Iron oxide-apatite (IOA) deposits of the Adirondack Mountains of New York locally contain elevated rare earth element (REE) concentrations (e.g. Taylor and others, 2019). Critical to evaluating resource potential is understanding the genesis of the IOA deposits that host the REE-rich minerals. As part of this effort, the U.S. Geological Survey (USGS) is conducting bedrock geologic mapping, geochronology, geochemistry, and geophysics in the region. Published and ongoing research demonstrates the spatial association of IOA deposits with the Lyon Mountain Granite Gneiss (LMG), and so understanding the relationship of the LMG to the IOA deposits is important for resource evaluation?however the age and origin of the LMG remain contentious. As a result, the USGS undertook a petrologic and geochronologic study of the LMG and Hawkeye Granite Gneiss to better understand the temporal relationship between ores and the host granites. Electron microprobe analyses of the feldspars in the sampled rocks was conducted as part of this research. Feldspar grains from 12 samples, including 4 samples of Hawkeye Granite Gneiss, 7 samples of Lyon Mountain Granite Gneiss, and one amphibolite were analyzed by electron microprobe to determine their major and minor element geochemistry. The electron microprobe data was collected by personnel of the Florence Bascom Geoscience Center in Reston, Virginia, for the U.S. Geological Survey (USGS) National Cooperative Geological Mapping Program (NCGMP). A fully automated JEOL 8900 Electron Microprobe with five wavelength dispersive analyzers operated at 15keV accelerating voltage, a 20-nA beam current, and an electron beam diameter of 5 to 20 micrometers was utilized. The microprobe was operated using Probe for EPMA software (Donovan, 2015). The feldspars of the analyzed samples included plagioclase, K-feldspar, microperthite, and microantiperthite. The latter two are fine-scale exsolution intergrowths of plagioclase and K-feldspar. The grain size of plagioclase was typically coarse (>100 ?m) and easily analyzed by electron microprobe methods. However, the microperthite and microantiperthite commonly had exsolution lamellae <10 ?m in width. As a result, some EMP analyses overlapped lamellae of different composition and are mixed analyses. All analyses with total elemental weight percentages between 98 and 102% are reported. These data were collected in two analytical sessions on 4/26/2019 and 4/30/2019. The spectrometer configuration including analyzing crystal, X-ray line, and on peak count times were as follows: Spectrometer 1 - TAP crystal, NaK&#945;, 20s, MgK&#945; 30s, Spectrometer 2 - LIFH crystal, FeK&#945; 20s, BaL&#945; 20s, Spectrometer 3 - TAP crystal, SiK&#945; 25s, AlK&#945; 25s, Spectrometer 4 - PETJ crystal, CaK&#945; 25s, TiK&#945; 25s, Spectrometer 5 - PETJ crystal, KK&#945; 20s, SrL&#945; 20s. All background corrections were done by linear interpolation of off-peak backgrounds, and off-peak background count times were half the on peak time for each background position. The matrix correction algorithm of Armstrong/Love Scott phi-rho-z (Armstrong, 1988) was used along with the mass absorption coefficients of Henke and others (1982) for all analyses. Detection limits (3&#963;) based on counting statistics were ~0.02 wt% for SiO2, CaO, MgO, Al2O3, K2O, 0.03 wt% for Na2O, 0.04 wt% for TiO2, 0.05 wt% for FeO, 0.06 wt% for SrO, and 0.07 wt% for BaO. Data are reported to the hundredths place and the above detect limits should be considered. Machine stability throughout the analyses and between sessions was documented by the analysis of secondary standard FSLC (Lake County, Plagioclase) in sets of 7 analyses roughly every 12 hours. For major oxides (>10 wt%) the total range of the average value from each of these sets varies by < ? 3% relative, within QC bounds of ? 3%, and for the minor oxide Na2O varies by 9% relative within the QC bound of ?10%. The published value for FSLC (Huebner and Woodruff, 1985) and the value and uncertainty (1&#963;) measured during this study are as follows SiO2, 51.42, 51.05 ? 0.47, Al2O3, 30.76, 30.67 ? 0.18, CaO, 13.42, 13.33 ? 0.14, Na2O, 3.52, 3.81 ? 0.09, FeO 0.39, 0.41 ? 0.02, MgO, 0.14, 0.14 ? 0.01, K2O, 0.12, 0.12 ? 0.01, SrO 0.07, 0.09 ? 0.01, TiO2, 0.04, 0.04 ? 0.01, BaO 0.007, 0.01 ? 0.02. Plagioclase grains from the Hawkeye Granite Gneiss have typical anorthite component values of An15-An20. In contrast, 5 of 7 Lyon Mountain Granite Gneiss samples have plagioclase grains with values <An10. Plagioclase from a sample near Moose Mountain (MR-15-001) has an anomalous composition (~An45-50). Plagioclase from an amphibolite has a composition of An25. The composition of the K-feldspar from the two units overlaps. However, the Hawkeye Granite Gneiss in general has a higher albite component. Backscattered electron and cathodoluminescence imaging indicates that in some cases there are relatively late generations of feldspars that can form veins (flame perthite) and/or patchy replacement of earlier feldspar generations. In some cases, the late feldspars are accompanied by microporosity and micrometer-sized inclusions of fluorite. These zones were generally avoided for the above analyses,but may explain part of the reason for overlap in the composition of K-feldspars from both units. * Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. References: Armstrong, J.T., 1988, Quantitative analysis of silicates and oxide minerals: Comparison of Monte-Carlo, ZAF and Phi-Rho-Z procedures, in Newbury, D.E., ed., Microbeam analysis: San Francisco, California, San Francisco Press, p. 239?246 Donovan, J. J., 2015, Probe for EPMA User Guide and Reference, online @ http://www.probesoftware.com/Technical.html Henke, B.L., Lee, T. P., Tanaka, T.J., Shimabukuro, R.L and Fijikawa, B.K., 1982, Atomic Data Nucl. Data Tables 27, 1. Huebner, J.S. and Woodruff, M.E., 1985, Chemical compositions and critical evaluation of microprobe standards available from the Reston microprobe facility. US Geological Survey Open File Report, 85-718, 45 p. Taylor, R.D., Shah, A.K., Walsh, G.J., and Taylor, C.D., 2019, Geochemistry and geophysics of iron oxide-apatite deposits and associated waste piles with implications for potential rare earth element resources from ore and historical mine waste in the eastern Adirondack Highlands, New York, USA. Economic Geology, v. 114, no. 8, p.1569-1598.