The thermodynamic properties of 254 end-members, including 210 mineral end-members, 18 silicate liquid end-members and 26 aqueous fluid species are presented in a revised and updated internally consistent thermodynamic data set. The PVT properties of the data set phases are now based on a modified Tait equation of state (EOS) for the solids and the Pitzer & Sterner (1995) equation for gaseous components. Thermal expansion and compressibility are linked within the modified Tait EOS (TEOS) by a thermal pressure formulation using an Einstein temperature to model the temperature dependence of both the thermal expansion and bulk modulus in a consistent way. The new EOS has led to improved fitting of the phase equilibrium experiments. Many new end-members have been added, including several deep mantle phases and, for the first time, sulphur-bearing minerals. Silicate liquid end-members are in good agreement with both phase equilibrium experiments and measured heat of melting. The new dataset considerably enhances the capabilities for thermodynamic calculation on rocks, melts and aqueous fluids under crustal to deep mantle conditions. Implementations are already available in thermocalc to take advantage of the new data set and its methodologies, as illustrated by example calculations on sapphirine-bearing equilibria, sulphur-bearing equilibria and calculations to 300 kbar and 2000 °C to extend to lower mantle conditions.

An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids

Phase stability in the Fe–Ni system: Investigation by first-principles calculations and atomistic simulations

Serpentinization of abyssal peridotites is known to produce extremely reducing conditions as a result of dihydrogen (H2,aq) release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by H2O. We have compiled and evaluated thermodynamic data for Fe^Ni^Co^O^S phases and computed phase relations in fO2,g^fS2,g and aH2,aq^aH2S,aq diagrams for temperatures between 150 and 4008C at 50 MPa. We use the relations and compositions of Fe^Ni^Co^O^S phases to trace changes in oxygen and sulfur fugacities during progressive serpentinization and steatitization of peridotites from the Mid-Atlantic Ridge in the 15820 0 N Fracture Zone area (Ocean Drilling Program Leg 209). Petrographic observations suggest a systematic change from awaruite^magnetite^pentlandite and heazlewoodite^magnetite^pentlandite assemblages forming in the early stages of serpentinization to millerite^pyrite^polydymite-dominated assemblages in steatized rocks. Awaruite is observed in all brucite-bearing partly serpentinized rocks. Apparently, buffering of silica activities to low values by the presence of brucite facilitates the formation of large amounts of hydrogen, which leads to the formation of awaruite. Associated with the prominent desulfurization of pentlandite, sulfide is removed from the rock during the initial stage of serpentinization. In contrast, steatitization indicates increased silica activities and that highsulfur-fugacity sulfides, such as polydymite and pyrite^vaesite solid solution, form as the reducing capacity of the peridotite is exhausted and H2 activities drop. Under these conditions, sulfides will not desulfurize but precipitate and the sulfur content of the rock increases. The co-evolution of fO2,g^fS2,g in the system follows an isopotential of H2S,aq, indicating that H2S in vent fluids is buffered. In contrast, H2 in vent fluids is not buffered by Fe^Ni^Co^O^S phases, which merely monitor the evolution of H2 activities in the fluids in the course of progressive rock alteration.The co-occurrence of pentlandite^awaruite^magnetite indicates H2,aq activities in the interacting fluids near the stability limit of water. The presence of a hydrogen gas phase would add to the catalyzing capacity of awaruite and would facilitate the abiotic formation of organic compounds.

Fe–Ni–Co–O–S Phase Relations in Peridotite–Seawater Interactions

Results of ab initio molecular orbital (MO) calculations provide a basis for the interpretation of structural and thermodynamic properties of crystals, glasses, and melts containing tetrahedrally coordinated Si, Al, and B. Calculated and experimental tetrahedral atom-oxygen (TO) bond lengths are in good agreement and the observed average SiO and AlO bond lengths remain relatively constant in crystalline, glassy, and molten materials. The TOT framework geometry, which determines the major structural features, is governed largely by the local constraints of the strong TO bonds and its major features are modeled well by ab initio calculations on small clusters. Observed bond lengths for non-framework cations are not always in agreement with calculated values, and reasons for this are discussed in the text. The flexibility of SiOSi, SiOAl, and AlOAl angles is in accord with easy glass formation in silicates and aluminosilicates. The stronger constraints on tetrahedral BOB and BOSi angles, as evidenced by much deeper and steeper calculated potential energy versus angle curves, suggest much greater difficulty in substituting tetrahedral B than Al for Si. This is supported by the pattern of immiscibility in borosilicate glasses, although the occurrence of boron in trigonal coordination is an added complication. The limitations on glass formation in oxysulfide and oxynitride systems may be related to the angular requirements of SiSSi and Si(NH)Si groups. Although the SiO and AlO bonds are the strongest ones in silicates and aluminosilicates, they are perturbed by other cations. Increasing perturbation and weakening of the framework occurs with increasing ability of the other atom to compete with Si or Al for bonding to oxygen, that is, with increasing cation field strength. The perturbation of TOT groups, as evidenced by TO bond lengthening predicted by MO calculations and observed in ordered crystalline aluminosilicates, increases in the series Ca, Mg and K, Na, Li. This perturbation correlates strongly with thermochemical mixing properties of glasses in the systems SiO2-M

