The iron oxide-apatite (IOA) deposits of the eastern Adirondack Mountains consist of intrusive sheets or dikes of magnetite, fluorapatite, augitic pyroxene, quartz, and microcline. Other trace mineral phases include ilmenite with hematite exsolution, V-rich titanite rimming magnetite, zircon, monazite-(Ce), stillwellite-(Ce), lanthanite-(Ce), allanite-(Ce), and thorite. Observations under transmitted light show polygonal and cumulate textures. The ore bodies, each with knife-edge contacts with the host gneisses, are closely associated in time with A-type leucogranites and granitic gneisses (ca. 1070–1050 Ma). Backscattered electron (BSE) images highlight the following types of fluorapatite-monazite-(Ce) relations that formed as a result of metamorphism and fluid-rock interaction: (1) areas of relatively low BSE intensities containing tiny secondary monazite-(Ce) and thorite crystals developed within brighter apatite grains and along crystal margins and fractures; (2) areas of low BSE intensity within larger fluorapatite grains; (3) oriented rods of quartz in fluorapatite; (4) monazite-(Ce) rimming fluorapatite; and (5) multi-domain clusters of fluorapatite in unzoned fluorapatite.