Alteration in geochemistry refers to processes by which the mineralogy, composition, and texture of a rock are changed as a result of re-equilibration under conditions of lower temperature and pressure or through interaction with aqueous or CO2-rich fluids. The minerals of the original rock, which may be magmatic, sedimentary, or metamorphic, are transformed into an assemblage of low-temperature, usually finer-grained minerals. A typical example is the replacement of magmatic minerals such as olivine, pyroxene, and feldspar by chlorite, clay minerals, or carbonates. Weathering is a type of alteration that takes place close to the surface through interaction of rock with the atmosphere and with ground- or surface waters. Alteration is also used in chemistry and biology (e.g., DNA alterations).