Authigenic clay minerals in the Nubia Sandstone of Kharga Oasis (Western Desert, Egypt) as revealed by scanning electron microscopy
Abu-Zeid, Mohamed M.
published: Aug 1, 1982
ArtNo. ESP154014402005, Price: 29.00 €
The use of the scanning electron microscope with non-dispersive elemental analyzer reveals that authigenic kaolinite, illite, and chlorite constitute a considerable portion of the cement in many parts of the Nubia Sandstone of Kharga Oasis. These minerals occur in the interstices between detrital sand grains, attached to their surfaces, in cracks, and as a product of alteration of feldspar grains and recrystallization of allogenic clay minerals. Kaolinite mostly forms aggregates of tightly-packed, pseudohexagonal flakes which are stacked in a face-to-face arrangement. Illite most commonly develops as packets of delicate sheets whose basal surfaces adjoin one another and their irregular edges may show a slight tendency to curl. Chlorite forms aggregates similar to those of kaolinite but the crystals are larger, though thinner. In few cases, an acicular variety of chlorite is reported fringing the sand grains or attached to their surfaces. Criteria for an authigenic origin which were observed include: a) Obvious crystal outlines, b) Delicacy of morphology, c) Undeformation, d) Pseudomorphous replacement, e) Large particle size, f) Fracture filling, g) Attachment to detrital grain surfaces and absence at grain contacts, h) Coverage of diagenetic components formed at earlier stages. The geochemical processes and main factors involved in the formation of the authigenic clay minerals are discussed.