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1.
The temperature dependence of cell parameters for three disordered, synthetic alkali feldspars (Or19, Or38, and Or100) has been determined up to 1,000 °C. The samples show no change in composition or degree of Si-Al disorder during the experiments. The triclinic-monoclinic inversion in the sample of composition Or19 occurs at 560 °±10 °C and is accompanied by changes in the rates of expansion of a, b and c; the rate for a increases and those for b and c decrease above the inversion. The b and c parameters in Or100 show small decreases with increasing temperature and this may be due to thermal motion effects causing a contraction of cell directions that are fully expanded at room temperature. Calculation of the thermal expansion ellipsoids for the monoclinic phases shows that the major expansion coefficients (1) for all three samples are more than an order of magnitude greater than the intermediate (2) and minor (3) coefficients. Thus the thermal expansion of these phases is dominated by that of 1 which makes an angle of 22 ± 4 ° with+a; this orientation is parallel to that of the short M-OA2 bonds. The thermal expansion mechanism for monoclinic, disordered alkali feldspars may involve tilting within the framework releasing compression along this direction and allowing the M-OA2 bonds to show high expansion rates. The stretching of the crankshaft units, which are parallel to a, may only play a subordinate role in controlling the expansion of the feldspar framework.  相似文献   

2.
Thermal expansion properties of synthetic orthopyroxenes (Fe0.20Mg0.80)SiO3, (Fe0.40Mg0.60)SiO3, (Fe0.50Mg0.50)SiO3, (Fe0.75Mg0.25)SiO3 and (Fe0.83Mg0.17)SiO3 were systematically studied by means of single-crystal x-ray diffraction in the temperature range from 296 to 1300 K. The measurements of unit cell dimensions as a function of temperature reveal that the a and c dimensions and the unit cell volume V increase nonlinearly with a positive curvature with rising temperature, whereas the b dimension behaves differently, depending on the total Fe content. For Mg-rich orthopyroxenes (Fe/(Fe+Mg)<30%), the b dimension expands similarly as the a and c dimensions, but it exhibits a nonlinear increase with a negative curvature for orthopyroxenes with Fe/(Fe+Mg)>30%. Together with the high temperature neutron diffraction data on enstatite (MgSiO3) (McMullan, Haga and Ghose, unpublished) and x-ray diffraction data on ferrosilite (FeSiO3) (Sueno et al. 1976), the measured unit cell dimensions were analyzed in terms of the Grüneisen theory of thermal expansion. The linear thermal expansion coefficients α a and α c both increase as temperature is elevated, with α c increasing faster, while α b changes gradually from increasing for Mg-rich orthopyroxenes to decreasing for Fe-rich orthopyroxenes. The relative magnitudes of linear thermal expansion coefficients are always in the order α b >α c >α a between 300 and 500 K, but at higher temperatures, the order changes to α c >α b >α a for Mg-rich orthopyroxenes and α c >α a >α b for Fe-rich ones. The linear thermal expansion behavior is interpreted on the basis of the structural mechanical model of Weidner and Vaughan (1982). The anomalous behavior of α b is mainly attributed to the changes in the Fe2+ population at the M2 site and the relative stiffness of the M2(Fe2+)-O bonds compared to the M2(Mg2+)-O bonds. The volume thermal expansion coefficients are nonlinear functions of temperature and lie between 23 and 49×10?6/K. The previously reported results of mean volume thermal expansion coefficients appear to represent the α V values characteristic of higher temperatures compared to our results. The thermal Debye temperatures are composition-dependent, decreasing linearly from 812 (MgSiO3) to 561 K (FeSiO3), and are systematically higher than the corresponding acoustic Debye temperatures. The Grüneisen parameters range from 0.85 to 0.89 and do not seem to vary with composition. The linear compressibilities derived from thermal expansion and elastic moduli data agree very well. The pressure derivatives of the isothermal bulk modulus (dK0/dP) are also composition-dependent and decrease from 11.2 (MgSiO3) to 8.77 (FeSiO3). Such large values indicate possible anomalous elastic behavior of orthopyroxenes at high pressures in the Earth's upper mantle.  相似文献   

