PREPARATION AND STRUCTURE REFINEMENT OF SYNTHETIC TI3-CONTAINING LINDSLEYITE, BAMN3TI18O38()

The stability field for lindsleyite in the system BaO-MnO-TiO2-Ti2O3 a t 1100 degrees C was studied using the quench method with H-2-CO2 gas mixtures to control the oxygen fugacity. Lindsleyite is stable at oxyg en fugacities lower than 10(-145) atm., down to the lowest fugacity us ed, 10-(17.5) atm. A lindsleyite phase with composition BaMn3Ti18O3, w as prepared at an oxygen fugacity of 10(-15.7) atm. for structural stu dies. The phase contains trivalent titanium, with Ti3+:Ti4+ = 4:14. It has rhombohedral symmetry, R (3) over bar, with a 10.4369(1) and c 20 .8871 (2) Angstrom. A Rietveld refinement of the structure was made us ing powder X-ray-diffraction data. The twelve-coordinated A site is th e largest observed for minerals of the crichtonite group. The mean A-O bond length is shown to have a linear relationship with the radii of the cations assigned to this site for all the minerals in the crichton ite group.