Abstract

A newly discovered deposit of aluminum sulfate minerals within kaolinite claystones of the Middle Jurassic Inmar Formation at Makhtesh Ramon is described.

The sulfate minerals as determined by X-ray diffraction, chemical analysis, and differential thermal analysis are predominately members of the alunite-natroalunite solid-solution series with subordinate jarosite (potassium-iron sulfate) and minor amounts of woodhouseite [CaAl3(PO4)(SO4)(OH)6]. Three distinct modes of occurrence were recognized for the sulfate minerals: (1) thin beds, 1 to 8 cm thick and laterally extensive, (2) pods concentrated in a horizon parallel to bedding and (3) fine disseminations within the claystone.

The host sequence is composed predominantly of kaolinite clay-stone with minor amounts of carbonaceous shale, flint clay, siltstone, and massive interbeds of quartzarenite. Two distinct facies, whose origins are related to the genesis of the sulfates, have been defined within the sequence. They are the coaly facies, characterized by organic-rich shaly claystone, and the flint-clay facies, composed of massive, homogeneous, and brecciated flint clay.

The pure well-ordered kaolinite of the claystone in the coaly facies has mineralogic properties similar to those of the flint-clay facies; both facies were probably formed by similar processes. In contrast to this, the other claystone parts of the host sequence contain higher quartz concentrations, disordered kaolinite, and minor amounts of illite.

Inasmuch as the sulfate mineralization occurred in both the flint-clay facies and coaly facies, their origin is postulated as being nonhydrothermal and closely related to the environmental conditions that accompanied deposition. The interpretation of an early diagenetic origin for the Na-alunite is based on the nature of textural fabrics within the alunite pods that were caused by segregation of a kaolinite + epiclastic quartz assemblage from a relatively homogeneous alunite assemblage. Fractionation into a kaolinite + quartz assemblage and an alunite assemblage could only have occurred before compaction, probably by a process of colloidal segregation during stages of dewatering.

The mineralogic characteristics of claystone within the flint-clay facies and coaly facies developed mainly in situ within swampy depressions by a process of upward leaching such as that proposed by Keller and others for the origin of Missouri flint clay. Furthermore, authigenic pyrite formed in response to an environment of negative Eh and was subsequently oxidized to H2SO4 by subaerial exposure of the swamp sediments. The diagenetic reorganization of colloidal clays by reaction with the sulfuric acid is considered to be the dominant process in the formation of the alunite.

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