 1
 
/+
 AlO2 and SiO2-M

n+O
n/2 (M=Li, Na, K, Rb, Cs, and Mg, Ca, Sr, Ba, Pb), with tendencies toward immiscibility in these systems, and with systematics in vibrational spectra. Trends in physical properties, including viscosity at atmospheric and high pressure, can also be correlated.

The tetrahedral framework in glasses and melts — inferences from molecular orbital calculations and implications for structure, thermodynamics, and physical properties

Thermodynamic review and calculations—alkali-metal oxide systems with nuclear fuels, fission products, and structural materials

There are two well-known ordered ferromagnetic intermetallic compounds in the iron-nickel system: tetrataenite, FeNi, and awaruite, FeNi3. Their thermodynamic properties are established from the various stable and metastable equilibria in which they are involved, and a review to make this data accessible is appropriate. The Fe-Ni phase diagram shows a paramagnetic face-centered cubic phase (gamma) with well-measured properties over the entire composition range from 1184 to 1662 K. Its properties can be extrapolated to lower temperatures, and known stable and metastable equilibria are used to establish models for the thermodynamic properties of the ferromagnetic alpha phase, a high-temperature paramagnetic alpha phase, a paramagnetic gamma-ordered Fe3Ni phase, the ferromagnetic disordered gamma phase, and both ordered ferromagnetic gamma phases, tetrataenite, FeNi, and awaruite, FeNi3. The overestimate of Chuang et al. for the entropy of ordering in awaruite is corrected. The standard enthalpies at 298.15 K of tetrataenite (0.5 FeNi) and awaruite (0.25 FeNi3) are −1170 and −3560±1000 J mol−1. The corresponding standard entropy values are 35.7 and 33.3±3 J mol−1 K−1. All the second-order transitions are accurately modeled and the full phase diagram is calculated. The spinodal regions and metastable equilibria responsible for the complex behavior on cooling of alloys with mole fractions of nickel between 0.26 and 0.40 are described in detail.

The thermodynamics of tetrataenite and awaruite: A review of the Fe-Ni phase diagram

Calculation of a phase diagram for the lithium oxide-aluminum oxide (LiO0.5-AlO1.5) system

Vapor pressure measurements, thermodynamic parameters, and phase diagram for the system potassium oxide-silicon oxide at high temperatures

Chemical form of copper(II) acetate in acetic acid solutions

Enthalpy and entropy values for solid calcium aluminates have been compared with activity measurements and the known phase diagram to obtain improved revised values and an accurate calculated phase diagram for this binary system. The revised entropies (J mol/sup -1/ K/sup -1/) at 298/sup 0/K are 205.05 +- 0.8 for Ca/sub 3/Al/sub 2/O/sub 6/, 114.22 +- 0.8 for CaAl/sub 2/O/sub 4/, and 188.73 +- 0.8 for CaAl/sub 4/O/sub 7/. The excess enthalpy of mixing of CaO and AlO/sub 1/ /sub 5/ liquids has been calculated and plotted. Heats of fusion can be calculated from these values, but all the pure calcium aluminates melt incongruently.

Reed A. Howald

Papers

The thermodynamics of tetrataenite and awaruite: A review of the Fe-Ni phase diagram

Calculation of a phase diagram for the lithium oxide-aluminum oxide (LiO0.5-AlO1.5) system

Vapor pressure measurements, thermodynamic parameters, and phase diagram for the system potassium oxide-silicon oxide at high temperatures

Chemical form of copper(II) acetate in acetic acid solutions

Thermodynamic properties of calcium aluminates