PETROGRAPHIC EXAMINATION OF THE BENNETT PEGMATITE, OXFORD COUNTY, MAINE

The granitic pegmatites of southwestern Maine are known to contain very coarse-grained and compositionally unique mineral assemblages. With the discovery of gem-quality tourmaline at Mt. Mica in 1820, a course was set for mining and exploration of the pegmatites in Oxford County that continues today. As a result, Maine has become a leading producer of high quality gems and minerals. Despite the popularity of mining and collecting within these pegmatite bodies, continued detailed research is still necessary to fully describe the observed mineral diversity of these rocks. The goal of this study was to petrographically examine the mineral assemblage of a pegmatite sample obtained from the Bennett Pegmatite body located in Oxford County, Maine. This sample was prepared for petrographic analysis using facilities and equipment available in the UALR Rock Preparation Laboratory. Examinations of prepared thin sections reveal a primary mineral assemblage of large (up to 2 cm in diameter), green apatite phenocrysts within a matrix of albite, quartz, microcline, feldspar, and muscovite. Many tourmaline (schorl) grains are also visible in thin section, having an average size of ~1-2 mm. These tourmalines appear as skeletal structures with visible zoning, showing predominantly brown cores with blue-green rims in plane-polarized light. Primary igneous textures in feldspars include twinning, poikilitic intergrowths, and exsolution lamellae. A high amount of strain is visible within some quartz grains, but it is not known if this strain occurred during or after crystallization. Interestingly, additional observations with shortwave ultra-violet light have indicated the presence of several fluorescent minerals, with the most notable being the gold-orange fluorescence of green apatite crystals.