Akimotoite, (Mg,Fe)SiO₃, a new silicate mineral of the ilmenite group in the Tenham chondrite

Abstract

Akimotoite, (Mg,Fe)SiO₃, a new silicate mineral of the ilmenite group, was found in the shock-metamorphosed Tenham chondrite. It occurs as aggregates adjacent to clinoenstatite in fragments within shock-induced melt veins. Chemical analyses show the simplified formula to be (Mg_0.79Fe_0.21)SiO₃,the same as for clinoenstatite. Selected-area electron diffraction (SAED) patterns correspond to the synthetic (Mg,Fe)SiO₃ ilmenite phase with space group R3̅ . Lattice parameters derived from SAED patterns are a = 0.478(5) nm, c = 1.36(1) nm and V = 0.269(8) nm³ in the hexagonal setting. The calculated density is 4.0(1) g/cm³. Akimotoite in this occurrence is thought to have been transformed from original orthoenstatite in a solid-state reaction produced by a shock event. Peak pressure and temperature generated by shock events in Tenham are estimated to be 22 GPa <Pmax <26 GPa and Tmax >2000 °C, assuming that equilibrium crystallization of aluminous majorite occurred in the melt veins. This new mineral was named after Syun-iti Akimoto.