3.
In order to fully assess the role of rutile in fractionation of Nb/Ta during partial melting of hydrous metabasalt, we have measured rutile - felsic melt partition coefficients (D values) for Nb and Ta with tonalitic to trondhjemitic compositions at 1.5-3.5 GPa, 900-1350 °C and ∼5.0-20 wt% H2O. DNb, DTa and DNb/DTa range from 17 ± 1 to 246 ± 13, 34 ± 2 to 232 ± 25 and 0.51 ± 0.04 to 1.06 ± 0.13, respectively. For the compositions investigated, melt composition appears to have no observable effect on the partitioning; the effect of pressure is also slight; whereas temperature and H2O have marked effects. DNb, DTa and DNb/DTa increase with decreasing temperature and H2O content, showing a reversal of DNb/DTa from <1.0 to >1.0. Using the data that approached equilibrium and obeyed Henry’s law, expressions describing the dependences of DNb, DTa and DNb/DTa on temperature, pressure and melt H2O content were obtained:
(1)  相似文献   

4.
A high temperature single-crystal X-ray diffraction study of enstatite, Mg2Si2O6 was undertaken at 296, 900, 1200, 1360 and 1400 K. During the X-ray data collection at 1360 K (T0), orthoenstatite (Pbca) transformed to protoenstatite (Pbcn). The unit cell parameters measured at T0 are a=18.456(4), b=8.960(2) and c=5.270(1) Å for ortho and a=9.306(1), b=8.886(1) and c=5.360(1) Å for proto. The discontinuous increase in c and decrease in b due to the ortho to proto transformation are associated with the drastic unkinking of the silicate chains, whereas the abrupt increase in a results from the large expansion of the M2 — O distances along a coupled with the increase in the out-of-plane tilting of the silicate tetrahedra. Stacking faults form in ortho prior to the phase transition, as well as in proto between 1360 and 1400 K. With increasing temperature, the silicate B chain in ortho straightens faster than the A chain as the configurations of the SiA and SiB tetrahedra tend to become similar. At T0, the A and B chains with the O3-O3-O3 angles (O3 being the bridging oxygen atom) of 163.0° and 149.5° in ortho, respectively, attain an identical angle of 168.4° in proto. The configuration of the silicate chain in proto resembles that of the A chain in ortho. Rigid-body thermal vibration analysis suggests that between 1200 and 1400 K the largest, the second largest and the smallest thermal librational motions of the [SiO4] tetrahedra in both ortho and proto are approximately around a, c and b, respectively. Below 1200 K, the largest thermal librational amplitudes of the SiA and SiB tetrahedra in ortho are quite different, but become nearly equivalent at T0. In contrast to the results reported for all iron-bearing orthopyroxenes at high temperature, switching of the O3B atoms coordinated with the M2 cation occurs during the ortho to proto transformation, but not in ortho below T0. The ortho-proto transition does not affect the configuration of the M1 octahedron significantly, but results in a decrease of the mean M2 — O bond distance by 0.043 Å and a highly distorted M2 octahedron in proto.  相似文献   

