Phase transitions in the FeTiO(3)-MgTiO(3)-CaTiO(3) ternary field at high pressures and temperatures.

摘要

High pressure and temperature phase transitions stabilize perovskite solid solutions and ordered perovskites in the FeTiO{dollar}sb3{dollar}-MgTiO{dollar}sb3{dollar}-CaTiO{dollar}sb3{dollar} ternary system. These reactions were studied through a combination of calorimetric measurements and high pressure studies.; On the pseudo-binary join CaTiO{dollar}sb3{dollar}-FeTiO{dollar}sb3,{dollar} enthalpies of formation from ilmenite, FeTiO{dollar}sb3,{dollar} and perovskite, CaTiO{dollar}sb3,{dollar} of two intermediate ordered perovskite phases, CaFeTi{dollar}sb2{dollar}O{dollar}sb6{dollar} and CaFe{dollar}sb3{dollar}Ti{dollar}sb4{dollar}O{dollar}sb{lcub}12{rcub},{dollar} were measured at 801{dollar}spcirc{dollar}C using oxide melt solution calorimetry. These data, in combination with experiments at high pressure and temperature, indicate that below 1518 {dollar}pm{dollar} 50{dollar}spcirc{dollar}C, CaFe{dollar}sb3{dollar}Ti{dollar}sb4{dollar}O{dollar}sb{lcub}12{rcub}{dollar} is stable at lower pressures ({dollar}sim{dollar}7 GPa at 1200{dollar}spcirc{dollar}C) than CaFeTi{dollar}sb2{dollar}O{dollar}sb6{dollar} ({dollar}sim{dollar}13 GPa at 1200{dollar}spcirc{dollar}C). This relationship should be reversed, and CaFeTi{dollar}sb2{dollar}O{dollar}sb6{dollar} should become stable at lower pressures than CaFe{dollar}sb3{dollar}Ti{dollar}sb4{dollar}O{dollar}sb{lcub}12{rcub},{dollar} at temperatures above 1518 {dollar}pm{dollar} 50{dollar}spcirc{dollar}C.; A high-pressure MgTiO{dollar}sb3{dollar} phase with the lithium niobate structure (R3c) was recovered from 21 GPa and 1200{dollar}spcirc{dollar}C. Phases with the lithium niobate structure were also synthesized for intermediate compositions between FeTiO{dollar}sb3{dollar} and MgTiO{dollar}sb3{dollar} and their lattice parameters determined. Through a series of multi-anvil experiments, a phase boundary was determined between MgTiO{dollar}sb3{dollar}I (ilmenite) and the new high pressure phase with the MgTiO{dollar}sb3{dollar} II (lithium niobate) structure after quenching. The boundary of the transition is described by the equation P(GPa) = {dollar}-{dollar}0.008(2)T(K) + 33(4).; The enthalpy of transformation of ilmenite to lithium niobate was determined for MgTiO{dollar}sb3{dollar}, FeTiO{dollar}sb3{dollar} and three intermediate compositions on the FeTiO{dollar}sb3{dollar}-MgTiO{dollar}sb3{dollar} join, Fe{dollar}sb{lcub}0.2{rcub}{dollar}Mg{dollar}sb{lcub}0.8{rcub}{dollar}TiO{dollar}sb3,{dollar} Fe{dollar}sb{lcub}0.5{rcub}{dollar}Mg{dollar}sb{lcub}0.5{rcub}{dollar}TiO{dollar}sb3{dollar} and Fe{dollar}sb{lcub}0.8{rcub}{dollar}Mg{dollar}sb{lcub}0.2{rcub}{dollar}TiO{dollar}sb3{dollar}; and was found to increase linearly with MgTiO{dollar}sb3{dollar} content.; In situ x-ray diffraction during diamond cell compression and heating of Fe{dollar}sb{lcub}0.5{rcub}{dollar}Mg{dollar}sb{lcub}0.5{rcub}{dollar}TiO{dollar}sb3{dollar} II to 802{dollar}spcirc{dollar}C at 21 GPa confirmed the transition to Fe{dollar}sb{lcub}0.5{rcub}{dollar}Mg{dollar}sb{lcub}0.5{rcub}{dollar}TiO{dollar}sb3{dollar} III (a GdFeO{dollar}sb3{dollar}-type perovskite) at simultaneous high pressure and high temperature. In combination these data confirm the stability of a continuous perovskite solid solution at high pressure and temperature for the FeTiO{dollar}sb3{dollar}-MgTiO{dollar}sb3{dollar} join. These FeTiO{dollar}sb3{dollar}-MgTiO{dollar}sb3{dollar}-CaTiO{dollar}sb3{dollar} high pressure phases are of petrological interest because titanates are commonly associated with kimberlites and ultra-high-pressure metamorphic rocks.