The chemical and mineralogical compositions of sediments and sedimentary rocks are extremely diverse. Sediments may be classified according to the origin of the minerals composing the bulk sediment, with end-members being referred to as terrigenous, biogenic or metalliferous sediment. Terrigenous sediments results from the processes of physical/chemical weathering and erosion acting upon upper crustal rocks exposed at the surface of the Earth. These processes give rise to fundamental changes in mineralogy and chemistry because most primary igneous and metamorphic minerals, originally formed and equilibrated at elevated pressures and temperatures, are thermodynamically unstable in the presence of most natural surface waters. Transport, either as particles or in solution, and deposition act to further alter the composition of sediments.