Wednesday, Nov. 26, 2014
49 ° Rain
|Student||Benjamin Bowes '14|
Dr. John Haley|
|Department||Earth and Environmental Sciences|
|Course||EES 489: Research in the Natural Sciences|
Minerals with densities greater than 2.9 g/cm3 are known as “heavy” minerals. They generally comprise the densest fraction of sand deposits, often forming economically important placer deposits in which uncommon elements such as gold, zirconium, and titanium are concentrated. Additionally, heavy minerals may be used to indicate the provenance of sediments and sedimentary rocks. Because many heavy minerals are opaque and otherwise similar in appearance, optical techniques are not always able to distinguish them. This project explored the use of scanning electron microscopy and x-ray energy dispersive spectrometry (EDS) as alternate means of identifying heavy minerals. Sands were collected from three locations: Belle Isle and Chippokes Plantation on the James River, and Cedar Island, a barrier island off the Eastern Shore of Virginia. Because heavy minerals typically make up less than 10 percent of a sand deposit, they must be concentrated to create a sample large enough to study. This was achieved by gravity separation of light minerals (mainly quartz) from the heavy minerals using tetrabromoethane, a liquid with high specific gravity. Energy dispersive x-ray spectrometry provided spectra and oxide compositions of grains which were compared with published spectra and compositions of known minerals appearing in the literature and standard texts. Additionally, the percent oxide data was entered into published algorithms to calculate mineral formulae. EDS provides relatively quick identification of simple, distinctive minerals like ilmenite or zircon. Its weaknesses, however, are apparent with more complex minerals that can have substitutions, exist over a range of compositions, or that, like tourmaline, contain elements undetectable by the EDS system.