Abstract
AbstractAkimotoite (Mg,Fe)SiO3 is one of the most common mineralogical indicators for high‐level shock metamorphism in meteorites. First described 1997, its occurrence has been amply confirmed in a number of highly shocked chondrites. Yet, a thorough structure analysis of natural akimotoite has remained extant. Here we report accurate cell parameters, fractional atomic coordinates, and site occupancies for natural akimotoite from the holotype specimen based on synchrotron microdiffraction. The variation of unit cell shape and volume with Fe content define mixing volumes. Based on the mixing volume relation for akimotoite and hemleyite, we constrain the unit cell volume of endmember hemleyite to 273.8 ± 1.0 Å3. We show that mixing is nearly ideal for low Fe content but evolves to positive excess volume toward the Fe endmember. Based on this finding and the actual composition of akimotoite in Tenham, we show that this mineral has formed by solid–solid transformation prograde from enstatite, not by crystallization from melt.
Highlights
Akimotoite is a high-pressure polymorph of (Mg, Fe)SiO3 isotypic with ilmenite
The unit cell parameters range from a = b = 4.736(1) and c = 13.570(4) A for Akim78 to 4.736(1) and 13.561(5) A for Akim82 which gives volumes between 263.6(2) to 263.8(2) A3, and c/a = 2.865(2) to 2.863(2)
Along with the endmember akimotoite (Horiuchi et al 1982) and type hemleyite (Akim48; Bindi et al 2017), the evolution of unit cell volume as function of Fe content is weakly quadratic but very close to linear within uncertainties
Summary
Akimotoite is a high-pressure polymorph of (Mg, Fe)SiO3 isotypic with ilmenite. The pure Mg endmember of akimotoite has a narrow stability range above 23 GPa and at temperatures below the solidus (Ito et al 1984; Sawamoto 1987; Ishii et al 2011). Fe-bearing akimotoite is a common phase in shockinduced melt veins in meteorites of sufficiently mafic composition such as L chondrites. It was described by Tomioka and Fujino (1997) and Sharp et al (1997) in two independent TEM studies as naturally occurring phase and approved as a mineral by the International Mineralogical Association (Tomioka and Fujino 1999). Akimotoite is a common phase in shock-induced melt veins in heavily shocked chondrites and serves as an excellent indicator for high-grade shock states (Sharp et al 1997; Tomioka and Fujino 1997; Ohtani et al 2004; Xie et al 2006, 2011; Xie and Sharp 2007; Chen and Xie 2015)
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