8.5 SANDSTONES
369
Fig. 8.27. Scanning electron micrographs illustrating different authigenic minerals in sandstone. (A) Authigenic quartz crystals encrusted by later kaolinite "accordian" crystals. (B) Fibrous crystals of illite, a major permeability diminisher. (C) Montmorillonite (smectite), the dreaded swelling clay whose lattice can absorb water. (D) Authigenic dolomite rhombs, with platey chlorite clay cement. (Courtesy of J. Huggett and Petroclays.)
sands tend to be subcommercial, and kaolinite-cemented ones economic (Stadler, 1973).
Thus it is important to be able to predict the distribution of authigenic kaolinite and
illite. Because of its stability in low pH waters, kaolinite tends to be deposited in continental environments; it may be converted to illite during diagenesis in the presence of
alkaline connate waters.
Illite, by contrast, is more commonly the detrital clay found in marine deposits. Recrystallization of illite from detrital flakes to furry coatings may take place in alkaline
connate waters. Conversely, kaolinite may form authigenically either from the breakdown of detrital grains, such as feldspar, or by the recrystallization of illite fibers to
369
Fig. 8.27. Scanning electron micrographs illustrating different authigenic minerals in sandstone. (A) Authigenic quartz crystals encrusted by later kaolinite "accordian" crystals. (B) Fibrous crystals of illite, a major permeability diminisher. (C) Montmorillonite (smectite), the dreaded swelling clay whose lattice can absorb water. (D) Authigenic dolomite rhombs, with platey chlorite clay cement. (Courtesy of J. Huggett and Petroclays.)
sands tend to be subcommercial, and kaolinite-cemented ones economic (Stadler, 1973).
Thus it is important to be able to predict the distribution of authigenic kaolinite and
illite. Because of its stability in low pH waters, kaolinite tends to be deposited in continental environments; it may be converted to illite during diagenesis in the presence of
alkaline connate waters.
Illite, by contrast, is more commonly the detrital clay found in marine deposits. Recrystallization of illite from detrital flakes to furry coatings may take place in alkaline
connate waters. Conversely, kaolinite may form authigenically either from the breakdown of detrital grains, such as feldspar, or by the recrystallization of illite fibers to