GEOCHEMICAL INVESTIGATION OF ADIRONDACK GARNETS

The garnets from the Adirondacks in upstate New York have been used economically over the past 125 years for their extreme hardness. They are also famous for because of their abnormally large size. Four main areas that produce these garnets of interest include Gore Mountain, Warrensburg, Ruby Mountain and the Old Hooper Mine. The focus of this study is to better understand the growth and generation of garnets by examining their geochemistry. The garnets were examined using an electron microprobe (EPMA) at Rutgers University to determine the crystal chemistry. Of particular interest was determining if the crystals were completely homogenous or if chemical zonation was present. Transects were made across the crystals to quantitatively examine potential zonation. EPMA analysis found mild zonation was present in some of the garnets, but these variations do not change in a simple pattern. One sample from Warrensburg displayed a composition along the rim of the crystal of Al_60.8Py_22.8Gr_12.1Sp_4.0, while the core of the crystal was Al_55.7Py_29.2Gr_12.7Sp_2.9. The composition change in the Hopper Mine samples was slightly smaller, but zonation was still present. The composition at the rim was Al_53.5Py_25.3Gr_19.0Sp_2.2, while the composition at the core was Al_55.2Py_23.0Gr_18.7Sp_3.1. The garnets from Gore Mountain again showed little to no evidence of chemical zonation, with the results showing that the chemical composition for one of the samples was Al_46.7Py_40.8Gr_11.4Sp_1.1, while the cores composition was Al_45.0Py_42.3Gr_11.4Sp_1.0. The data was subsequently used in geochemical modeling to better constrain the garnet growth history.