Abstract Perthitic K-feldspar in Brazilian alkaline rocks covered with a thick weathering rind shows evidence for the formation of iron-rich transitional precursors of clay minerals. High-resolution transmission electron microscopy, energy dispersive X-ray analyses (EDX), and electron diffraction patterns clearly show the decomposition of K-feldspar and growth of these transitional products. They occur as long, curled fiber forms or circular forms on the altered feldspar surface. The electron diffraction patterns show diffuse rings at 4.41, 2.65, 1.56, and 1.38 Å, suggesting poor crystallinity or random orientation. These circular structures, typically 150–200 Å in diameter, contain duplicate to quintuplicate 7-, 14-, and 20-Å lattice images. Step scanning EDX analysis shows that the concentrations of Si, Al, and K tend to decrease and Fe increase from unaltered parts to altered parts of the clay precursors. Crystalline halloysite(7Å) appears to form from them.