Abstract

Garnet is a key rock-forming phase of the Earth’s crust and upper mantle, where it represents the paradigmatic cubic mineral, optically isotropic. Although strain-induced birefringence is observed in the rare Ca-Fe3+ hydrogarnets, the optical anisotropy rarely documented in common Fe2+-Mg-Mn garnets has not found a general and robust explanation so far.

Here we show that anisotropic garnets are much more widespread than previously thought, in particular in blueschist-facies rocks. We unequivocally demonstrate that the origin of birefringence is the tetragonal nature of garnet, that it is unrelated to strain, and that crystals are twinned according to a merohedral law. We also show the absence of a hydrogarnet component, and that the tetrahedral form appears to be typical of Fe-Ca-rich compositions, with very low Mg contents.

The widespread occurrence of optically anisotropic OH-free garnets in blueschists implies that this phase does not commonly grow with cubic symmetry at low-temperature conditions (< 400 °C). The crystal-chemistry of garnet, its thermodynamics and, in turn, its use in unravelling petrogenetic processes in cold metamorphic environments, such as in subduction zones, need to be re-assessed.