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1.
Plagioclase is the major rock-forming mineral constituting the Earth’s crust, whereas anorthite (CaAl2Si2O8) is a common minerals in lunar highlands crust, meteorites, possibly in some comets and on Mercury. Besides anorthite, two high-temperature polymorphs of CaAl2Si2O8 are known: dmisteinbergite and svyatoslavite, which are found in burnt coal dumps, meteorites and pseudotachylytes. Here we present the results of detailed studies (quenching experiments, elemental analysis, Raman spectroscopy and in situ high temperature single crystal X-ray diffraction (up to 1000 °C)) on naturally co-occurring CaAl2Si2O8 polymorphs (anorthite, dmisteinbergite and svyatoslavite) from a burnt coal dump in Kopeisk, Russia. New polymorphs were found in all natural samples and obtained upon heating of dmisteinbergite (unquenchable β-dmisteinbergite and quenchable γ-dmisteinbergite). It was shown that Ca coordination differs significantly in CaAl2Si2O8 polymorphs, resulting in a different capacity to host Ba and possibly other large ion lithophile elements. Combining our data on natural samples with the previously published data on natural and synthetic compounds, we propose a new scheme of CaAl2Si2O8 polymorphs stability. Our results indicate that CaAl2Si2O8 polymorphs could be used for temperature estimations for both Earth and planetary sciences.  相似文献   

2.
Stoichiometric anorthite, CaAl2Si2O8, Pˉ1, with sharp a, b, c, and d diffractions was grown, using a CaV2O6 solvent, by cooling at 2 ° C per hour from 1450 to 750 ° C in air. Euhedral crystals up to 5 × 3× 0.5 mm, with prominent {010} and well-developed {110} and {001}, were obtained by spontaneous nucleation. Nonstoichiometric anorthite with excess SiO2 (CaAl2Si2O8+Si2Si2O8) was grown on the join CaAl2Si2O8-SiO2. Chemical analysis of the synthetic anorthite, having the highest SiO2 content, with maximum vacancies on M-site gave □0.110 Na0.006Ca0.884Al1.80Si2.20O8, and X-ray diffraction showed a trend from stoichiometric Pˉ1 through diffuse Pˉ1 to body-centered Iˉ1 probably reflecting increasing disorder caused by a higher Si/Al ratio in the T-sites and the positional disorder accompanied by M-site vancancies. Annealing of the nonstoichiometric diffuse Iˉ1 anorthite in air at 1050 ° C for 14 days resulted in exsolution of minute SiO2 inclusions due to probable ordering in the T-sites and filling of M-sites by Ca. Stoichiometric Pˉ1 anorthite was not obtained by annealing at 1050 ° due to appreciable solubility of SiO2 in CaAl2Si2O8. Metastable hexagonal CaAl2Si2O8 was found to be a twinned monoclinic crystal with cell dimensions: a = 10.24 (2), b = 17.74 (3), c= 14.99 (5) ?, β = 92.05 (5) °, space group C2.  相似文献   

3.
The phase transition between the and phases of anorthite has been studied at elevated pressure by single-crystal X-ray diffraction in a diamond-anvil cell. The phase transition is shown to be first-order in character for both end-member anorthite (CaAl2Si2O8) and for an anorthite with a small amount of albite component (NaAlSi3O8) in solid solution. Reversals of the transition across the phase boundary at three other compositions show that the transition pressure (P Tr) increases with increasing albite content. This behaviour is compared with that observed at elevated temperatures, and is analysed in terms of Landau theory.  相似文献   

4.
Transmission electron microscopic study of synthetic CaAl2Ge2O8-feldspar revealed two types of antiphase domains: type “b”-antiphase domains with the displacement vector 1/2 [110] and type “c”-antiphase domains with the displacement vector 1/2 [111]. The “b”-domains were on the order of 0.1 μm in size, while the “c”-domains displayed wall to wall distances mostly between 0.1 and 1 μm. The formation of the two types of antiphase domains is due to reductions in translational symmetry associated with the phase transitions \(C\bar 1 \to I\bar 1 \to P\bar 1\) . The antiphase domain textures of CaAl2Ge2O8-feldspar are very similar to the textures found in natural and synthetic anorthite, CaAl2Si2O8.  相似文献   

