MODULATED MICROSTRUCTURE IN SYNTHETIC MAJORITE

Transmission electron microscopy on Mg end-member majorite crystals sy nthesized at 22 GPa and above 2350-degrees-C reveals a modulated micro structure, characterized by mutually intersecting linear contrasts par allel to the tetragonal {101} planes. with a typical wavelength of the modulation between 150 and 300 angstrom. Streaking perpendicular to t he {101} planes is observed in electron diffraction patterns. The modu lated microstructure coarsens with decreasing synthesis and quench tem perature and, by 2050-degrees-C, is completely replaced by twin domain s related by reflection operations across the tetragonal {101} planes. These observations suggest that the structural modulation is due to a phase transformation in majorite from cubic (space group Ia3d) to tet ragonal (I4(1)/a) near 2350-degrees-C, probably through ordering of th e [6]Mg and [6]Si atoms. The observed microstructure is a result of pa rtial inversion during quenching from above 2350-degrees-C and reflect s a transitional state in which the cubic structure is perturbed by a number of intersecting wave systems of strain modulation. Majorite spe cimens found in meteorites reportedly have cubic symmetry, whereas mos t of those synthesized in the laboratory are tetragonal; this discrepa ncy may be reconciled by the phase transformation.