Calculation of temperature - oxygen fugacity tables for H₂-CO₂ gas mixtures at one atmosphere total pressure, and an investigation of the zoisite-clinozoisite transition

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Virginia Polytechnic Institute and State University

Part I: Calculation of Temperature - Oxygen Fugacity Tables for H₂ - CO₂ Gas Mixtures at One Atmosphere Total Pressure.

The initial CO₂ volumes yielding selected oxygen fugacities at specified temperatures have been computed for H₂ - CO₂ gas mixtures at one atmosphere total pressure. The calculations are based on the thermochemical data of the JANAF (1971) Tables for the species CO₂-H₂-H₂O-CO-O₂-CH₄ and are carried out using the Newton-Raphson method of successive approximation. The new tables replace older ones (Dienes, Nafziger, Ulmer and Woermann, 1974) in which the effects of CH₄ on the system were treated only in an approximate manner.

Part II: An Investigation of the Zoisite - Clinozoisite Transition

Available data on natural coexisting zoisite - clinozoisite (Ca₂Al3-xFex[Si₂O₇][SiO₄]O(OH) with x ≤ 0.33) assemblages imply that the two phases are related by a solid solution transition loop, with zoisite being the low-iron, high-temperature, high-pressure phase. Reversibility experiments at 6500 bars constrain the iron-free equilibrium clinozoisite zoisite to lie below 407°C. Electron microprobe and single crystal x-ray diffraction studies of epitactically related zoisite-clinozoisite crystals grown from seeded mixtures of anorthite, calcite, hematite and silica glass at 6500 bars and 650°C are shown to bracket the transition loop between Pistacite 3.2 (Ca₂Al2.90Fe0.10Si₃O₁₂(OH)) and Pistacite 12.6 (Ca₂Al2.62Fe0.38Si₃O₁₂(OH)). Experimental techniques used in this investigation, which focus on detalied analysis of single crystals, offer promise as a means for determining equilibrium phase relations of epidotes and other frequently zoned mineral groups.