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1.
The crystal structures of the two hydrous wadsleyite crystals with formulae, Mg1.75SiH0.50O4 (0.5H–β) and Mg1.86SiH0.28O4 (0.3H–β) have been analyzed in this study. The single-crystal X-ray diffraction data showed that the unit cells of the 0.3H–β and the 0.5H–β are metrically monoclinic with a slight distortion from the orthorhombic cell but their intensity distributions conform to the orthorhombic symmetry within the limit of experimental errors. The Fourier and the difference Fourier syntheses were calculated. Small but significant Fourier peaks were found at the site, Si2, in a normally vacant tetrahedral void adjacent to Mg3 site as reported for the monoclinic hydrous wadsleyite by Smyth et al.. From the comparison of the hydrous and anhydrous wadsleyite structures, the Mg-vacant structural modules were found to be the building units for the structure of hydrous wadsleyite. The dilution of symmetry from orthorhombic to monoclinic in the hydrous wadsleyite structure is interpreted qualitatively due to lack of mirror perpendicular to the a axis in the module. The mode of arrangement of the Mg-vacant structural modules interprets the symmetry and hydrogen content of the hydrous wadsleyite and gives the structural relationship between hydrous wadsleyite and hydrous ringwoodite. Received: 8 May 1998 / Revised, accepted: 3 October 1998  相似文献   

2.
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3.
The sequential displacement mechanism based on the oxygen-lattice cubic closest packing (c.c.p.) in the < 2 0 1 > direction was proposed in this study. All displacements of cations are within the OT and the O layers with the length of displacement vector being around 1.7 or 2.9 Å, contrary to displacement of around 5.0 Å for models proposed previously. The difference in atomic arrangement between hydrous wadsleyite and hydrous ringwoodite is small. The atomic arrangement of the O layer of hydrous wadsleyite is essentially the same as that of hydrous ringwoodite when Mg vacancies preferentially exist in the O layer. The partial occupancies of normally vacant tetrahedral sites reported in the hydrous-β and hydrous-γ structures may possibly be caused by the existence of Mg vacancies at the octahedral sites through phase transition from hydrous-β to hydrous-γ or from hydrous-γ to hydrous-β phases.  相似文献   

4.
The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000° C, have been solved and refined in space group $\bar 3$ . The compositions and unit cells for the two materials, assuming six oxygens per cell, are Mg2.08Si1.16H3.20O6, a=2.9701(1) Å, c=13.882(1) Å V = 106.05(4) Å3 for sample 1, and Mg2.17Si1.01H3.62O6, a=2.9853(6) Å, c=13.9482(7) Å, V= 107.65(4) Å3 for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlay er octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no long-range order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hågg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.  相似文献   

5.
The structure of phase F, a non-stoichiometric hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions of 17 GPa and 1000° C, has been solved and refined in space group P63cm, using synchrotron X-ray data for a single crystal of a size 18 × 24 × 30 μm. The composition and unit cell for phase F are Mg3.35Si5.51H7.26O18, a=5.073(3) Å, c=14.013(9) Å, 7= 312.3(5) Å3. The structure contains layers with many similarities to superhydrous phase B. The layers of oxygen atoms are stacked in the ABCBAC-type double cubic closest packing arrangement. The bulk modulus of phase F was estimated from the structural and compositional relationship to superhydrous phase B and periclase.  相似文献   

6.
A possible structure for hydrous modified olivine (HyM-α) has been obtained by the subtraction of Mg3SiO5 from forsterite by crystallographic shear along a direction parallel to the [010] direction of olivine. The subtraction of Mg3SiO5 results in the subtraction of MgO from bulk chemistry (?Mg3SiO5=?Mg2SiO4?MgO). A possible structure for HyM-α thus obtained has the chemical formula Mg9Si5H2O20 (= 5 × Mg1.8SiH0.4O4) with monoclinic unit cell a=4.754 Å, b=10.19 Å, c=29.90 Å, ρ=3.126 g cm?3, and space group=Ac2m (no. 39). Since the X-ray powder diffraction pattern of HyM-α proposed in this study is very close to that of clinohumite, there is the possibility of this phase having been undiscovered. The humite group minerals and HyM-α proposed in this study make a homologous series as recombination structures: Mg(2 m + n )Si m H2 n O2(2 m + n ) for the humite group and Mg(2 m + n )Si( m + n )H2 n O4( m + n ) for HyM-α A characteristic feature is that Mg/Si > 2 for the humite group and Mg/Si < 2 for HyM-α. Forsterite specimens containing around 100 ppm H2O reported in mantle xenoliths might be the disordered case with n=1 and m=1200 of the humite group or HyM-α.  相似文献   

