DEFINITION
Hornfels is a very fine-grained metamorphosed shale rock, usually gray to black in color. Mineral composition in variable but is usually made up of some combination of equidimensional mosaic of quartz or calcite along with inequidimensional sheet silicates in random orientation or sometimes containing andalusite porphyroblasts.
OCCURANCE
Hornfels forms when shale rocks come into contact with a hot igneous intrusion body at lower pressures and at the equilibrium temperature of the igneous-country rock contact.
TYPES
Hornfels can be classified in two different ways. They can be identified by the parent rock in which it was metamorphosed or it can be identified by its major mineral end-members. Pyroxene and hornblende