A PETROGRAPHIC AND GEOCHEMICAL DATABASE FOR COUNTERTOPS AS A TEACHING RESOURCE

A petrographic and geochemical database (http://educ.jmu.edu/~johns2ea/countertops.htm ) was created for commercial countertop rocks installed as desks in the Mineralogy/Petrology classroom in the Department of Geology and Environmental Science at JMU. Leftover countertop materials from a local supplier were used to make twenty desks from ten commercially available igneous and metamorphic countertops: Baltic Brown, Bethel Ash, Giallo Fiorito, Golden Leaf, Green Butterfly, Rainbow Red, Santa Cecilia, Tan Brown, Tropical Brown, and Ubatuba. These countertops should be widely available in home improvement stores and countertop suppliers, and samples are therefore accessible by other instructors across the country. The cost of purchasing and installing the countertops was relatively affordable and comparable to the cost of installing standard black chemical-resistant laboratory countertops. It is not necessary to permanently install large countertops to use these materials in class activities; smaller pieces can be installed or can be used as hand samples. Images of whole thin sections in both unpolarized and cross-polarized light were obtained using a photo scanner. Mineral assemblages were determined using a combination of optical techniques on a polarizing microscope and EDS analysis using the JEOL-8900 electron microprobe at the USGS in Reston. Representative feldspar compositions for each rock were also determined using the electron microprobe. Modal abundances of major minerals were calculated by counting 340-400 points on an evenly spaced grid across each thin section. Representative samples were sent to a commercial laboratory for whole-rock major and trace element geochemistry using ICP-AES analyses. Instructors can use this data to create their own course activities and labs, or can use or modify example exercises listed on the database website. These include a jigsaw activity to create a poster for each countertop containing petrographic and geochemical data and interpretations; point-counting on the countertops themselves using plastic fencing as a grid; and comparing feldspar compositions obtained optically using the Michel-Levy method to those obtained with the electron microprobe.