ZEOLITIC FACIES

one of the major divisions of the mineral facies classification of metamorphic rocks, the rocks of which formed at the lowest temperatures and pressures associated with regional metamorphism. It represents the transition between the sedimentary processes of diagenesis and the distinct regional metamorphism exhibited by the greenschist facies. This facies was first proposed for rocks that were buried at depths of 6 to 9 km (approximately 10 to 14 miles), where they are subject to a load pressure of about one-fifth of a kilobar (about 3,000 pounds per square inch) and temperatures of 200 to 300 C (400 to 575 F). Minerals typically found under these conditions include the zeolites, albite, quartz, and prehnite. Volatile components are very important in the chemistry of this facies because, under zeolitic facies, the temperature and pressure can change the chemical potentials of water and carbon dioxide so as to produce the mineralogy of the greenschist facies.