66 SEDIMENTARY ROCKS/Clays and Their Diagenesis Figure Fracture surface scanning electron microscopy (SEM) image of pore filling kaolinite Figure Backscattered scanning electron microscopy (SEM) image of kaolinite pseudomorph after mica in the London Clay Formation (Eocene) fluid/rock ratio is still high, and the Kỵ can be removed from the site of dissolution, allowing the reaction to continue A tentative reaction is deduced from the common association of kaolinite pseudomorphs after mica with mica/quartz pressure solution contacts: muscovite ỵ H4 SiO4 ỵ Kỵ ỵ Hỵ ! kaolinite ỵ K feldspar ỵ Naỵ ỵ Mg2ỵ ỵ Fe2ỵ ỵ H2 O However, in the Eocene of the London Basin, where burial has never been greater than km, probably much less, kaolinite pseudomorphs after mica are widespread (Figure 5), and evidence for pressure solution is minimal This particular reaction mechanism may be more applicable to sandstones than mudrocks The fate of the Kỵ that is released at shallow depths is not clear; certainly, it is not normally needed for the illitization of smectite at such low temperatures Clay Diagenesis in Sandstones Kaolin Clays Authigenic kaolinite in sandstones forms stacks of euhedral pseudohexagonal platelets, with the c axis parallel to the stacking direction (Figure 6) Very long stacks are called vermicules (Figure 6).Typically, this clay has a pore-filling habit Kaolinite also forms Figure Fracture surface scanning electron microscopy (SEM) image of wispy illite in Rotliegendes Sandstone (Permian) The clay has constricted the pore throat pseudomorphs after detrital mica, usually muscovite and chlorite It should be noted that this kaolinite forms particles far larger than the mm maximum defined for clay particles Pore-filling authigenic kaolinite is often linked to the dissolution of feldspar at temperatures of less than 100 C Although it is commonly assumed that a high aHỵ is necessary for significant feldspar and mica leaching, the stability field of kaolinite extends to the greatest range of [Kỵ]/[Hỵ] values at a pH close to neutral It should be noted that, for significant kaolinite to precipitate, Kỵ and Naỵ from feldspar and mica need to be removed, or illite rather than kaolinite will become the stable clay mineral The implication is that kaolinite will form at higher fluid flow rates (or a higher water/ rock ratio) than illite, i.e., in the most porous parts