Abstract

Mid to late Cretaceous arenites and calcarenites from Northeastern Algeria were found to contain chlorites, frequently in globular form, usually expressed as shell fillings. Although the present polymorph is a 14 A type, it is believed that this material originated as test fillings in a shallow water marine environment (less than 200 m. deep). The original polymorph being 7 A, the polymorphic transition could have occurred from 25 degrees C to somewhat above 100 degrees C. This conclusion is based on phase changes deduced from the clay mineralogy in shales of the same stratigraphic series and from the presence of authigenic albite and quartz in the chlorite-bearing rocks. Microprobe analysis of six chlorite-bearing rock samples indicates a rather homogeneous chemical composition for different grains in the same rock, and little variation between the six rock samples. The chlorites are aluminous (near 20% Al 2 O 3 ) and contain about 50% Fe atoms in octahedrally coordinated sites. The ratio Fe/Fe + Mg is near 0.8.

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