Hornfels

Hornfels is the name given to fine-grained, non-foliated rocks formed by thermal (contact) metamorphism.

The two pictures above (both under crossed polarizers) show the typical random orientations and fine grain size of hornfels. The rock on the left falls within the hornblende hornfels facies, with hornblende and plagioclase; the rock on the right is in the pyroxene hornfels facies and contains plagioclase, cordierite (a few of the first order yellow grains), and biotite.