This chapter, like Churchman (2000), seeks to bring readers up to date with information and understanding about the alteration of minerals and the nature of their products in the context of the formation and development of soils. It complements various articles by Bergaya et al. (2006), and the recent books by Velde and Meunier (2008) and Velde and Barré (2010). This current chapter differs from Churchman (2000) in that it discusses the manner in which minerals, and especially secondary minerals, actually occur in soils, i.e., it deals (in Section 20.3) with the occurrence, as well as the alteration and formation of minerals in soils.