7.
Infrared absorption spectra are presented for four hydrous magnesian silicates which are stable at high pressures and high temperatures characteristic of the earth's mantle: phase A (Mg7Si2O14H6), phase B (approx. Mg23Si8O42H6), chondrodite (Mg5Si2O10H2), clinohumite (Mg9Si4O18H2). The results show that phase B as well as phase A, chondrodite and clinohumite contain hydroxyl groups in their structures. It is also suggested that phase B, in addition to phase A, could be of potential importance in the upper mantle and transition zone as a dense host mineral for hydroxyl ions. Chemical composition of phase A was reexamined by microprobe analysis: it agrees well with Mg7Si2O14H6, estimated by Yamamoto and Akimoto (1974, 1977), but disagrees with a newly proposed formula Mg2SiO5H2 by Benimoff and Sclar (1984).  相似文献   

8.
The heat capacity of synthetic hydrous cordierite (Mg2Al4Si5O18·nH2O) has been determined by differential scanning calorimetry (DSC) from 295 to 425 K as a function of H2O content. Six samples with H2O contents ranging from 0 to 0.82 per formula unit were examined. The partial molar heat capacity of H2O in cordierite over the measured temperature interval is independent of composition and temperature within experimental uncertainty and is equal to 43.3 ±0.8 J/mol/ K. This value exceeds the molar heat capacity of gaseous H2O by 9.7 J/mol/K, but is significantly smaller than the heat capacity of H2O in several zeolites and liquid H2O. A statistical-mechanical model of the heat capacity of adsorbed gas species (Barrer 1978) is used to extrapolate the heat capacity of hydrous cordierite to temperatures greater than 425 K. In this model, the heat capacity of hydrous cordierite (Crd·nH2O) is represented as follows: Cp(Crd · nH2O) = Cp(Crd)+ n{Cp(H2O, gas)+ R(gas constant)} (1) An examination of calorimetric data for hydrous beryl, analcime, mordenite, and clinoptilolite (Hemingway et al. 1986; Johnson et al. 1982, 1991, 1992) demonstrates the general applicability of the statistical-mechanical model for the extrapolation of heat capacity data of zeolitic minerals. The heat capacity data for cordierite are combined with the data of Carey and Navrotsky (1992) to obtain the molar enthalpy of formation and enthalpy of hydration of hydrous cordierite as a function of temperature.  相似文献   

9.
Raman spectra of monoclinic Fo90 hydrous wadsleyite with 2.4 wt% H2O have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium to 58.4 GPa at room temperature. The most intense, characteristic wadsleyite modes, the Si–O–Si symmetric stretch at 721 cm−1 and the symmetric stretch of the SiO3 unit at 918 cm−1, shift continuously to 58.4 GPa showing no evidence of a first order change in the crystal structure despite compression well beyond the stability field of wadsleyite in terms of pressure. The pressure dependence of these two modes is nearly identical for Fo90 hydrous and Fo100 anhydrous wadsleyite. A striking feature in the high-pressure Raman spectra of Fo90 hydrous wadsleyite is the appearance of new Raman modes above 9 GPa in the mid-frequency range (300–650 cm−1 at 1-bar and shifted to 500–850 cm−1 at 58.4 GPa) accompanied by a significant growth in their intensities under further compression. In the OH stretching frequency range Fo90 hydrous wadsleyite exhibits a larger number of modes than the Mg end-member phase. The higher number of modes may be due to either additional protonation sites or simply that we observe a different subset of all possible OH modes for each sample. The high-pressure behaviour of the OH stretching modes of Fo90 and Fo100 hydrous wadsleyite is consistent: OH stretching modes with frequencies <3,530 cm−1 decrease with increasing pressure whereas the higher-frequency OH modes show a close to constant pressure dependence to at least 13.2 GPa. The approximately constant pressure dependence of the OH modes above 3,530 cm−1 is consistent with protons being located at the O1···O edges around M3.  相似文献   

