Abstract

Weathering of chlorite adjacent to macro- and micro-passages (fissures) in regolith units has been studied to determine the alteration mechanisms and products and compare them to those that prevail at a distance from the open passages. The micro-fissures in the saprolith units evolve as a function of weathering with an increase in micro-fissure density and their infilling with ferriargillans corresponding to an increase in weathering intensity. In the saprock and saprolite units, goethite-rich bands with surrounding reddish haloes invade the chlorite adjoining the micro-fissures. In the reddish haloes surrounding the goethite bands and fissures, the phyllosilicates alter to an intimate mixture of goethite, kaolinite, feroxyhyte and halloysite with the possible nanoscale presence of other fine-grained iron aluminosilicates such as hisingerite and smectite. In the fine saprolite, the weathering products after original chlorite adjoining the micro-fissures bearing ferriargillans, alter directly to ferriargillan products – kaolinite and goethite. The lack of orientation of the products with the parent phyllosilicates indicates the operation of a dissolution-precipitation mechanism which is in contrast to the topotactic alteration mechanism functioning at a distance within the phyllosilicate grain assemblage. The differences in alteration mechanisms and products of chlorite weathering in different microsites suggest the rate of weathering of chlorite can differ in microsites within individual regolith units. The presence of fine-grained metastable products in the form of feroxyhyte and halloysite adjoining the fissures suggests an Ostwald Step Rule sequence during alteration of phyllosilicates with rapid oxidation of Fe and the presence of Si in the microsite considered the main factor favoring fine-grained metastable products.

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