--> Illite Crystal Morphology as Indicator of Diagenetic Environment, by J. Reed Glasmann; #91024 (1989)

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Illite Crystal Morphology as Indicator of Diagenetic Environment

J. Reed Glasmann

The morphology of diagenetic illite crystals is indicative of the environment of diagenesis and major growth mechanisms. Illite in the < 0.1-µm fraction of shales and sandstones from a variety of geologic settings was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), x-ray diffraction (XRD), and K-Ar analyses. Euhedral lath-shaped illite is very common in sandstones in which illite precipitates from solution and crystal growth is unconstrained by pore size. Length-to-width ratios of diagenetic illite laths commonly exceed 60:1, and particle thicknesses generally range from 2 to 8 nm. When illite crystal growth is restricted by pore size, such as when illite precipitates in micropores within masses of pore-filling kaoli ite, anhedral illite results. Lath-shaped illite (length-to-width ratio < 20:1) is also abundant in shallowly buried (< 1 km) hydrothermally altered Pleistocene shales from Imperial Valley, California, and in other shallow hydrothermal environments where illite precipitated from advecting pore fluids and crystal growth occurred in shale mesopores (pores > 5 µm in diameter). In contrast, the dominance of anhedral illite in deeply buried Gulf Coast shales indicates that illite growth occurred after compaction had destroyed shale mesoporosity. Crystal growth was probably controlled by diffusion, and crystal habit was influenced by shale micropore geometry. The abundance of euhedral illite laths in Mesozoic shales in the northern North Sea suggests that diagenesis occurred prio to major loss of porosity associated with deep Tertiary burial (2-4 km) and is consistent with measured K-Ar ages suggesting Maastrichtian to early Paleogene diagenesis.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.