THE STABILITY AND CRYSTAL-CHEMISTRY OF SYNTHETIC LOVERINGITE IN THE SYSTEM CA-MN-TI-O UNDER STRONGLY REDUCING CONDITIONS

The stability of loveringite. Ca(Ca,Mn,Ti3+,Ti4+)(21)O-38, in the syst em CaO-MnO-Ti2O3-TiO2 was studied as a function of oxygen fugacity at 1100 degrees C. Loveringite was prepared as a purr or dominant phase a t oxygen fugacities in the range 10(-15) to 10(-18) atm. At higher fug acities, the stable assemblage is rutile + pyrophanite + perovskite, w hereas at lower fugacities, loveringite becomes unstable relative to m ixtures of perovskite + pyrophanite + MnxTi3-xO5. Results of electron- microprobe analyses and of wet-chemical analyses for Ti3+ show that th e compositional variations in loveringite (in atoms per formula unit, apfu) are 1.1 < Ca < 1.7, 1.8 < Mn < 3.2, 2.4 < Ti3+ < 5.6 and 13.2 < Ti4+ < 14.4. The dominant mode of compositional variation involves the charge-coupled mechanism of substitution Mn2+ + Ti4+ reversible arrow 2Ti(3+). Crystal-chemical variations in synthetic loveringite were st udied using Rietveld refinement of powder X-ray data. The unit-cell pa rameters are strongly correlated with the Ca and Mn contents. Calcium in excess of 1 apfu is ordered at the largest octahedral site, M1. Sam ples with higher Mn contents have Mn ordered at the M1 site and the te trahedral site T. Strongly reduced samples, with low manganese content s, have Mn preferentially at the T site, and Ti3+ at M1.