Extreme twin densities in calcite - A shock indicator

In abrupt and extreme loading events, such as meteorite impact or explosions, minerals undergo unique crystallographic changes known as shock metamorphism. The discovery of terrestrial impact craters relies on the unequivocal documentation of such shock effects. While shock effects for most of the rock-forming silicates have been intensely studied and calibrated experimentally against shock pressure, unambiguous shock effects for calcite, one of the most abundant minerals in Earth's upper continental crust, are rare. Here, we examined the potential of high twin densities as a shock effect in calcite. We found that in marble shocked experimentally to shear stresses of 1.4 GPa and peak pressures of 3.9 GPa, the calcite twin density can exceed values of 1000 twins/mm. The average width of twins is below the wavelength of visible light, and thus the optical properties of shocked calcite are uniquely affected. We show that calcite twin piezometry can be applied to evaluate shock deformation in calcite. A numerical modeling approach indicates calcite is capable of sustaining shear stresses of ~1.4 GPa under highly dynamic loading conditions. The application of the new shock criteria may enable the confirmation of meteorite impact craters formed in limestone or marble targets, and potentially improve the shock classification of calcite-bearing meteorites.