The behavior of rock decomposition products has been studied by many authors throughout the country. These works have great importance for the geotechnical engineering, mainly in tropical regions, with hot and rainy climates, like it is found in Brazil, since atmospheric conditions have a great influence in the soil formation originated from the mother rock. Therefore the detailed knowledge of the soil origin provides a better understanding of its geomechanical behavior. This work deals with a physical- chemicalmineralogical and micromorphological study of arbitrary soil points on the Tanguá alkaline complex, at the Tanguá Municipality (Rio de Janeiro State, Brazil). The main subject of the present work is to analyze effects of weathering on the physical-chemical- mineralogical and structural characteristics of the residual soil derived from the alkaline rock found in Tanguá, as well as on some simple hydro-mechanical characteristics of the material. It`s important to note here that works on this type of rock and/or soil are rarely found in the literature. Thus, this work contributes towards a better understanding on the behaviour of residual soils as a whole.This study has been done within the context of the PRONEX-Rio project named Geotechnics Applied to the Evaluation, Prevention and Remediation of Environmental Problems, under development within the Environmental Geotechnics Group of DEC/PUC-Rio.