Thermal expansivity of geikielite and ilmenite utilizing in-situ synchrotron X-ray diffraction at high temperature

The unit-cell parameters and volumes of geikielite (MgTiO3) and ilmenite (FeTiO3) were investigated at high temperatures up to 700 K and ambient pressure, using in-situ angle-dispersive synchrotron X-ray diffraction. No phase transition was detected over the experimental temperature range. Using (Berman in J Petrol 29:445-522, 1988. 10.1093/petrology/29.2.445) equations to fit the temperature-volume data, the volumetric thermal expansion coefficients at ambient conditions (& alpha;(V0)) of MgTiO3 and FeTiO3 were obtained as follows: 2.55 (6) x 10(-5) K-1 and 2.82 (10) x 10(-5) K-1, respectively. We infer that the larger effective ionic radius of Fe2+(VI) (0.78 & ANGS;) than that of Mg2+(VI) (0.72 & ANGS;) renders FeTiO3 has a larger volumetric thermal expansivity than MgTiO3. Simultaneously, the refined axial thermal expansion coefficients under ambient conditions are & alpha;(a0) = 0.74 (3) x 10(-5) K-1 and & alpha;(c0) = 1.08 (5) x 10(-5) K-1 for the a-axis and c-axis of MgTiO3, respectively, and & alpha;(a0) = 0.95 (5) x 10(-5) K-1 and & alpha;(c0) = 0.92 (12) x 10(-5) K-1 for the a-axis and c-axis of FeTiO3, respectively. The axial thermal expansivity of MgTiO3 is anisotropic, but that of FeTiO3 is nearly isotropic. We infer that the main reason for the different axial thermal expansivity between MgTiO3 and FeTiO3 is that the thermal expansion mode of the Mg-O bond in MgTiO3 is different from that of the Fe-O bonds in FeTiO3.