a mixture of amorphous material and chlorite. Cordierite and quartz are terminated by crystal faces in contact with amorphous material. Chlorite is sometimes found on cordierite surfaces and penetrating into cordierite grains along defects. Quartz contains (former) fluid inclusions 10–20 nm in maximum dimension. The presence of planar interfaces between cordierite and the amorphous phase is reminiscent of those between crystals and glass in volcanic rocks, but in the absence of compelling evidence that the amorphous material represents former melt, it is interpreted as a reaction product of cordierite. Plagioclase–quartz intergrowths occur in a number of microstructural variants and are commonly associated with cordierite–quartz intergrowths. The plagioclase–quartz intergrowths display simple, non‐planar interfaces between plagioclase and quartz. Quartz contains (former) fluid inclusions of dimensions similar to those observed in cordierite–quartz intergrowths. The boundary between quartz and enclosing K‐feldspar is cuspate, with quartz cusps penetrating a few tens of nanometres into K‐feldspar, commonly along defects in K‐feldspar and sometimes with very low dihedral angles at their tips. This cuspate microstructure is interpreted as melt pseudomorphs. The plagioclase–quartz intergrowths share some features with myrmekite, but differ in some respects: the composition of the plagioclase (An37Ab62Or1–An38Ab61Or1); the association with cordierite–quartz intergrowths; and microstructures that are atypical of myrmekite (e.g. quartz vermicules shared with cordierite–quartz intergrowths). It is inferred that the plagioclase–quartz intergrowths may have formed from, or in the presence of, melt. Inferred melt‐related microstructures preserved on the nanometre scale suggest that melt on grain boundaries was more pervasive than is evident from light optical and SEM observations.  相似文献   

20.
 One of the main uncertainties in mineralogical models of the Earth's lower mantle is the nature of the aluminous mineral: it is not clear whether Al forms its own minerals or is mainly contained in (Mg,Fe)SiO3-perovskite. This question is very important, since it is known that if Al were mainly hosted by perovskite, it would radically change Fe/Mg-partitioning and phase equilibria between mantle minerals, and also alter many physical and chemical properties of perovskite, which is currently believed to comprise ca. 70% of the volume of the lower mantle. This, in turn, would require us to reconsider many of our geochemical and geophysical models for the lower mantle. This work considers the possibility of a V3O5-type structured modification of Al2SiO5 to be the main host of Al in the lower mantle, as proposed by previous workers. We report ab initio calculations, based on density functional theory within the generalised gradient approximation (GGA) with plane wave basis set and nonlocal pseudopotentials. We consider polymorphs of Al2SiO5 (kyanite, andalusite, sillimanite, and hypothetical V3O5-like and pseudobrookite-like phases), SiO2 (stishovite, quartz) and Al2O3 (corundum). Computational conditions (e.g., plane-wave energy cutoff, Brillouin zone sampling) were carefully chosen in order to reproduce small energy changes associated with phase transitions between the Al2SiO5 polymorphs. Good agreement of crystal structures, bulk moduli, atomisation energies and the phase diagram of Al2SiO5 with experimental data was found. Strong disagreement between the calculated lattice parameters and density of V3O5-like phase of Al2SiO5 and experimental values, assigned to it by previous workers, suggests that a V3O5-structured phase of Al2SiO5 was never observed experimentally. In addition, we found that the most stable high-pressure assembly in Al2SiO5 system is corundum+stishovite, and the value of the transition pressure at T = O K (113 kbar) is in excellent agreement with experimental estimates (95–150 kbar). We explain the instability of octahedrally coordinated silicates of Al to decomposition on the basis of Pauling's second rule. Received: 18 May 1999 / Accepted: 5 November 1999  相似文献   

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