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):