Characteristics of shocked quartz in late Eocene impact ejecta from Massignano (Ancona, Italy): Clues to shock conditions and source crater

The microstructure and optical properties of shocked quartz, discovered in a distal ejecta horizon of the stratigraphically continuous upper Eocene section at Massignano, Italy, have been investigated in detail. The microstructural observations confirm the impact origin of the shocked quartz grains and reveal that the optically visible planar lamellae are well-preserved, amorphous to partially recrystallized planar deformation features. Refractive index measurements of shocked quartz and analysis of the orientation and thickness of planar deformation features indicate shock pressures of 10 to 26 GPa at ambient preshock temperature. Altogether, the characteristics of shocked quartz from Massignano imply a nonporous, quartz-rich near-surface rock as its source. Popigai, Russia, is favored as the source crater because of the presence of compact quartzites in its preimpact cover.