Specific features of structure and composition of the weathering crust of kimberlite rocks

This article presents the regularities revealed by complex study of mineralogical-geochemical alteration of kimberlites in hypergenetic conditions. In the most developed weathering crust strata, the significant transformation is observed for rock-forming minerals and for stable primary minerals as well. The content of olivine, pyroxene, pyrope sufficiently decreases at the upper part of weathered crust profiles, but the role of picroilmenite increases. Represented by layers of structural type A and B serpentine dominates in lower part of the weathering profiles. The rounded, globular grains are characteristic for the layers of primarily formed type A, which are also observed on grains of other more stable minerals. At early stages of kimberlites' weathering, alteration of serpentine is related to its recrystallization accompanied by polytypic transformation. At the same time, the laminated minerals of trioctahedral type either decompose (Mg- and Fe-Mg-chlorites), or partially covert into dioctahedral forms (phlogopite to hydromica, etc). These processes causes the significant increase of porosity and permeability of primary rocks. It leads to significant increase of surface open for contacting to pneumatolytic-hydrothermal fluids and weathered horizons drainage intensity. Mineral composition of secondary forms and distribution of pH and Eh values at eluvial profiles suggest that this stage of kimberlites' weathering corresponds to an alkaline type. Eluvi-al profiles discussed in the article may be referred to the type of the residual local weathering crusts at the stage of initial hydration of primary minerals and leaching of the less stable components.