Analcime and Albite in Altered Jurassic Tuff in Idaho and Wyoming

A sodium-rich altered tuff bed, approximately 5 feet thick, in the Jurassic Twin Creek limestone occurs over an area of more than one thousand square miles in Idaho and Wyoming. Microcrystalline quartz, analcime, albite, and calcite are the major components of the typically hard pale-green or grayish-green aphanite. One chemically analyzed analcime-rich rock contains 6.2 per cent Na₂O. At one locality the altered tuff is composed nearly entirely of albite. A shard texture is preserved in both the analcime-rich and albite-rich rocks. Analcime is considered to have developed from a reaction of volcanic glass and sea water (possibly hypersaline) at some diagenetic stage and was converted in variable amount to albite at higher temperatures and pressures due to deep burial.