10.
 We carried out a series of melting experiments with hydrous primitive mantle compositions to determine the stability of dense hydrous phases under high pressures. Phase relations in the CaO–MgO–Al2O3–SiO2 pyrolite with ˜2 wt% of water have been determined in the pressure range of 10–25 GPa and in the temperature range between 800 and 1400 °C. We have found that phase E coexisting with olivine is stable at 10–12 GPa and below 1050 °C. Phase E coexisting with wadsleyite is stable at 14–16 GPa and below 900 °C. A superhydrous phase B is stable in pyrolite below 1100 °C at 18.5 GPa and below 1300 °C at 25 GPa. No hydrous phases other than wadsleyite are stable in pyrolite at 14–17 GPa and 900–1100 °C, suggesting a gap in the stability of dense hydrous magnesium silicates (DHMS). We detected an expansion in the stability field of wadsleyite to lower pressures (12 GPa and 1000 °C). The H2O content of wadsleyite was found to decrease not only with increasing temperature but also with increasing pressure. The DHMS phases could exist in a pyrolitic composition only under the conditions present in the subducting slabs descending into the lower mantle. Under the normal mantle and hot plume conditions, wadsleyite and ringwoodite are the major H2O-bearing phases. The top of the transition zone could be enriched in H2O in accordance with the observed increase in water solubility in wadsleyite with decreasing pressure. As a consequence of the thermal equilibration between the subducting slabs and the ambient mantle, the uppermost lower mantle could be an important zone of dehydration, providing fluid for the rising plumes. Received: 9 September 2002 / Accepted: 11 January 2003 Acknowledgements The authors are thankful to Y. Ito for the assistance with the EPMA measurement, A. Suzuki, T. Kubo and T. Kondo for technical help with the high-pressure experiments and Raman and X-ray diffraction measurements and C.R. Menako for technical support. K. Litasov thanks H. Taniguchi for his continuous encouragement and the Center for Northeast Asian Studies of Tohoku University and the Japanese Society for the Promotion of Science for the research fellowships. This work was partially supported by the Grant-in-Aid of Scientific Research of the Priority Area (B) of the Ministry of Education, Science, Sport, and Culture of the Japanese government (no. 12126201) to E. Ohtani.  相似文献   

11.
The new synthetic phase Mg2Al3O[BO4]2(OH) provisionally named “pseudosinhalite” is optically, chemically, and structurally similar to the mineral sinhalite, MgAl[BO4], isostructural with forsterite. It grows hydrothermally from appropriate bulk compositions in the range 4–40?kbar at temperatures that increase with pressure (～650?→?900?°C), and it breaks down at higher temperatures to sinhalite?+?corundum?+?H2O. At P?≥?20?kbar single-phase products of euhedral twinned crystals could often be obtained. Pseudosinhalite is monoclinic with a?=?7.455 (1) Å, b?=?4.330 (1) Å, c?=?9.825 (2) Å, β?=?110.68 (1)°, and space group P21/c. Crystal structure analysis reveals that pseudosinhalite is also based on hexagonal close packing (hcp) of oxygen atoms with Mg and Al in octahedral and B in tetrahedral coordination. In pseudosinhalite the winged octahedral chains in the plane of hcp are not straight as in sinhalite but have a zigzag, 3-repeat period (Dreierkette), and only 1/10 instead of 1/8 of all tetrahedral sites are filled by boron. Hydrogen is located at a split position between two oxygen atoms O5—O5, which are only 2.550 Å apart and thus generate strong hydrogen bonding. This may be responsible for the absence of an hydroxyl absorption band between 2800?cm?1 and 3500?cm?1 in the powder IR spectrum. The equilibrium breakdown curve of pseudosinhalite to form sinhalite, corundum, and water was determined by bracketing experiments to pass through 10?kbar, 745?°C and 35?kbar, 950?°C, giving a slope of about 8?°C/kbar, similar to dehydration curves of some silicates at high pressure. In nature pseudosinhalite could have been misidentified as sinhalite. A possible appearance, like sinhalite in boron-rich skarns, would require more aluminous bulk compositions than for sinhalite at relatively low temperatures. However, pseudosinhalite might also form as a hydrous alteration product of sinhalite at low temperatures, perhaps in association with szaibelyite, MgBO2(OH).  相似文献   

