A COMMON ROCK-FORMING MINERAL UNDER SHOCK: A STUDY OF PLAGIOCLASE AMORPHIZATION IN NATURE AND EXPERIMENTS

The process of plagioclase amorphization in response to shock metamorphism is not yet fully constrained, even though this mineral is quite abundant in nature. Plagioclase is so common that its degree of amorphization in response to shock has been used to evaluate the shock stage in meteorites. The best opportunity to investigate the progression of amorphization is provided in cases of incomplete transformation of crystalline plagioclase into amorphous material. In this work, we focus on experimentally and naturally shocked plagioclase, showing incomplete amorphization. The selected samples derive from both a shock recovery experiment, performed at 28 GPa with a troctolite, and involving An₅₅ plagioclase, and a shatter cone from the Manicouagan impact structure, Québec, Canada, formed in a garnet-bearing metagranite and containing An₂₄ plagioclase. The amorphization is generally evaluated with the optical microscope, in transmitted light and on petrographic thin section. In this study, we used several spectroscopic techniques (such as Raman spectroscopy, photoluminescence, and cathodoluminescence), to evaluate plagioclase amorphization whenever the preparation of a petrographic thin section is not permitted, and electron back-scatter diffraction (EBSD) to evaluate the progression of amorphization within individual crystals.

The amorphization of plagioclase in the investigated experimentally shocked sample is incomplete and its occurrence is likely affected by (i) the crystallographic orientation of each grain with respect to the shock wave reflection in the used experimental set up, (ii) the presence of crystal anisotropies, such as inclusions, cracks, and cleavage and their orientation, and (iii), the nature of neighboring phases (size, orientation, mineralogy, and corresponding physical properties, such as shock impedance). On the other hand, the progression of amorphization of the naturally shocked plagioclase from the Manicouagan impact structure is controlled by the formation of microtwins. Our observations imply that the formation of microtwins is the first response to shock compression, followed by amorphization of those previously formed twins conveniently oriented with respect to the locally scattered shock wave.