5.
The compressibility at room temperature and the thermal expansion at room pressure of two disordered crystals (space group C2/c) obtained by annealing a natural omphacite sample (space group P2/n) of composition close to Jd56Di44 and Jd55Di45, respectively, have been studied by single-crystal X-ray diffraction. Using a Birch–Murnaghan equation of state truncated at the third order [BM3-EoS], we have obtained the following coefficients: V 0 = 421.04(7) Å3, K T0 = 119(2) GPa, K′ = 5.7(6). A parameterized form of the BM3 EoS was used to determine the axial moduli of a, b and c. The anisotropy scheme is β c  ≤ β a  ≤ β b , with an anisotropy ratio 1.05:1.00:1.07. A fitting of the lattice variation as a function of temperature, allowing for linear dependency of the thermal expansion coefficient on the temperature, yielded αV(1bar,303K) = 2.64(2) × 10−5 K−1 and an axial thermal expansion anisotropy of α b  ≫ α a  > α c . Comparison of our results with available data on compressibility and thermal expansion shows that while a reasonable ideal behaviour can be proposed for the compressibility of clinopyroxenes in the jadeite–diopside binary join [K T0 as a function of Jd molar %: K T0 = 106(1) GPa + 0.28(2) × Jd(mol%)], the available data have not sufficient quality to extract the behaviour of thermal expansion for the same binary join in terms of composition.  相似文献   

6.
Fe-bearing dense hydrous magnesium silicate Phase A, Mg6.85Fe0.14Si2.00O8(OH)6 has been studied by single-crystal X-ray diffraction at ambient conditions and by high-pressure powder diffraction using synchrotron radiation to 33 GPa. Unit cell parameters at room temperature and pressure from single crystal diffraction are a=7.8678 (4) Å, c=9.5771 (5) Å, and V=513.43 (4) Å3. Fitting of the P–V data to a third-order Birch-Murnaghan isothermal equation of state yields V 0=512.3 (3) Å3, K T,0=102.9 (28) GPa and K′=6.4 (3). Compression is strongly anisotropic with the a-axes, which lie in the plane of the distorted close-packed layers, approximately 26% more compressible than the c-axis, which is normal to the plane. Structure refinement from single-crystal X-ray intensity data reveals expansion of the structure with Fe substitution, mainly by expansion of M-site octahedra. The short Si2–O6 distance becomes nearly 1% shorter with ~2% Fe substitution for Mg, possibly providing additional rigidity in the c-direction over the Mg end member. K T obtained for the Fe-bearing sample is ~5.5% greater than reported previously for Fe-free Phase A, despite the larger unit cell volume. This study represents a direct comparison of structure and K T–ρ relations between two compositions of a F-free dense hydrous magnesium silicate (DHMS) phase, and may help to characterize the effect of Fe substitution on the properties of other DHMS phases from studies of the Fe-free end-members.  相似文献   

7.
Three natural Mg(Al2-yCry)O4 spinels (y 0.07–0.16), highly ordered in terms of Mg–Al, and one Mg(Al2–yFe3+y)O4 spinel (y0.08), highly ordered also in terms of Fe3+, were studied by means of X-ray single-crystal diffraction. All samples were heated in situ from 25 to 1000 °C in order to follow both thermal expansion and evolution of the structural state of spinel with temperature. Thermal expansion was monitored by means of the variation of cell edge a with temperature, and found to be well represented throughout the temperature range by a regression line a = a0 (1+T), slightly different at lower and higher temperatures. Thermal expansion coefficient 1, referring to the lower temperature range (i.e. during pure thermal expansion), was slightly lower than 2, calculated only over the highest temperatures. The trend showed different slopes for individual crystals. Structural evolution with temperature was studied by means of the variation of oxygen positional parameter u, which is strongly influenced by intersite cation exchange and thus closely correlated with inversion parameter x. In particular, in the three Cr samples, in which Cr resides only in the octahedral site, u parameter variations and hence the order–disorder process, started at about 700 °C. Instead, in the Fe3+ sample, this process was triggered at lower temperatures, starting at 550 °C with Fe3+–Mg exchange followed at higher temperatures by that of Mg–Al. Cr contents in the Cr samples affected the occupancy of Al in the tetrahedral site at the highest temperatures. In both Mg–Al–Cr and Mg–Al–Fe3+ compositions, if CrFe3+, parameter u reached the same value only when the Mg–Al exchange was dominant, i.e. at the highest temperatures, but not before. Cation distribution at each temperature was obtained by the bond-length model, applying thermal expansion to pure bond lengths. This method is applied here to complex compositions for the first time.  相似文献   

