There are two major hypotheses of origin for the rare occurrence of analcime phenocrysts in sodic volcanic rocks. They are either primary analcime or replacement pseudomorphs of leucite. The preponderance of experimental, isotopic, chemical, textural, and field evidence supports the primary analcime hypothesis for the analcime from the Crowsnest Formation, Alberta, Canada. O isotope data are permissive evidence for either replacement of leucite or isotopic alteration of analcime, as is the case with sanidine phenocrysts, which coexist with the analcime but are igneous in origin. Most of the analcime in the Crowsnest Formation shows signs of alteration; only a small amount is pristine. Textural observations of thermal alteration place the analcime in the volcanic environment immediately after eruption (while the rocks were still hot) and are the most persuasive evidence of the sequence of events by which the primary analcime was partially preserved in the Crowsnest flows, tuffs, and agglomerates.