Analcime Crystals Research Articles

AUTHIGENIC SILICATE MINERALS IN SEARLES LAKE, CALIFORNIA

SUMMARYMonoclinic K‐feldspar, analcime, searlesite and phillipsite of diagenetic origin are present in samples of clay and tuff from U. S. Geological Survey cores of Pleistocene and Recent sediments of Searles Lake, California. Authigenic K‐feldspar was identified in 37 of the 72 samples studied; analcime was identified in 21, searlesite in 8, and phillipsite in 3. Most of the K‐feldspar, analcime, and searlesite occur in non‐tuffaceous claystones, which form about 46% of the 875‐ft. thickness of sediments that was cored. Phillipsite was found only in three rhyolitic tuff laminae within the upper 110 ft. of beds. Most of the K‐feldspar and analcime crystals are less than 2 μ in diameter, and maximum diameters are 0.10 mm for K‐feldspar and 0.02 mm for analcime. Phillipsite occurs as lath‐shaped crystals 0.01–0.02 mm long, and searlesite principally forms spherulites 0.02–0.2 mm in diameter.Microcline, orthoclase, albite, andesine, and quartz predominate in the sand and silt fraction of the clays; illite, montmorillonite, chlorite, and kaolinite predominate in the detrital clay fraction. Grains of quartz and andesine are extensively etched at various levels below a depth of 220 ft., and only skeletal crystals of andesine remain in some clays. Volcanic glass has been completely dissolved or replaced by authigenic silicate minerals in the few tuffs. Distribution of clay minerals in the core suggests that montmorillonite and chlorite have been destroyed in many of the clays.Alkaline brine saturates the surface and subsurface sediments, and the high activity ratio of alkali ions to hydrogen ions in this brine probably accounts for the widespread formation of K‐feldspar, zeolites, and searlesite. Na, K, and B for the authigenic silicates were probably supplied by the brine and by saline minerals in contact with the brine. Montmorillonite may have provided additional K. Solution of glass provided the Si and Al required to form authigenic silicate minerals in the tuffs, and solution of quartz, plagioclase, and montmorillonite most likely supplied the Si and Al for authigenic silicates in the clays.

A Zeolite-filled Cavity in the Igneous Rock of Croft, Leicestershire

During an intensive investigation of the joint phenomena of the South Leicestershire igneous intrusions a widened joint cavity eighteen inches long and five wide apparently filled with “calcite or quartz” was noted. On closer examination the mineral showed perfect trapezohedra of analcime and cube-like rhombohedra of chabazite. The analcime crystals attained a diameter of 0·4 inches: they comprised the bulk of the specimen, the chabazite being very subordinate and only in small sizes. As is usual the analcime had preceded the chabazite in order of crystallization. The cubic mineral was colourless, milky white, and also reddish through impregnation with iron oxide. The chabazite was opaque white but translucent on fracture.