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1.
An end member of the tourmaline series with a structural formula □(Mg2Al)Al6(BO3)3[Si6O18](OH)4 has been synthesized in the system MgO-Al2O3-B2O3-SiO2-H2O where it represents the only phase with a tourmaline structure. Our experiments provide no evidence for the substitutions Al → Mg + H, Mg → 2H, B + H → Si, and AlAl → MgSi and we were not able to synthesize a phase “Mg-aluminobuergerite” characterized by Mg in the (3a)-site and a strong (OH)-deficiency reported by Rosenberg and Foit (1975). The alkali-free tourmaline has a vacant (3a)-site and is related to dravite by the □ + Al for Na + Mg substitution. It is stable from at least 300°C to about 800°C at low fluid pressures and 100% excess B2O3, and can be synthesized up to a pressure of 20 kbars. At higher temperatures the tourmaline decomposes into grandidierite or a boron-bearing phase possibly related to mullite (“B-mullite”), quartz, and unidentified solid phases, or the tourmaline melts incongruently into corundum + liquid, depending on pressure. In the absence of excess B2O3 tourmaline stability is lowered by about 60°C. Tourmaline may coexist with the other MgO-Al2O3-B2O3-SiO2-H2O phases forsterite, enstatite, chlorite, talc, quartz, grandidierite, corundum, spinel, “B-mullite,” cordierite, and sinhalite depending on the prevailing PTX-conditions.The (3a)-vacant tourmaline has the space group R3m with , , and . However, these values vary at room temperature with the pressure-temperature conditions of synthesis by , , and , probably as a result of MgAl order/disorder relations in the octahedral positions. Despite these variations intensity calculations support the assumed structural formula. Refractive indices are no = 1.631(2), nE = 1.610(2), Δn = 0.021. The infrared spectrum is intermediate between those of dravite and elbaite. The common alkali and calcium deficiencies of natural tourmalines may at least partly be explained by miscibilities towards (3a)-vacant end members. The apparent absence of (3a)-vacant tourmaline in nature is probably due to the lack of fluids that carry boron but no Na or Ca. 相似文献
2.
The mineral solubility model of Harvie and Weare (1980) is extended to the eight component system, Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O at 25°C to high concentrations. The model is based on the semi-empirical equations of Pitzer (1973) and co-workers for the thermodynamics of aqueous electrolyte solutions. The model is parameterized using many of the available isopiestic, electromotive force, and solubility data available for many of the subsystems. The predictive abilities of the model are demonstrated by comparison to experimental data in systems more complex than those used in parameterization. The essential features of a chemical model for aqueous electrolyte solutions and the relationship between pH and the equilibrium properties of a solution are discussed. 相似文献
3.
John Longhi 《Geochimica et cosmochimica acta》2005,69(3):529-539
Graphical analysis of free-energy relationships involving binary quadruple points and their associated univariant equilibria in the system CO2-H2O suggests the presence of at least 2 previously unrecognized quadruple points and a degenerate binary invariant point involving an azeotrope between CO2-rich gas and liquid. Thermodynamic data extracted from the equilibrium involving clathrate (hydrate), gas, and ice (H = G+I) are employed along with published data to calculate the P-T range of the 3-ice equilibrium curve, S+I = H, where S is solid CO2. This equilibrium curve intersects the H = G+I curve approximately where the latter curve intersects the S+H = G curve, thus confirming the existence of one of the inferred quadruple points involving the phases S, G, H, and I. Recognition of some binary equilibria probably have been hampered by extremely low mutual solubilities of CO2 and H2O in the fluids phases which, for example, render the S+H = G virtually indistinguishable from the CO2-sublimation curve.To make the published portion of the L(liquid CO2)-G-H equilibrium “connect” with the other new quadruple point involving S, L, G, and H, it is necessary to change the sense of the equilibrium from L = G+H at higher pressures to L+H = G at lower pressures by positing a L = G azeotrope at very low concentrations of H2O. At the low-pressure origin of the azeotrope, which is only a few bars above the CO2-triple point, the azeotrope curve intersects the 3-phase curve tangentially, creating a degenerate invariant point at which the 3-phase equilibrium changes from L+H = G at lower pressures to L = G+H at higher pressures. The azeotrope curve is offset at slightly lower temperature from the L = G+H curve until the 3-phase equilibrium terminates at the quadruple point involving G, L, H, and W (water). With further increase in pressure the azeotrope curve tracks the L = G+W equilibrium and apparently terminates at a critical end point in close proximity to critical endpoints for the CO2-saturation curve and the L = G+W curve.Thermodynamic data for clathrate extracted from the slope of the H = G+I curve are consistent with a solid-state phase transformation in CO2-clathrate between 235 and 255 K. Published work shows that the type-I clathrate phase, whose atomic structure is a framework of water molecules with CO2 molecules situated in large “guest” sites within the framework, is variable in composition with ∼1 guest site vacancy per unit cell at the high-temperature limit of its stability; the number of water molecules, however, remains constant. The formula (CO2)8-y·46H2O, where y is the number of vacancies per unit cell, is in keeping with the atomic structure, whereas the traditional formula, CO2·nH2O, where n (hydration number) = 5.75, is misleading.Ambient P-T conditions in the Antarctic and Greenland ice sheets are compatible with sequestering large amounts of carbon as liquid CO2 and/or clathrate. 相似文献
4.
Chemical equilibrium between sodium-aluminum silicate minerals and chloride bearing fluid has been experimentally determined in the range 500–700°C at 1 kbar, using rapid-quench hydrothermal methods and two modifications of the Ag + AgCl acid buffer technique. The temperature dependence of the thermodynamic equilibrium constant (K) for the reaction Albite Andalusite Qtz. can be described by the following equation: log k = ?4.437 + 5205.6/T(K) The data from this study are consistent with experimental results reported by Montoya and Hemley (1975) for lower temperature equilibria defined by the assemblages albite + paragonite + quartz + fluid and paragonite + andalusite + quartz + fluid. Values of the equilibrium constants for the above reactions were used to estimate the difference in Gibbs free energy of formation between NaClo and HClo in the range 400–700°C and 1–2 kbar. Similar calculations using data from phase equilibrium studies reported in the literature were made to determine the difference in Gibbs free energy of formation between KClo and HClo. These data permit modelling of the chemical interaction between muscovite + kspar + paragonite + albite + quartz assemblages and chloride-bearing hydrothermal fluids. 相似文献
5.
S.K. Saxena 《Geochimica et cosmochimica acta》1981,45(6):821-825
Using the model of fictive ideal components, Gibbs free energies of formation of pyrope and Al2O3-enstatite have been determined from the experimental data on coexisting garnet and orthopyroxene and orthopyroxene and spinel in the temperature range of 1200–1600 K. The negative free energies in kJ/mol are:
TK | 1200 | 1300 | 1400 | 1500 | 1600 |
Pyrope | 4869.92 | 4747.05 | 4614.26 | 4462.63 | 4311.00 |
Al2O3-enstatite | 1257.25 | 1244.28 | 1191.93 | 1158.67 | 1125.64 |