Analcime from the L. Lias on the W. Somerset coast

Abstract

SummaryAnalcime, occurring both as small, pale-brown trapezohedral crystals and pale-brown slaggy masses, is reported growing into and replacing aragonitic shell debris in a heavily faulted, 15-km long, coastal exposure of L. Jurassic marine sediments. Associated minerals include authigenic baryte, sphalerite, pyrite (some framboidal), celestine, aragonite, calcite, dolomite, quartz, and solid hydrocarbon. No other zeolites have been found. Its modes of occurrence, crystal morphology, and relationships to these minerals are described. The analcime appears to be restricted in occurrence to the W. Somerset coast. Partial chemical analysis of one sample is given. The brown colouration is suggested to be due to organic compounds. XRD analysis of five samples reveals progressive decrease in silica content towards western extremity of the exposure. No evidence of volcanic ash contamination of the sediments is found. A hydrothermal origin is discounted; formation of the analcime by purely diagenetic processes is suggested.