8.
The high-temperature thermoelastic behavior of a natural cancrinite has been investigated by in situ single-crystal X-ray diffraction. The unit-cell volume variation as a function of temperature (T) exhibits a continuous trend up to 748 K (hydrous expansion regime). The unit-cell edges expansion clearly shows an anisotropic expansion scheme (α a  < α c ). At 748 K, a dehydration process takes place, and a series of unit-cell parameter measurements at constant temperature (748 K) for a period of 12 days indicate that the dehydration process continued for the entire period of time, until the cell parameters were found to be constant. After the dehydration process is completed, the structure expands almost linearly with increasing temperature up to 823 K, where a sudden broadening of the diffraction peaks, likely due to the impending decomposition, did not allow the collection of further data points. Even with a very limited temperature range for the anhydrous regime, we observed that the behavior of the two (i.e., hydrous and anhydrous) high-temperature structures is similar in terms of (1) volume thermal expansion coefficient and (2) thermoelastic anisotropy. The structure refinements based on the data collected at 303, 478 and 748 K (after the dehydration), respectively, showed a change in the mechanism of tilting of the quasi-rigid (Si,Al)O4 tetrahedra, following the loss of H2O molecules, ascribable to the high-temperature Na+ coordination environment within the cages.  相似文献   

9.
Thermal behaviour of γ-anhydrite (γ-CaSO4, soluble anhydrite) has been investigated in situ real-time using laboratory parallel-beam X-ray powder diffraction data. Thermal expansion has been analysed from 303 to 569 K with temperature steps of 4 K. Lattice parameters and volume were fitted with a second-order polynomial to calculate thermal expansion coefficients. Thermal expansion of γ-anhydrite is anisotropic being larger along the c axis. Within the 343–383 K thermal range, γ-anhydrite has been found to partially re-hydrate to bassanite CaSO4·0.5H2O. At 455 K the transformation γ-CaSO4 → β-CaSO4, insoluble anhydrite, starts reaching completion at 653 K.  相似文献   

10.
Thermal expansion has been measured by laboratory and synchrotron X-ray powder diffraction for end-member åkermanite (ak, Ca2MgSi2O7) and gehlenite (ge, Ca2Al2SiO7) in the range 20–1,500 K. In ak in the range 340–390 K, there is a negative linear thermal expansion in [001] direction. This is related to the phase transition from an incommensurate modulated structure (IC) to a normal one (N). The volumetric mean thermal expansion coefficients for ak and ge, obtained with a linear fit of the experimental data in the temperature range 298–1,400 K, are respectively 32.1×10–6 and 28.3×10–6 K–1 . The variation of the c/a ratio with temperature, due to different thermal expansion along the crystallographic axes, can be related to the different behaviour of the tetrahedral layers in the N and IC phases. Analysis of the variation of the superstructure peaks intensity across the phase transition confirms the tricritical behaviour of the IC/N transition in ak.  相似文献   

11.
Sub-potassic nephelines in the system NaAlSiO4(Ne)-KAlSiO4(Ks) were synthesized under a variety of conditions and studied at room temperature and up to 1000 °C using an X-ray powder diffractometer. At low temperatures they do not have the hexagonal structure determined by Hahn and Buerger (1955) for natural nepheline. Samples with 0.7 to 2.5 mole % Ks have an orthorhombic supercell with parameters equivalent to a, 3a, 3c where a and c are Hahn and Buerger structure cell parameters. Nephelines with 0 to 0.7% Ks consist of two phases with different c axes; one of these phases has the orthorhombic supercell.Pure-Na nephelines (NaAlSiO4) invert to a hexagonal phase with the Hahn and Buerger structure at 190 °±10 °C; this inversion temperature decreases with increasing Ks and a sample with 0.5% Ks inverts at 170 °±5 °C. The inversion is reversible and is displacive. Another reversible inversion begins at 875 °±10 °C in pure-Na nepheline; this inversion increases in temperature with increasing Ks and a sample with 1.8% Ks begins to invert at 960 °±10 °C.Superstructures with anomalous low-temperature cell parameters in sub-potassic nephelines are attributed to reversible collapse of the framework about the larger cation sites which must be occupied by small Na in subpotassic nephelines. Superstructures in natural nephelines are also related to framework collapse at a displacive inversion.  相似文献   