5.
Liquidus phase equilibria have been determined in the system CaAl2Si2O8-NaAlSi3O8-KAlSi3O8-NaAlSiO4-KAlSiO4 (An-Ab-Or-Ne-Ks) at a pressure of water of 5 kb, for low anorthite contents. The main effects of increasing anorthite content on phase relationships in the system Ab-Or-Ne-Ks include the expansion of the plagioclase stability field towards the potassium-rich part of the system, and an accompanying contraction of the alkali feldspar, leucite, nepheline and kalsilite stability fields; and an increase in liquidus temperatures throughout most of the compositional range. Two quaternary invariant points have been identified in the system, one a reaction point between the fields of alkali feldspar, plagioclase, nepheline and kalsilite at approximately An4, and the other probably a quaternary eutectic between the fields of alkali feldspar, plagioclase, leucite and kalsilite at approximately An6. A shallow minimum trough in liquidus temperatures occurs on the two-feldspar surface, and this would be expected to control the paths of liquids cooling under equilibrium conditions. Phase relationships in this quaternary system have been applied to the interpretation of the histories of the potassium-rich rocks of the Roman Volcanic Region, Italy. Differentiation of the phonolitic series in this region may have occurred by two-feldspar fractionation.  相似文献   

6.
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7.
For a pure phase at equilibrium with a polycomponent melt, two sets of expressions can be derived; one expressing its activity as a function of enthalpy, entropy, heat capacity and temperature, and the other by coupling a Flory-Huggins' polymerisation model with the van Laar heat of mixing term. Interaction parameters for binary and ternary systems have been computed at 1 bar by equating these two expressions. Assuming the interaction parameter to be independent of temperature, equilibrium temperatures at higher pressures can be calculated by an iterative procedure. Such retrieval calculations were carried out in simple eutectic, volatile-free systems like CaAl2Si2O8-CaMgSi2O6, Mg2SiO4-TiO2, MgSiO3-TiO2, Mg2SiO4-CaMgSi2O6, NaAlSi3O8-SiO2 and CaAl2Si2O8-CaMgSi2O6-Mg2SiO4. The close agreement between the theoretically retrieved and the experimentally determined equilibrium temperatures testifies to the validity of the model at higher pressures. The successful application of the model to simple eutectic, binary and ternary systems involving vastly dissimilar phases without imposing added constraints implies that it can be possibly extended to hitherto unknown systems provided the thermodynamic parameters of the phases involved are known.  相似文献   

8.
29Si MAS NMR experiments have been carried out to determine the silica species distribution (Q distribution) in albite, NaAlSi3O8, and anorthite, CaAl2Si2O8, composition glasses (designated albite and anorthite glass). Our results indicate that the Q distribution of albite glass contains all five possible silica species and shows a tendency towards high Q3 and Q4 concentrations, whereas anorthite glass does not contain Q4 and has a high Q0 concentration. Rationalizations are made in terms of the observed Q distributions to explain differences in devitrification behavior of these two glasses. 27Al MAS NMR data for these glasses suggest that differences in devitrification behavior between these two glasses should be ascribed to small growth rates rather than small nucleation rates of crystalline albite from albite glass.  相似文献   

9.
One atmosphere liquid-present experiments were carried out in the CMAS system using an ordinary quench furnace apparatus. The runs, including reversal and duplicate experiments, describe the univariant curve l=fo+an+ di between the invariant points Q: l+fo+an=di+sp and F: l+fo=an+di+oen, located respectively at 1245±1° C and 1244±1° C. The thermal divide on this curve M3: l= fo+an+di is located at 1275±1° C and plots well within the silica-saturated field, in agreement with Longhi's (1987) experiments. Along the univariant curve l=fo+an+di, liquid composition evolves away from the thermal divide either toward invariant points Q or F and pierces the silica saturation plane, i.e., the join Di-An-En, in the silica saturated field. In this compositional range, the Al solubility in clinopyroxene changes drastically from one side of the thermal divide to the other, with great increase of Al solubility in the silica-undersaturated field. Four endmembers must be used to describe the complex solid solution of anorthite: CaAl2Si2O8, CaMgSi3O8, MgAl2Si2O8 and [] Si4O8. The last two of these are present only within the silica-saturated field. Unlike clinopyroxene, the Mg content of anorthite is insensitive to the thermal barrier but is only sensitive to silica-saturation plane. Olivine composition can be described by a binary solid solution of forsterite and monticellite with no Ca in the M2 site. As with anorthite, olivine compositions exhibit a marked change with crossing of the silica-saturation plane. The above features imply that the solubility of minor elements in crystalline phases (Al in clinopyroxene, Ca in olivine and Mg in anorthite) selectively respond to only one or another of these particular plane. Results have many important consequences. One is the likelihood of changes in melt speciation depending on position with respect to the thermal divide and the silica-saturation plane.  相似文献   

