Armalcolite stability as a function of pressure and oxygen fugacity |
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| Authors: | John J Friel RIan Harker Gene C Ulmer |
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| Institution: | Department of Geology, University of Pennsylvania, Philadelphia, Pennsylvania 19174, U.S.A.;Department of Geology, Temple University, Philadelphia, Pennsylvania 19122, U.S.A. |
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| Abstract: | The stability of synthetic armalcolite of composition (Fe0.5Mg0.5Ti2O5 was studied as a function of total pressure up to 15 kbar and 1200°C and also as a function of oxygen fugacity (?O2) at 1200°C and 1 atm total pressure. The high pressure experiments were carried out in a piston-cylinder apparatus using silver-palladium containers. At 1200°C, armalcolite is stable as a single phase at 10 kbar. With increasing pressure, it breaks down (), to rutile, a more magnesian armalcolite, and ilmenite solid solution. At 14 kbar, this three-phase assemblage gives way () to a two-phase assemblage of rutile plus ilmenite solid solution.A zirconian-armalcolite was synthesized and analyzed; 4 wt % ZrO2 appears to saturate armalcolite at 1200°C and 1 atm. The breakdown of Zr-armalcolite occurs at pressures of 1–2 kbar less than those required for the breakdown of Zr-free armalcolite. The zirconium partitions approximately equally between rutile and ilmenite phases.The stability of armalcolite as a function of ?O2 was determined thermogravimetrically at 1200°C and 1 atm by weighing sintered pellets in a controlled atmosphere furnace. Armalcolite, (Fe0.5Mg0.5)-Ti2O5, is stable over a range ?O2 from about 10?9.5to 10?10.5 atm. Below this range to at least 10?12.8 atm, ilmenite plus a reduced armalcolite are formed. These products were observed optically and by Mössbauer spectroscopy, and no metallic iron was detected; therefore, some of the titanium must have been reduced to Ti3+. This reduction may provide yet another mechanism to explain the common association of ilmenite rims around lunar armalcolites. |
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