The Semilir Formation in southern Central Java, consisting of various volcanic rocks with a thickness of over 460 m, has not been previously studied for soil mineralogy and engineering properties. Four soil samples from this formation, representing different weathering stages, were analyzed using X-ray diffraction to identify clay minerals and assess engineering properties like specific gravity, particle gradation, and shear strength. The results showed that clay minerals like saponite and metahalloysite were present in most samples, except Ponorogo, which lacked clay minerals, indicating it was still in the physical weathering stage. Ponorogo soil had higher specific gravity and the lowest cohesion value due to its non-plastic characteristics and absence of clay minerals.