SCANNING ELECTRON MICROSCOPY OF A BEACH SAND FROM PUGET SOUND, WASHINGTON

The sand described in this paper comes from a low-energy beach along the shore of Puget Sound in Western Washington State. It is derived from the erosion, mainly by waves, of cliffs composed of glacial and interglacial sediments. The beach is washed by wind, waves, and tidal currents. The sand is medium gray in color and moderately sorted. Quartz grains picked from the sand are mostly angular, with sharp corners and edges, and show obvious conchoidal fractures typical of glacial processing.

The sample was prepared for the scanning electron microscope study by cementing the grains together with epoxy. The resulting epoxy block was then cut, mounted on a microscope slide, polished, and carbon coated. The grains were imaged with a backscattered electron (BSE) detector and analyzed with an x-ray (EDS) microanalizer. The mineral and chemical composition of the sample is very diverse. More than half of the grains consist of only one mineral, but the remainder are made of two or more components. Some of these polymineralic grains are obviously fragments of fine grained rocks, others are not. The most common monomineralic grains are, in order of abundance, silica, feldspars, pyroxenes, and amphiboles. Most of the silica is quartz. Some may be tridymite. Feldspars are k-spar, sanadine, and plagioclase. There is a variety of plagioclase, but most are less than AN#50. Pyroxenes and amphiboles also show a variety of compositions. The most common polymineralic grains are quartz & feldspar, pyroxene & an oxide, feldspar& amphibole, and fine-grained rock fragments. In addition to the common components, the sand contains a variety of trace minerals such as iron/titanium oxides, titanite, zircon, monazite, allenite (pentahydrite), calcite (fragments of mollusks and barnacles that inhabit the beach environment) and garnet. We conclude that the composition of this beach sand is very diverse. This diversity represents many different source rock types. Scanning electron microscopy is a very useful technique for studying this kind of sediment.