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

Abstract

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 (α_V0) of MgTiO₃ and FeTiO₃ were obtained as follows: 2.55 (6) × 10^–5 K⁻¹ and 2.82 (10) × 10^–5 K⁻¹, respectively. We infer that the larger effective ionic radius of Fe²⁺(VI) (0.78 Å) than that of Mg²⁺(VI) (0.72 Å) renders FeTiO₃ has a larger volumetric thermal expansivity than MgTiO₃. Simultaneously, the refined axial thermal expansion coefficients under ambient conditions are α_a0 = 0.74 (3) × 10^–5 K⁻¹ and α_c0 = 1.08 (5) × 10^–5 K⁻¹ for the a-axis and c-axis of MgTiO₃, respectively, and α_a0 = 0.95 (5) × 10^–5 K⁻¹ and α_c0 = 0.92 (12) × 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.