10.
We have performed first-principles molecular dynamics simulations of CaAl2Si2O8 (anorthite) liquid at pressures up to 120 GPa and temperatures of 3000, 4000 and 6000 K. At the lowest degrees of compression the liquid is seen to accommodate changes in density through decreasing the abundance of 3- and 4-membered rings, while increases in coordination of network forming cations take effect at somewhat higher degrees of compression. Results are fit to a fundamental thermodynamic relation with 4th order finite strain and 1st order thermal variable expansions. Upon compression by a factor of two, the Grüneisen parameter (γ) is found to increase continuously from 0.35 to 1.10. Weak temperature dependence in γ is thermodynamically consistent with a slight decrease in isochoric heat capacity (CV), for which values of between 4.4 and 5.2 NkB are obtained, depending on the temperature. Pressure and temperature dependence of self-diffusivities is found to be well represented by an Arrhenius relation, except at 3000 K and pressures lower than 5 GPa, where self-diffusivities of Si, Al, and O increase with pressure. Analysis of the lifetimes of individual coordination species reveals that this phenomenon arises due to the disproportionately high stability of 4-fold coordinated Si, and to a lesser extent 4-fold coordinated Al. Our results represent a marked improvement in accuracy and reliability in describing the physics of CaAl2Si2O8 liquid at deep mantle pressures, pointing the way to a general thermodynamic model of melts at extreme pressures and temperatures relevant to planetary-scale magma oceans and deep mantle partial melting.  相似文献   

11.
Beginning of melting and subsolidus relationships in the system K2O-CaO-Al2O3-SiO2-H2O have been experimentally investigated at pressures up to 20 kbars. The equilibria discussed involve the phases anorthite, sanidine, zoisite, muscovite, quartz, kyanite, gas, and melt and two invariant points: Point [Ky] with the phases An, Or, Zo, Ms, Qz, Vapor, and Melt; point [Or] with An, Zo, Ms, Ky, Qz, Vapor, and Melt.The invariant point [Ky] at 675° C and 8.7 kbars marks the lowest solidus temperature of the system investigated. At pressures above this point the hydrated phases zoisite and muscovite are liquidus phases and the solidus temperatures increase with increasing pressure. At 20 kbars beginning of melting occurs at 740 °C. The solidus temperatures of the quinary system K2O-CaO-Al2O3-SiO2-H2O are almost 60° C (at 20 kbars) and 170° C (at 2kbars) below those of the limiting quaternary system CaO-Al2O3-SiO2-H2O.The maximum water pressure at which anorthite is stable is lowered from 14 to 8.7 kbars in the presence of sanidine. The stability limits of anorthite+ vapor and anorthite+sanidine+vapor at temperatures below 700° C are almost parallel and do not intersect. In the wide temperature — pressure range at pressures above the reaction An+Or+Vapor = Zo+Ms+Qz and temperatures below the melting curve of Zo+Ms+Ky+Qz+Vapor, the feldspar assemblage anorthite+sanidine is replaced by the hydrated phases zoisite and muscovite plus quartz. CaO-Al2O3-SiO2-H2O. Knowledge of the melting relationships involving the minerals zoisite and muscovite contributes to our understanding of the melting processes occuring in the deeper parts of the crust. Beginning of melting in granites and granodiorites depends on the composition of plagioclase. The solidus temperatures of all granites and granodiorites containing plagioclases of intermediate composition are higher than those of the Ca-free alkali feldspar granite system and below those of the Na-free system discussed in this paper.The investigated system also provides information about the width of the P-T field in which zoisite can be stable together with an Al2SiO5 polymorph plus quartz and in which zoisite plus muscovite and quartz can be formed at the expense of anorthite and potassium feldspar. Addition of sodium will shift the boundaries of these fields to higher pressures (at given temperatures), because the pressure stability of albite is almost 10kbars above that of anorthite. Assemblages with zoisite+muscovite or zoisite+kyanite are often considered to be products of secondary or retrograde reactions. The P-T range in which hydration of granitic compositions may occur in nature is of special interest. The present paper documents the highest temperatures at which this hydration can occur in the earth's crust.  相似文献   