12.
Hydrogen site positions and occupancy in the crystal structure of dense hydrous magnesium silicate (DHMS) phase E were determined for the first time by pulsed neutron powder diffraction. A fully deuterated pure phase E powder sample, which had space group \(R\overline{3} m\) and lattice parameters of a = 2.97065(8) Å and c = 13.9033(4) Å, was synthesized at 15 GPa and 1100 °C. Through quantitative evaluation of refined structure parameters obtained with sufficient spatial resolution and very high signal-to-background ratio, we conclude that the O–D dipoles in the refined phase E structure are tilted by 24° from the direction normal to the layers of edge-shared MgO6 octahedra (octahedral layers). The tilted dipole structure of phase E is in remarkable contrast to that of brucite, Mg(OH)2, which has dipoles exactly normal to the octahedral layer. This contrast exists because the O–Si–O bonding unique in the phase E structure connects two adjacent octahedral layers and thereby reduces the interlayer O···O distance. This shrinkage of the interlayer distance induces the tilting of the O–D dipole and also generates unique O–D···O hydrogen bonding connecting all the layers in the phase E structure.  相似文献   

13.
Crystal chemistry of wadsleyite II and water in the Earth’s interior   总被引：1，自引：1，他引：0  
Wadsleyite II is a variably hydrous magnesium-iron silicate phase similar to spinelloid IV and a potential host for H in the Transition Zone of the Earths mantle. Two separate samples of wadsleyite II synthesized at 17.5 GPa and 1400°C and at 18 GPa and 1350°C have been characterized by electron microprobe, single-crystal X-ray diffraction, visible, IR, Raman, and Mössbauer spectroscopies, and transmission electron microscopy including electron energy-loss spectroscopy. The two samples have the following chemical formulae: Mg1.71Fe0.18Al0.01H0.33Si0.96O4 and Mg1.60Fe0.22Al0.01 H0.44Si0.97O4. Mössbauer spectroscopy and electron energy loss spectroscopy (EELS) indicate that about half of the iron present is ferric. Refinement of the structures shows them to be essentially the same as spinelloid IV. Calculated X-ray powder diffraction patterns show only subtle differences between wadsleyite and wadsleyite II. The hydration mechanism appears to be protonation of the non-silicate oxygen (O2) and possibly the oxygens surrounding the partially vacant tetrahedral site Si2, charge-balanced by cation vacancies in Si2, M5 and M6. The unit cell volume of this phase and its synthesis conditions indicate that it may be an intermediate phase occurring between the fields of wadsleyite and ringwoodite, if sufficient trivalent cations are available. The unit cell parameters have been refined at pressures up to 10.6 GPa by single-crystal X-ray diffraction in the diamond anvil cell. The refined bulk modulus for the sample containing 2.8 wt% H2O is 145.6 ± 2.8 GPa with a K of 6.1 ± 0.7. Similar to wadsleyite and ringwoodite, hydration has a large effect on the bulk modulus. The presence of this phase in the mantle could serve to obscure the seismic expression of the phase boundary between wadsleyite and ringwoodite near 525 km. The large apparent effect of hydration on bulk modulus is consistent with hydration having a larger effect on seismic velocities than temperature in the Transition Zone.  相似文献   

