Composition and Genesis of Analcime in the Barstow Formation, San Bernardino County, California

Abstract

Analcime is a common authigenic silicate mineral in rhyolitic tuffs and mudstones of the Miocene Barstow Formation. Optical and X-ray studies show that analcime is associated with clinoptilolite, phillipsite, clay minerals, and (or) potassium feldspar in the altered tuffs, and clay minerals and potassium feldspar in the mudstones. Analcime is nowhere associated with relict glass. The analcime is isotropic, subhedral to euhedral, 0.02–0.3 mm in size, and generally crowded with inclusions of opal or clay minerals. Vitroclastic texture in analcime-rich tuff is vague or nonexistent.