BLUE HILLS IGNEOUS COMPLEX: MODAL ANALYSIS BY X-RAY DIFFRACTION

The Blue Hills igneous complex consists of Avalonian granite, porphyry, pyroclastic flows, and rhyolite flows. A major difference between units is the riebeckite amphibole content, which is significant in the granite and porphyry, but absent in the volcanic units. Modal analysis allows differentiation of chemically similar units, and allows direct comparison between texturally different rocks. Use of Rietveld analysis for quantitative analysis provides both modal and crystallographic information useful in interpreting the differences, even for very fine grained rocks. Sample preparation by McCrone micronizing mill to produce optimal grain sizes, and spray-drying for randomization of orientation, provide excellent quantitative data. Use of standard addition methods verifies the accuracy of analysis, which starts with 12-14 minerals commonly found and reduces to a subset for each sample.