An autocryst is a mineral grain that crystallized in equilibrium with the final melt that transported and deposited it. Unlike crystals inherited from older magma pulses or ripped from the surrounding country rock, autocrysts represent the primary, final cooling phase of the igneous rock in which they are found.
Heard on the job
The zoning on these feldspar autocrysts indicates a steady cooling rate during the final ascent of the magma.
Do not mix it up with
- Antecryst: Autocrysts form from the final host melt, while antecrysts form from an earlier pulse of magma in the same system.