Abstract: Single grain X-ray and electron diffraction patterns of weathered biotite flakes exhibit groupings of 001 and hk reflections of biotite, vermiculite, mixed-layer clay minerals, and kaolinite indicating that the secondary minerals are in parallel crystallographic orientation to the parent biotite. Asterism of biotite reflections is enhanced by weathering. Gibbsite crystals developed in parallel basal orientation to biotite flakes. Most goethite in weathered biotite occurs as aggregates of randomly oriented crystals in cleavages and on grain surfaces. Some goethite is present on micaceous fragments as 0.05-µm size, lathlike crystals in a hexagonal arrangement with their (100) face resting on the (001) biotite face. Selected area electron diffraction patterns of aggregates of lathlike goethite crystals contain 0kℓ, 1kℓ, and 2kℓ reflections due to undulation of the aggregates and the extreme thinness of the crystals. These patterns indicate that the close packed anion layers in goethite coincide with the brucite-like layer of the micaceous minerals.