12.
The solid solution between lead fluorapatite and lead fluorvanadate apatite, Pb10[(PO4)6-x (VO4) x ]F2 with x equal to 0, 1, 2, 3, 4, 5 and 6, was synthesized by solid-state reaction at 1 atm and 700°C for 72 h and characterized by scanning electronic microprobe, electronic microprobe analysis, micro-Raman spectroscopy, and powder X-ray diffraction. The volume-composition relationship at ambient temperature does not show significant deviation from the Vegard’s Law. The Raman spectrum data suggest that both P and V are identical on a C s site and both end-members show no apparent factor-group effect. The Raman frequency shift of the symmetric stretching vibration is linearly dependent on the composition. High temperature X-ray diffraction data, up to 600°C, suggest that the thermal expansion coefficients α a , α c , and α V also vary linearly with the compositions of the apatites.  相似文献   

13.
Summary At the northeastern flank of Gebel Yelleq, northern Sinai, pure limestones of Upper Cretaceous age were subjected to a thermal overprint, caused by a c. 80m thick Tertiary olivine dolerite sill. Metasomatic supply of Si, Al, Fe, Mg and Ti was greater to the c. 7m wide upper than to the c. 25m wide lower thermal aureole. The greater width of the lower aureole is possibly due to a longer duration of the thermal overprint at this contact. Mineral assemblages in both aureoles are (from the contact outward):(i) clinopyroxene + garnet ± wollastonite + calcite(ii) garnet ± wollastonite + calcite;(iii) wollastonite + calcite.In places, late stage xenoblasts of apophyllite and witherite overgrow these assemblages. Garnets are grandites to melanites with Grs56–86Adr14–42Sch0–2Sps0–0.2Prp0 in the lower, and Grs29–94Adr5–64Sch0–12Sps0–0.2Prp0–1.7 in the upper aureole. Close to the upper contact, clinopyroxene is virtually pure diopside with X Mg = Mg/(Mg + Fe2+) = 0.97–1.0, whereas clinopyroxenes farther away from the upper contact and in the lower aureole have X Mg-values of 0.49 and 0.53, respectively.The minimum temperatures reached during contact metamorphism in the upper and lower aureole are defined by the lower stability limit of wollastonite. The temperatures are inferred with a calculated T-X(CO2) projection in the system CMASCH and are estimated at c. 290 °C and 380 °C for X(CO2) values of 0.05 and 0.25, respectively. A pressure of roughly 100 bar is estimated for the lower dolerite-limestone contact. As indicated by one-dimensional thermal modelling, a maximum temperature of 695 °C was attained at this contact, assuming a magma temperature of 1150 °C. Further modelling results indicate (i) wollastonite, which occurs first 13 m away from the lower contact, formed at a maximum temperature of c. 575 °C, (ii) there, wollastonite formation lasted for approximately 170 years and, (iii) at the outer rim of the lower aureole, the maximum temperature reached was 480 °C, and temperatures sufficient for wollastonite formation lasted for about 140 years.  相似文献   

