What is "pneumatolysis"

n. (context geology English) the alteration of rocks and minerals by gases or magmatic fluid within volcanos etc

Pneumatolysis is the alteration of rock or mineral crystallization affected by gaseous emanations from solidifying magma. The term was coined by Robert Bunsen in 1851.

Magmatic gases may through pneumatolysis create skarn in interaction with limestone, or greisen with granite or schist.