MINERALOGY OF DUNE SANDS ALONG THE SOUTHEASTERN SHORE OF LAKE MICHIGAN

The mineralogy of dune sand was investigated from 3 large dune complexes (north: Hoffmaster State Park; central: Holland; south: Warren Dunes State Park) within a 130 km stretch along the southeastern coast of Lake Michigan. Pairs of dark and light colored sands were collected from patches in layers in the same area. Sand size distributions were determined by sieve analysis. Dark sands were always finer grained (range in median grain size 120-230 μm, average 182) than light sands (range in median grain size 205 -310 μm , average 252). We studied the mineralogy in up to 5 size fractions (500-355, 300-250, 180-150, 150-125, 125-88 μm) in 4 to 8 samples per complex by making multiple traverses across grain mounts on an electron microprobe. We used the energy dispersive spectrum to determine the mineralogy of between 100 to 300 grains per size fraction. The 300 to 250 μm fractions of all 16 samples were dominated by quartz (80-90%) and feldspar (8-18%). The amount of quartz decreases progressively with grain size as the mineralogy becomes more diverse. The important minerals in the 150-125 μm size range include quartz (0.4 to 42%), feldspar (2 to 13%), Fe and Fe-Ti oxides (7 to 82%), carbonate (2 to 25%), garnet (2 to 17%), pyroxene (1 to 6%), hornblende (1 to 5%) and epidote group minerals (1 to 7%). Dolomite was always much more abundant than calcite and K-feldspar was usually more abundant than plagioclase. The smaller size fractions in both light and dark colored sands tend to be dominated by dark minerals: the color of dark sands is largely due to the fact that smaller grain sizes make up a larger proportion.

Variations in mineralogy within dune complexes are as large as variations between complexes and much larger sample sizes would be necessary to establish clear geographical trends. The mineralogy of the finer size fractions reflects the diverse mineralogy of the glacial deposits from which they were derived. The tendency for oxides, ferromagnessium silicates, epidote group minerals and carbonates to be concentrated in the finer grain sizes reflects in part sorting and mechanical breakdown during eolian transport. Trace element analyses of quartz grains in the coarser fractions to study provenance of the quartz, are planned using Proton Induced X-ray Emission analysis at the Hope College Ion Beam Analysis Laboratory