TESTING VARIOUS METHODS FOR MINERAL ABUNDANCES IN GRANITOID ROCKS

Two methods of determining modal abundances of minerals in two granite specimens were tested. It was hypothesized that ImageJ software would be more accurate in showing modal abundance due to its ability to analyze a larger portion of the specimen at one time. Additionally, this study utilized the modal abundances to classify the rocks using the IUGS Classification of Igneous Rocks The first specimen, referred to as Specimen A, is Pikes Peak Granite from Pikes Peak, Colorado. The second specimen, Specimen B, was found along a roadcut northeast of Thunder Bay, Ontario, Canada. For Specimen A, ImageJ software was used to determine the modal abundances of quartz and feldspar based on the surface area of the rock. For both Specimen A and Specimen B, 10x10 box grids were overlaid on top of powerpoint images of the rock surfaces and Excel’s random number generator was used to randomly select 30 boxes. Within those boxes, the presence of feldspar was recorded as either a yes (Y) or no (N). Specimen A had a %Area ratio of 53.8% quartz and 46.2% feldspar according to the ImageJ method. The powerpoint method resulted in a %Area ratio of 13% quartz and 87% feldspar. Specimen B had a %Area ratio of 96.667% feldspar and 3.333% mafic minerals due to the fact that Specimen B did not have any visible quartz grains. The study concluded that the ImageJ software gave more accurate results for the %Area ratio. The IUGS classification named Specimen A a granite based on the selected sample although other published research on Pikes Peak granite names it a syenogranite. This is most likely due to variations in the samples. The IUGS classification for Specimen B remained unknown since the quartz grains were not visible in hand sample. This study was largely used as a teaching experiment on using point-counting to determine the modal abundance of granites. Other learning outcomes of this study were that the IUGS classification largely depends on the sample itself as well as other factors such as location.