Akimotoite, a new mineral discovered in a shocked meteorite

Abstract

Akimotoite (IMA1997-044), (Mg,Fe)SiO3, a new silicate mineral of the ilmenite group, has been discovered in the shocked Tenham chondritic meteorite. It occurs as aggregates adjacent to clinoenstatite in host-rock fragments trapped into shock-induced melt veins. Chemical analysis gives the simplified formula to be (Mg0.79Fe0.21)SiO3, the same as for clinoenstatite. Selected-area electron diffraction (SAED) patterns correspond to the synthetic (Mg,Fe)SiO3 ilmenite phase with the space group R3. The lattice parameters estimated from the SAED patterns are a = 0.478(5) nm, c = 1.36(1) nm in the hexagonal setting. The calculated density is 4.0(1) g/cm3. In a shock-induced melt vein, akimotoite and clinoenstatite are intergrown with a topotaxial relationship in which close-packed oxygen layers of both phases are preserved. This occurrence suggests that the enstatite-akimotoite transition was caused by a shear transformation mechanism without long-range atomic diffusion.