Surface textures on pyrope, picroilmenite and chrome diopside from Kimberlite

Abstract

The examination and interpretation of surface features on pyrope, picroilmenite and chrome diopside from kimberlite gives valuable information on the history of these minerals both during and after kimberlite emplacement. Descriptions and scanning electron photo-micrographs of surface features observed on these mantle-derived minerals from thirty-four kimberlites having a global distribution are presented. Careful observations established the age relationships between the different surface features on each of the minerals studied, and enabled the compilation of a classification scheme of these surface features. This proposed classification scheme is based on the chronological sequence of formation of th.e surface morphological characteristics observed on these mantle-derived minerals and separates thoie which are formed during kimberlite emplacement (primary features), from those resulting ffom in-situ weathering and diagenetic etch processes (secondary features). The mineralogy, structure and mode of formation of kelyphite are discussed in relation to kimberlite emplacement. Pyrope encrusted by kelyphite is commonly found in most garnet peridotites, some eclogite xenoliths from kimberlite, as well as a few garnet-bearing serpentinites. Although the relative abundance of kelyphite-encrusted garnet varies from one kimberlite to another, kelyphite shells are most commonly developed on mauve garnets which are most probably derived from peridotite and are least commonly found on orange varieties from eclogites.