Armalcolite stability as a function of pressure and oxygen fugacity

Abstract

The stability of synthetic armalcolite of composition (Fe 0.5Mg 0.5Ti 2O 5 was studied as a function of total pressure up to 15 kbar and 1200°C and also as a function of oxygen fugacity (ƒO 2) 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 ( dT dP = 20°C/ kbar ), to rutile, a more magnesian armalcolite, and ilmenite solid solution. At 14 kbar, this three-phase assemblage gives way ( dT dP = 30°C/ kbar ) to a two-phase assemblage of rutile plus ilmenite solid solution. A zirconian-armalcolite was synthesized and analyzed; 4 wt % ZrO 2 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 ƒO 2 was determined thermogravimetrically at 1200°C and 1 atm by weighing sintered pellets in a controlled atmosphere furnace. Armalcolite, (Fe 0.5Mg 0.5)-Ti 2O 5, is stable over a range ƒO 2 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 Ti 3+. This reduction may provide yet another mechanism to explain the common association of ilmenite rims around lunar armalcolites.