Pressure, temperature, and the availability of the most abundant chemical components are the parameters of the geochemical environment that determine which mineral phases are stable. On the basis of these variables, it is possible to classify all the natural geochemical environments of the Earth into two major groups--primary and secondary. The primary environment extends downward from the lower levels of circulating meteoric water to the deepest level of the crust and may extend into the mantle. It is an environment of high temperature and pressure, restricted circulation of fluids, and relatively low free-oxygen content. The secondary environment is the environment of weathering, erosion, and sedimentation at the surface of the Earth. It is characterized by low temperatures, nearly constant low pressure, free movement of solutions, and abundant free oxygen, water, and carbon dioxide.