What is an episyenite in geology ?

Episyenite : definiton

An episyenite is a grainy rock with high porosity, with an alkaline syenite composition.

The porosity is secondary and results from a late hydrothermal alteration causing selective dissolution of quartz, and albitization of plagioclases. The voids created can subsequently fill with hydrothermal minerals. Episyenites are typical of certain uranium deposits (Limousin, France).