Shale samples ranging in depth from 305 m to 3658 m (Miocene to Late Cretaceous) from a single well in DeWitt County, Texas were examined by x-ray diffraction (XRD), scanning-transmission electron microscopy (STEM), and analytical electron microscopy (AEM) techniques to detail the mineralogical, textural, and chemical changes during diagenesis. XRD data suggest that from the surface down to 2134 m, illite/smectite (I/S) is < 50% illite. AEM data show that this smectite-rich I/S contains more K than Na or Ca, suggesting that at least some K exchange with pore fluids occurred well before the transition to illite-rich I/S, and implying a lesser role for detrital K-feldspar and mica in the transition. In addition, direct STEM/AEM observations of separates show that these smectite-rich samples contain equidimensional, euhedral, authigenic illite-rich I/S crystallites at depths as shallow as 610 m; electron-diffraction yields single-crystal patterns. However, such crystallites are rare compared with common anhedral flakes; electron-diffraction patterns for anhedral masses show powder rings, indicating random orientation of layers. Al/Si ratios for anhedral material imply a smectite-like high Si content for tetrahedral sites; Al/Si ratios for the euhedral crystallites suggest an illite-like lower Si content for tetrahedral sites. Between 2134 m and 2438 m (83 degrees to 88 degrees C), the smectite to illite (S-I) transition abruptly occurs, such that at 2438 m and deeper the I/S content remains constant at 75-90% illite. Direct STEM/AEM observations of separates of these illite-rich samples show abundant, equidimensional, euhedral, authigenic, illite-rich I/S crystals; electron-diffraction shows single-crystal patterns. XRD data show that mixed-layer I/S is dominantly R1 ordered. We interpret these data to suggest that, initially, anhedral smectite-rich material is dominant with significant K in exchangeable cation positions; rare illite-rich I/S crystals form in the early stages of diagenesis in Gulf Coast pelitic rocks. During the abrupt S-I transition, the original random orientation of smectite layers is lost and a coherent arrangement of illite packets is formed. The S-I transition proceeds by dissolution of smectite-rich material and utilization, at least in part, of exchangeable K to develop abundant, euhedral, dominantly R1-ordered, illite-rich I/S crystals. The development of illite-rich I/S crystals proceeds via an Ostwald-step-rule process: dissolution of metastable smectite-rich I/S and growth of abundant, euhedral, R1-ordered, metastable illite-rich I/S crystals.