Weathering of chlorite: reactions and products in microsystems controlled by the primary mineral

Abstract

The weathering of chlorite in hydrothermally-altered basalt was studied with XRD, TEM and electron microprobe to determine the type and orientation of secondary minerals. Optical examination indicated chlorite assemblages to have altered in two distinct microsites: one microsite near micro-fissures traversing the regolith units, and the other away from the continuous passages. In this paper, weathering mechanisms and products of chlorite present in microsites distant from the micro-fissures are reported. In all the regolith units the original chlorite grain remained intact and was pseudomorphed by secondary products. In the saprock, chlorite altered to corrensite with possible random interstratifications of chlorite and corrensite and corrensite and vermiculite. In the saprolite, corrensite altered to vermiculite. Parallelism of two axes of the products with the host indicated topotactic alteration. In the fine saprolite, vermiculite was found to alter to kaolinite via a randomly interstratified kaolinite-vermiculite stage with a high proportion of kaolinite. Goethite crystallized in between packets of kaolinite, vermiculite and kaolinite-vermiculite. Though the disruption of the crystal structure of vermiculite is necessary in its alteration to kaolinite, the reaction was such as to maintain parallelism of the c axis. The alteration of chlorite to vermiculite was characterized by the loss of Mg and Fe and minor A1, all ions considered to be lost from the brucite-like sheet of chlorite. The Fe released during the alteration of vermiculite to kaolinite is likely to have migrated to micropores to form goethite. The presence of interstratifications of the end-members of layer silicates involved in the reaction sequence suggests that interstratifications are common during layer silicate weathering in environments where space is limited and consequently solution and ionic transport passages are restrictive.