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Twins in zircon are increasingly being used as an indicator of shock metamorphism on Earth and other planetary bodies, yet the conditions of their formation have not been quantified. We modeled the shear stress involved with homogeneous nucleation of the most common zircon twin (112)[(1) over bar(1) over bar1]. This stress is between 1.7 to 2.6 GPa for the range of crystal sizes common to the crust, stresses that are restricted to impact deformation regimes as opposed to tectonism. The twin nucleation stress is largely pressure independent, and it is likely that twins can form over a much larger range of shock pressures than is currently appreciated. This prediction is validated by the occurrence of twins in shocked zircon from distal regions of the 2.02 Ga Vredefort impact structure.

Plain Language Summary The mineral zircon can deform plastically by the mechanism of twinning, which is characterized by a large angle change in lattice orientation. This is typically achieved by shear. The criteria for the occurrence of twins in zircon are not well known but potentially useful since these twins are found in rocks affected by meteor impact. It is likely that previous estimates of these conditions involve a quantity that is not intrinsic to the formation of the twin itself and are being used to erroneously interpret the severity of meteorite impacts. We evaluate a theory on the twin nucleation based on the free energy required to stabilize a nascent twin "embryo" assuming homogeneity at the microscale. We assess the forces required in terms of what that would mean for the deformation conditions that caused the twin and discuss some implications for the interpretation of these features in the geologic record.