HEAVY MINERALS IN BEACH AND DUNE SAND ALONG SOUTHERN LAKE MICHIGAN

Seven samples of sand along the southern shores of Lake Michigan and one sample of crushed slag were analyzed for heavy mineral content. Very fine sand (3.5 phi) was separated for minerals heavier than 2.95 g/cm³ using sodium polytungstate liquid. Optical mineral identification and point counts were made using a petrographic microscope,while chemical compositions were analyzed with EDX on SEM. The objective of this research was to compare heavy mineral assemblages between beaches and dunes of various ages, and to examine samples for possible anthropogenic particles.

Three eolian samples were taken from the ancient dune (~ 3.5 Ka), recent dune (~0.8 Ka), and the surface of a modern dune. Four samples were from modern beaches in eastern and central sectors of the Indiana shoreline. The samples from the ancient and recent dune contain abundant (~85%) hematite, magnetite, very round ilmenite, and garnet (almandine). Apatite, augite, epidote, hornblende, titanite, and zircon were also present in both samples. In the recent dune sample hornblende and ilmenite grains appeared relatively fresh, but in the ancient dune sample those minerals appeared more weathered. The sample from the modern dune contained less abundant (72%) hematite/magnetite/ilmenite than ancient and recent dune, but more abundant garnets (almandine and andradite), epidote, zircon, and monazite.

The modern beach samples contained 92% hematite, magnetite, and angular ilmenite. Zircon, augite, apatite, enstatite, epidote, garnet (andradite), and monazite were also present. There were several composite grains in the modern beach samples, with sections higher in carbon imbedded in a grain with high concentrations of iron; likely anthropogenic in origin.

The sample of slag contained a higher concentration of carbon than others, likely from carbonates added in the steelmaking process. There was a presence of molybdenum in here undetected in other samples. The modern dune sample shows similarities between both ancient dune sands and modern beach sands with the combination of almandine and andradite garnets. Ilmenite is much more rounded in ancient and recent dune samples than in beach samples. While anthropogenic particles within composite grains were detected in the modern beach samples, they were not found in dune samples.