The mineralogy and chemistry were determined of spodosols in 3 Adirondack lake-watersheds by X-ray and electron microprobe analysis, and by optical microscopy. The major minerals present in the soils are quartz, K-feldspar, plagioclase feldspar and hornblende. Accessory minerals identified with the petrographic microscope include magnetite, ilmenite, hypersthene, garnet, tourmaline, epidote and zircon. Vermiculite is the dominant clay mineral in the < 2-.mu.m fraction. Illite, kaolinite, mixed-layer illite/vermiculite, smectite and chlorite are also present, but are less abundant. Vermiculite in the soil profiles contains Al interlayer contaminants that prevent the mineral from collapsing to 10 .ANG. upon K-saturation. Apparently, vermiculite in the soil horizons is derived mainly from the weathering of hornblende. The high content of extractable Fe in these Adirondack soils (up to 6 wt % Fe) results from the chemical dissolution of Fe rich (ferrohastingsitic) hornblende.