12.
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   

13.
BaAl2Ge2O8-Feldspar undergoes an order-disorder phase transition I2/c↔C2/m at T tr ≈1690 K. The thermodynamics of the Al,Ge cation ordering process is described in terms of the compressible Ising model in mean field approximation. The mean field potential predicts a first order character of the phase transition. This is compared to antiferromagnetic ordering in a two-dimensional square Ising model with NN-pair interactions and four-spin interactions on alternating squares. Calculated order parameters and short range ordering are in good agreement with the corresponding properties observed in BaAl2Ge2O8-feldspar by means of X-ray diffraction, hard mode infrared spectroscopy and TEM. Using known calorimetric data a similar model is postulated for Al,Si ordering in anorthite, CaAl2Si2O8, for which the derived potential describes a transition with slightly stronger first order character at T tr ≈1928 K. Received: 30 January 1998 / Revised, accepted: 29 August 1998  相似文献   

14.
Enthalpy of formation and third law entropy for andradite (Ca3Fe2Si3O12) and hedenbergite (CaFeSi2O6) at standard state have been retrieved from experimental P-T-f(O2) data on the stability relations of these phases. The data for hedenbergite were combined with the thermodynamic data for related phases (Berman 1988) to formulate the geobarometers based on the reactions CaFeSi2O6+2CaAl2Si2O8+2FeTiO3= Ca3Al2Si3O12+Fe3Al2Si3O12+2TiO2 (CPG-IR), 3CaFeSi2O6+3CaAl2Si2O8+3FeTiO3=Ca3Al2Si3O12 +2Fe3Al2Si3O12+3CaTiSiO5 (CPG-IS) and 3CaFeSi2O6+3CaAl2Si2O8=2Ca3Al2Si3O12+ Fe3Al2Si3O12+3SiO2 (CPG-O). The geobarometers involving rutile and sphene are proposed for the first time. In several granulite terranes, pressures have been estimated from the three geobarometers for basic granulites and charnockitic suite of rocks containing the above assemblages. In clinopyroxene-garnet-plagioclase-quartz-ilmenite-rutile assemblages, pressures obtained from CPG-IR are within ±500 bars of pressures calculated from the CPG-Q barometer. However, if rutile is absent, the pressures computed from the CPG-IR barometer are 300 to 1,200 bars higher than those calculated from the CPG-Q barometer. For maximum differences in pressures obtained from the two equilibria TiO2 activity [a(TiO2)] in the rocks is calculated to be 0.8. The sphene-bearing geobarometer (CPG-IS) also registers pressures comparable (<±500 bars) to those obtained from the CPG-Q barometer. The close agreement in pressure values obtained from the CPG-IR and the CPG-IS equilibria with those registered by the CPG-Q geobarometer in assemblages containing quartz implies that the rutile- and sphene-bearing geobarometers which do not involve quartz, can also be applied to estimate pressures from quartz-absent assemblages-assemblages which are not amenable to quantitative geobarometry using existing formulations.  相似文献   

15.
We show from elastic neutron diffraction data that anorthite CaAl2Si2O8 (An100) undergoes a primitive to body-centered phase transition at T c =237±1°C. The transition is reversible, and T c is well defined. Our measurements demonstrate that the I-lattice at the high temperature phase applies to all structural elements; in other words, the time-averaged lattice is exactly body-centered and not just in the space average of An100, as concluded earlier by other authors.  相似文献   

16.
The effect of Cr on the silicate system has been studied in air at 1 atm by adding a small amount of MgCr2O4 (0.2–0.5 wt.%) to the join Mg2SiO4 (forsterite) — CaAl2Si2O8 (anorthite) — CaMgSi2O6 (diopside), which has been considered to form a thermal divide in the system CaO-MgO-Al2O3-SiO2. The spinel primary field is enlarged compared with that in the Cr-free join at the expense of the anorthite primary field. The piercing points forsterite+anorthite+diopside+liquid and forsterite+anorthite+spinel+liquid approach each other with increasing MgCr2O4, meet at the join with 0.25 wt.% MgCr2O4 (0.20 wt.% Cr2O3) to form the ‘isobaric quaternary invariant point’ forsterite+anorthite+diopside+spinel+liquid, and then separate again as new ‘piercing points’ of diopside+spinel+anorthite+liquid and forsterite+diopside+ spinel+liquid. This process indicates that the join Mg2SiO4-CaAl2Si2O8-CaMgSi2O6 containing more than 0.2 wt.% Cr2O3 cannot be a thermal divide in the basalt tetrahedron. The results of the present study show that the presence of a minor amount of Cr causes a significant effect on the phase relations and therefore, the role of Cr must be taken into account in the formulation of a petrologic model.  相似文献   

