Rock Group

Metamorphic

Grain Size

Fine-grained biotite mica and quartz matrix, surrounding coarse andalusite crystals (<1cm long) that display crystal faces. The texture of having coarse grains in a finer grained matrix is called porphyroblastic, i.e. the andalusite grains are porphyroblasts.

Composition

This hornfels is likely derived from fine-grained sedimentary rock such as claystone or mudstone. Andalusite (the coarse elongated crystals) is one of the three aluminosilicate polymorphs, the others being sillimanite and kyanite. Each polymorph has the same mineral chemistry but is stable under different metamorphic conditions.

Key Features

The occurrence of andalusite indicates that this sample formed in a low pressure, low-medium temperature environment.