Diagenesis and the formation of authigenic illite-rich I/S crystals in Gulf Coast shales; TEM study of clay separates

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