17.
The surface tension between silicate melts and air has been measured for melt compositions lying on the diopside-anorthite (Di-An) join from 1300° C to 1580° C. It ranges from 300 dyne/cm to 400 dyne/cm, and decreases with increasing temperature, except for a pure diopside composition. At relatively high temperatures, the surface tension decreases as the anorthite content increases, whereas at lower temperature it is almost constant. These results suggest that diopside melt has a more discrete structure at higher temperatures, whereas, anorthite-bearing melts do not dissociate in the temperature range studied. They also suggest that the structure of both the surface and interior parts of the melt are almost identical at lower temperatures, but at higher temperatures, the surface part has a more polymerized structure with Al2O3 enrichment. The surface energy, obtained from the relationship between surface tension and temperature, increases from 294 erg/cm2 (Di composition) to 1013 erg/cm2 (Di40An60) with increasing anorthite content.  相似文献   

18.
The dielectric constants and dissipation factors of LiAlSi2O6, CaAl2Si2O8 and CaMgSi2O6 in both the crystalline (α-spodumene, anorthite, and diopside) and amorphous forms were determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: spodumene κ′ 11=7.30 tan δ= 0.0007 κ′22=8.463 tan δ= 0.0002 κ′33 =11.12 tan δ= 0.0007 anorthite κ′ a *=5.47 tan δ= 0.0009 κ′b *=8.76 tan δ= 0.0010 κ′c *=7.19 tan δ= 0.0013 diopside κ′11=9.69 tan δ= 0.0016 κ′22 = 7.31 tan δ= 0.0007 κ′33=7.29 tan δ= 0.00019 LiAlSi2O6 κ′=8.07 tan δ= 0.047 amorphous CaAl2Si2O8 κ′=7.50 tan δ= 0.0024 amorphous CaMgSi2O6 κ′=8.89 tan δ= 0.0021 amorphous The dielectric properties of a spodumene glass, progressively crystallized at different conditions, were also determined. As the crystallization temperature was increased from 720 to 920° C, κ′ increased from 6.22 to 6.44. The dissipation factor, tan δ, remained constant at 0.020. Similarly, as the crystallization time at 750° C increased from 0.5 hr to 6.0 hr, κ′ increased from 6.28 to 6.35. The deviations of the measured dielectric polarizabilities as determined from the Clausius-Mosotti equation from those calculated from the sum of oxide polarizabilities according to α D(mineral, glass) = σ α D(oxides) are +7.4% for α-spodumene, +1.2% for diopside, and +28.0, +19.6 and +15.9% for amorphous spodumene, anorthitie and diopside compositions, respectively. Positive deviations in α-spodumene and anorthite are consistent with lower than normal apparent cation bond valence sums and are believed to be evidence for loosely bonded “rattling” Li and Ca ions. Diopside, with Ca and Mg ions having normal bond valence sums, exhibits no abnormal deviation from additivity. Larger positive deviations in amorphous SiO2, LiAlSi2O6, CaAl2Si2O8 and CaMgSi2O6 are postulated to arise from a combination of loosely bonded cations and disordered O= ions where the oxygen dielectric polarizability increased from its normal value of 2.0 Å3 in well-behaved oxides to 2.2–3.0 Å3 in the amorphous phases.  相似文献   

19.
We have measured in-situ Raman spectra of aluminosilicate glasses and liquids with albite (NaAlSi3 O8) and anorthite (CaAl2Si2O8) compositions at high temperatures, through their glass transition range up to 1700 and 2000 K, respectively. For these experiments, we have used a wire-loop heating device coupled with micro-Raman spectroscopy, in order to achieve effective spatial filtering of the extraneous thermal radiation. A major concern in this work is the development of methodology for reliably extracting the first and second order contributions to the Raman scattering spectra of aluminosilicate glasses and liquids from the high temperature experimental data, and analyzing these in terms of vibrational (anharmonic) and configurational changes. The changes in the first order Raman spectra with temperature are subtle. The principal low frequency band remains nearly constant with increasing temperature, indicating little change in the T-O-T angle, and that the angle bending vibration is quite harmonic. This is in contrast to vitreous SiO2, studied previously. Above Tg, intensity changes in the 560–590 cm?1 regions of both sets of spectra indicate configurational changes in the supercooled liquids, associated with formation of additional Al-O-Al linkages, or 3-membered (Al, Si)-containing rings. Additional intensity at 800 cm?1 reflects also some rearrangement of the Si-O-Al network.  相似文献   

20.
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