Antiphase domains and reverse thermoremanent magnetism in ilmenite-hematite minerals.

Examination of synthetic ilmenite-hematite samples by transmission electron microscopy has for the first time revealed the presence of well-defined antiphase domains and antiphase domain boundaries in this mineral system. Samples quenched from 1300 degrees C have a high density of domain boundaries, whereas samples quenched from 900 degrees C have a much lower density. Only the high-temperature samples acquire reverse thermoremanent magnetism when cooled in an applied magnetic field. The presence of a high density of domain boundaries seems to be a necessary condition for the acquisition of reverse thermoremanent magnetism.

Shock-produced olivine glass: first observation.

Transmission electron microscope (TEM) observations of an experimentally shock-deformed single crystal of natural peridot, (Mg(0.88)Fe(0.12))(2)SiO(4), recovered from peak pressures of about 56 x 10(9) pascals revealed the presence of amorphous zones located within crystalline regions with a high density of tangled dislocations. This is the first reported observation of olivine glass. The shocked sample exhibits a wide variation in the degree of shock deformation on a small scale, and the glass appears to be intimately associated with the highest density of dislocations. This study suggests that olivine glass may be formed as a result of shock at pressures above about 50 to 55 x 10(9) pascals and that further TEM observations of naturally shocked olivines may demonstrate the presence of glass.