14.
After its initial synthesis as the new compound Mg2Al3B2O9(OH) (Daniels et al. 1997) pseudosinhalite has now been discovered as a new mineral. It occurs, together with hydrotalcite, as a replacement product of sinhalite, MgAlBO4, in an impure marble of the contact metasomatic iron boron deposit of Tayozhnoye in the Aldan Shield of Siberia. Its chemical composition determined by electron microprobe is (wt%): Al2O3 46.88; MgO 25.12; FeO 1.99; B2O3 (calculated) 21.75; H2O (calculated) 2.81 giving a total of 98.55 and leading to the empirical formula (Mg2.00 Fe2+ 0.09)Σ=2.09 Al2.94 B2O9(OH). The small deviation from the ideal stoichiometry with (Mg?+?Fe2+):Al?≠?2:3 may be caused by either solid solution towards, or submicroscopic interlayering with lamellae of, the structurally similar mineral sinhalite. The underlying substitution involving also B and H would be (Mg?+?Fe)+?B=Al+2H. Pseudosinhalite is monoclinic, space group P21/c, with a=7.49(1), b=4.33(1), c=9.85(2) Å; β=110.7(1)°; V?=?299(1) Å3; Z?=?2. Calculated density is 3.508?g/cm3. Pseudosinhalite is colourless with white streak and has a vitreous lustre. It is transparent; no fluorescence was detected. There is no cleavage and parting; fractures are concoidal. Optical constants could not be measured properly due to polysynthetic microtwinning, but α<1.72<γ. For synthetic pseudosinhalite α=1.691(1); β=1.713(1); γ=1.730(1); Δ=0.039; 2?V=80°. The temperature of pseudosinhalite formation was below about 400?°C at low pressures and with a hydrous, CO2-bearing fluid participating in the reaction.  相似文献   

15.
The structure of a single crystal hydrous ringwoodite, Mg1.89Si0.98H0.30O4 synthesized at conditions of 1300?°C and 20?GPa has been analyzed. Crystallographic data for hydrous ringwoodite obtained are; Cubic with Space group: Fd3m (no. 227). a= 8.0693(5)?Å, V=526.41(9)?Å3, Z=8, Dcalc= 3.48?g?cm?3. The results of site occupancy refinement using higher angle reflections showed the existence of a small degree of Mg2+-Si4+ disorder in the structure such as (Mg1.84Si0.05□0.11)(Si0.93Mg0.05□0.02)H0.30O4. The IR and Raman spectra were measured and OH vibration spectra were observed. A broad absorption band was observed in the IR spectrum and the maxima were observed at 3160?cm?1 in the IR and at 3165?cm?1 and 3685?cm?1 in relatively sharp Raman spectra, which suggest that locations between O-O pairs around the octahedral 16c and 16d sites are possible sites for hydrogen.  相似文献   

16.
A new superstructure was found in bafertisite [(Ba0.98Na0.02)1.00(Fe1.71Mn0.26Mg0.01)1.98 TiO[(Si1.82Ti0.04Al0.03Cr0.01)1.90O7](OH1.40F0.53Cl0.03)1.96] from Donghai County, Jiangsu Province, China. The occurrence of the superstructure reflections were observed by single crystal diffraction using a SMAR APEX CCD. The a*, b*and c* axis directions revealed extra weak reflection spots of the superstructure. The apparent 2a, 2b and 2c superstructure is monoclinic with unit cell a＝10.6502(15)?, b＝13.7233(19)?, c＝21.6897(3)?, α＝90o, β＝94.698(3)o, γ＝90o,space group Cm,Z=16. If c* extra weak reflections are ignored, the secondary supercell gave a cell a＝10.6548(15)?, b＝13.7284(19)?, c＝11.6900(17)?, α＝90o, β＝112.322(28)o, γ＝90o,space group Cm,Z=8. The basic subcell was obtained by ignoring all extra weak reflection spots and gave: a＝5.3249(17)?, b＝6.8669(22)?, c＝10.8709(36)?, α＝90o, β＝94.740(62)o, γ＝90o,space P21/m,Z=2. The superstructure has been refined to R = 0.063 for 7805 [R(int) = 0.0266] unique reflections I>2δ(I). The structure consists of an octahedra (O) sheet sandwiched between two heteropolyhedral (H) sheets. These sheets consist of Ti–octahedra and twin tetrahedral disilicate groups [Si2O7]. The O sheet comprises (Fe,Mg)O4 octahedra. The large Ba cation is located in the interlayer area. The refined structure shows Fe, Mg are partly ordered. The shifting of the TiO6 octahedron and SiO4 tetrahedron sites in the sheet may be a consequence of the superstructure.  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.
New germanate analogs of the mineral surinamite, Mg3Al4BeSi3O16, have been synthesized with composition Mg4A4Ge3O16 (A=Al, Ga) and have been characterized by powder X-ray diffraction and transmission electron microscopy. The Al surinamite phase crystallizes with a primitive unit-cell (P2/n, a=10.153(1), b=11.708(2), c=9.920(1) Å, β=110.18 (2)° and Z=4) similar to that of the silicate mineral. The Ga surinamite-like phase crystallizes with a larger unit-cell (C2/c, a=10.308(2), b=23.690(5), c=10.057(l) Å, β=110.23 (2)° and Z=8). High-resolution electron microscopy has shown the common formation of intergrowths between the surinamite and sapphirine structures, illustrating the polysomatic structural relationship between them. Observations of disordered microstructures in the Al surinamite suggest the occurrence of a P2/n?C2/c transformation.  相似文献   

