Petrochemical and mineralogical characteristics of fresh basalt and its saprolite from Touzhai landslide in Zhaotong, Yunnan Province, are described in terms of chemical analyses, optical observations of thin sections and SEM (scanning electron microscopic) observations. Chemical analysis showed that the fresh basalts and corestones slightly changed in composition except that variations in Fe2+ to Fe3+ are most noticeable. In the course of corestones changing to saprolite elements of Si, Ca, Na and Mg, the ratio of SiO2/Al2O3 decreased gradually, while the elements of Fe, Al and the values of LOI and CIA significantly increased. The weathering front under SEMs is tortuous, and physical weathering cracks which are of easier emergence along the cleavage plane can be observed in the vicinity of the weathering front. The major minerals follows the weathering order of dissolution of glass, pyroxene and feldspar, which is in agreement with the sequence of weathering-resistant minerals. The main secondary minerals are smectite and chlorite. Rock coupling chemical weathering with physical weathering processes not only exists on a macro-scale, but also on a meso- and micro-scale. Changes in the valence of iron took place in the whole formation process of basalt saprolite, thus leading to a volume increase, giving rise to physical cracks.
