Spheroidal weathering of basalt from Gebel Qatrani, Fayum Depression, Egypt

Abstract

The core-stones of the basalt spheroids are composed of phenocrystals of plagioclase feldspars and clinopyroxenes set in a groundmass of tiny feldspars and pyroxenes, relatively altered olivine and opaque minerals. The basalt is subalkali (tholeiitic). The outer weathered shells surrounding the core-stones are composed of partly altered feldspars and pyroxenes. The calculated weathering indices show that there is marked weathering trend from the core-stones of the spheroids to the outer shells. The chemical mobility of the elements shows marked depletion of Mg, Ca, Na and K from the core-stones to the outer shells due to the weathering of olivine, pyroxene and feldspars. The trace elements Rb, Sr, Ni, V, Cr and Zn are also depleted.