19.
A thermodynamic formulation of hydrous Mg-cordierite (Mg2Al4Si5O18·nH2O) has been obtained by application of calorimetric and X-ray diffraction data for hydrous cordierite to the results of hydrothermal syntheses. The data include measurements of the molar heat capacity and enthalpy of hydration and the molar volume. The synthesis data are consistent with a thermodynamic formulation in which H2O mixes ideally on a single crystallographic site in hydrous cordierite. The standard molar Gibbs free energy of hydration is-9.5±1.0 kJ/mol (an average of 61 syntheses). The standard molar entropy of hydration derived from this value is-108±3 J/mol-K. An equation providing the H2O content of cordierite as a function of temperature and fugacity of H2O is as follows (n moles of H2O per formula unit, n<1): $$\begin{gathered}n = {{f_{{\text{ H}}_{\text{2}} O}^{\text{V}} } \mathord{\left/{\vphantom {{f_{{\text{ H}}_{\text{2}} O}^{\text{V}} } {\left( {f_{{\text{ H}}_{\text{2}} O}^{\text{V}} + {\text{exp}}\left[ { - {\text{3}}{\text{.8389}} - 5025.2\left( {\frac{1}{T} - \frac{1}{{298.15}}} \right)} \right.} \right.}}} \right.\kern-\nulldelimiterspace} {\left( {f_{{\text{ H}}_{\text{2}} O}^{\text{V}} + {\text{exp}}\left[ { - {\text{3}}{\text{.8389}} - 5025.2\left( {\frac{1}{T} - \frac{1}{{298.15}}} \right)} \right.} \right.}} \hfill \\{\text{ }}\left. {\left. { - {\text{ln}}\left( {\frac{T}{{{\text{298}}{\text{.15}}}}} \right) - \left( {\frac{{298.15}}{T} - 1} \right)} \right]} \right) \hfill \\\end{gathered}$$ Application of this formulation to the breakdown reaction of Mg-cordierite to an assemblage of pyrope-sillimanite-quartz±H2O shows that cordierite is stabilized by 3 to 3.5 kbar under H2O-saturated conditions. The thermodynamic properties of H2O in cordierite are similar to those of liquid water, with a standard molar enthalpy and Gibbs free energy of hydration that are the same (within experimental uncertainty) as the enthalpy and Gibbs free energy of vaporization. By contrast, most zeolites have Gibbs free energies of hydration two to four times more negative than the corresponding value for the vaporization of water.  相似文献   

20.
The crystal structure of α-CaSi2O5 synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P , using single crystal X-ray diffraction data. The composition (Z=4) and unit cell are Ca1.02Si1.99O5 by EPMA analysis and a=7.243(2) Å, b=7.546(4) Å, c=6.501(4) Å, α=81.43(5)°, β=84.82(4)°, γ=69.60(3)°, V=329.5(3) Å3, yielding the density value, 3.55 g/cm3. The structure is closely related to that of titanite, CaTiSiO5 and features the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 Å. The substantial deviation of valence sum for Ca indicates the existence of local strain and the instability of α-CaSi2O5 at room pressure.  相似文献   

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