Reaction Rims

When a crystal is out of equilibrium with the surrounding magma (now seen as the finer grained groundmass), it starts to react with it and develops a "reaction rim".  This situation can occur due to an influx of new magma, magma evolves out of a particular minerals stability field, or the crystal is an accidental inclusion or "xenocryst" (i.e., it was not formed by this particular batch of magma).





 In extreme cases, the crystal becomes completely resorbed and only relicts or ghost shapes remain (from Sample F84, hornblende andesite):