14.
  相似文献   

15.
The structure of deuterated portlandite, Ca(OD)2, has been investigated using time-of-flight neutron diffraction at pressures up to ∼4.5 GPa and temperatures up to ∼823 K. Rietveld analysis of the data reveals that with increasing pressure, unit-cell parameter c decreases at a rate about 4.5 times larger than that for a, which is largely due to rapid contraction of the interlayer spacing in this pressure range. Fitting of the determined cell volumes to the third-order Birch–Murnaghan equation of state yields a bulk modulus (K 0) of 32.2 ± 1.0 GPa and its first derivative (K 0′) of 4.4 ± 0.6. Moreover, on compression, hydrogen-mediated interatomic interactions within the interlayer become strengthened, as reflected by decreases in interlayer D···O and D···D distances with increasing pressure. Correspondingly, D–D, the distance between the three equivalent sites over which D is disordered, increases, suggesting a pressure-induced hydrogen disorder. This behavior is similar to that reported in brucite at elevated pressure. On heating at ∼2.1 GPa, cell parameter c increases more rapidly than a, as expected. However, because of the pressure effect, the thermal expansion coefficients, particularly along c, are much smaller than those at ambient pressure. With increasing temperature, the three partially occupied D sites become further apart, and the D-mediated interactions, mainly the interlayer D···D repulsion, become weakened.  相似文献   

16.
Summary The structure type of joaquinite has been derived from precession photographs (Mo radiation) of a polysynthetic twin. Space group: C 2/m; lattice constants:a=10.50Å,b=9.58Å;c=11.78Å;=109°32;cell content: Ba4Fe2RE4Ti4O4[Si4O12]4.The atomic arrangement was determined by Patterson-, Fourier-, and (F0–Fc)-syntheses. The least squares refinement (with isotropic temperature factors) led toR=0.16. The authors are of the opinion that this relativeley highR-value is due to the poor quality of the crystal.The main structural feature are double layers of [Si4O12]-rings parallel to (001) which are connected by 10-coordinated Ba and octahedrally coordinated Ti. These double layers have pseudo-symmetry C mmm, with mirror planes perpendicular toa, b, andc *. They are stacked alongc * and translated –3/8a. The orthorhombic pseudo-symmetry allows also the translation +3/8a which is responsible for both the formation of twins and the frequent stacking disorder. The described double layers are connected by 7-coordinated RE and 4-coordinated Fe.The rôles of Fe, which has an occupancy of only 1/2, and of Na and OH. which are reported in published analyses, are discussed.
Der Strukturtyp von Joaquinit
Zusammenfassung Der Strukturtyp von Joaquinit wurde aus Präzessionsaufnahmen (MoStrlg.) eines polysynthetischen Zwillings abgeleitet. Raumgruppe: C 2/m; Gitterkonstanten:a=10,50Å;b=9,58Å,c=11,78Å; =109°32, Zellinhalt: Ba4Fe2SE4Ti4O4[Si4O12]4.Die Atomanordnung wurde aus Patterson-, Fourier- und (F0–Fc)-Synthesen abgeleitet. Die Verfeinerung mit der Methode der Kleinsten Quadrate führte (mit isotropen Temperaturfaktoren) aufR=0,16. Die Autoren sind der Ansicht, daß dieser relativ hoheR-Wert von der schlechten Qualität des Kristalles herrührt.Wesentliches Strukturmotiv sind Doppelschichten von [Si4O12]-Ringen parallel (001), die durch 10-koordiniertes Ba und oktaedrisch koordiniertes Ti verknüpft werden. Diese Doppelschichten haben die Pseudosymmetrie C mmm mit Symmetrieebenen senkrechta, b undc *. Sie werden nachc * gestapelt und um –3/8a translatiert. Die rhombische Pseudosymmetrie läßt andererseits die Translation +3a zu, was für die Zwilligsbildung und die häufige Stapelungsunordnung verantwortlich ist. Diese Doppelschichten werden weiter durch 7-koordinierte SE und 4-koordinertes Fe verknüpft.Die Rollen des Fe, welches eine Punktlage nur zur Hälfte besetzt, und des Na und OH, welche in publizierten Analysen angegeben werden, werden diskutiert.

Mit 10 Abbildungen  相似文献   

17.
The thermal behaviour of microsommite (MC), davyne from Vesuvius (DV) and from Zabargad (DZ) was determined by X-ray single crystal data obtained employing a microfurnace connected to a four-circle diffractometer. Upon heating, the a parameter increased linearly, with similar thermal expansion rates for the three samples: the mean linear expansion coefficients, a , were 10.2(3)·10-6, 13.4(7)·10-6, 15.1(8)·10-1 K-1 for MC, DV and DZ respectively.At about 473 K both MC and DZ showed a discontinuity in the expansion of the c parameter. The mean linear expansion coefficient, c , changed abruptly from 16(4)·10-6 K-1 for both minerals below the discontinuity to 2(1)·10-6 and 3(1)·10-6 K-1 for MC and DZ, respectively, above the discontinuity. In DV, however, the c coefficient was constant between 293 und 827 K and equal to 1(2)·10-6 K-1.  相似文献   

18.
The structure of deuterated jarosite, KFe3(SO4)2(OD)6, was investigated using time-of-flight neutron diffraction up to its dehydroxylation temperature. Rietveld analysis reveals that with increasing temperature, its c dimension expands at a rate ~10 times greater than that for a. This anisotropy of thermal expansion is due to rapid increase in the thickness of the (001) sheet of [Fe(O,OH)6] octahedra and [SO4] tetrahedra with increasing temperature. Fitting of the measured cell volumes yields a coefficient of thermal expansion, α = α0 + α1 T, where α0 = 1.01 × 10−4 K−1 and α1 = −1.15 × 10−7 K−2. On heating, the hydrogen bonds, O1···D–O3, through which the (001) octahedral–tetrahedral sheets are held together, become weakened, as reflected by an increase in the D···O1 distance and a concomitant decrease in the O3–D distance with increasing temperature. On further heating to 575 K, jarosite starts to decompose into nanocrystalline yavapaiite and hematite (as well as water vapor), a direct result of the breaking of the hydrogen bonds that hold the jarosite structure together.  相似文献   

19.
The high temperature volume and axial parameters for six C2/c clinopyroxenes along the NaAlSi2O6–NaFe3+Si2O6 and NaAlSi2O6–CaFe2+Si2O6 joins were determined from room T up to 800°C, using integrated diffraction profiles from in situ high temperature single crystal data collections. The thermal expansion coefficient was determined by fitting the experimental data according to the relation: ln(V/V 0) = α(T − T 0). The thermal expansion coefficient increases by about 15% along the jadeite–hedenbergite join, whereas it is almost constant between jadeite and aegirine. The increase is related to the Ca for Na substitution into the M2 site; the same behaviour was observed along the jadeite–diopside solid solution, which presents the same substitution at the M2 site. Strain tensor analysis shows that the major deformation with temperature occurs in all samples along the b axis; on the (010) plane the higher deformation occurs in jadeite and aegirine at a direction almost normal to the tetrahedral–octahedral planes, and in hedenbergite along the projection of the longer M2–O bonds. The orientation of the strain ellipsoid with temperature in hedenbergite is close to that observed with pressure in pyroxenes. Along the jadeite–aegirine join instead the high-temperature and high-pressure strain are differently oriented.  相似文献   

20.
The high-temperature cell parameters of lime (CaO), periclase (MgO), corundum (Al2O3), and spinel (MgAl2O4) have been determined from 300 up to 3000 K through X-ray diffraction experiments with synchrotron radiation. The good agreement found with dilatometric results suggests that vacancy-type defects do not make a large contribution to thermal expansion for these oxides, even near the melting point, justifying the use of X-ray diffraction for determining volume properties up to very high temperatures. Thermal expansion coefficients were determined from the measured cell volumes with equations of the form α=α0 + α1 T + α2/T 2. Along with available isobaric heat capacity and compressibility data, these derived coefficients clearly show that anharmonic effects contribute little to the isochoric heat capacities (C v ) of CaO, MgO, and Al2O3, which do not depart appreciably from the 3nR Dulong and Petit limit. Received: 31 March 1999 / Revised, accepted: 23 June 1999  相似文献   

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