Magnesium (Mg) is one of the eight main elements of Earth’s crust and one of the four major elements making up the mass of the whole Earth. Mg is one of the main constituents of silicate minerals that build up Earth, like olivine, pyroxenes, serpentine, talc, phyllosilicates (clay minerals), etc. The concentration of divalent magnesium (Mg2+) in ocean water is 52.8 mmol/kg. The intra- and extracellular concentration in organisms, on the other hand, is about 1 mmol/kg. The coordination geometry of magnesium is strictly octahedral, i.e. the Mg atom coordinates six atoms – almost always oxygen – around itself in its first coordination shell. In the marine geochemical environment magnesium is particularly important since the tri-octahedral layer of the common phyllosilicates in sediments consist primarily of brucite, the mineral name of magnesium hydroxide (Mg(OH)2). The ocean floor beneath the sediment layers consists of basalts and ultramafic rocks that have relatively low silica contents (45-52% and <45%, respectively) but have a high content of primary ferromagnesian silicate minerals (olivine and pyroxenes). Alteration of these minerals in contact with water leads to ‘serpentinization’, a process in which olivine and pyroxenes are transformed to serpentine. The serpentine cannot accommodate all of the magnesium of the primary minerals, so dissolved magnesium is abundant in serpentinization environments and the mineral brucite is formed as a separate mineral phase at temperatures below about 315°C. Brucite may be transformed into double-layer hydroxides (DLH) if a fraction of the divalent Mg2+ is replaced by common trivalent cations such as Al3+, Fe3+, and Cr3+. Polyphosphate generation often has an absolute requirement for divalent metal ions, typically Mg(II). It has been shown by several investigators that magnesium pyrophosphate (MgPPi) is easily formed under mild abiotic hydrothermal conditions (165-180°C) from magnesium salts and orthophosphate (Pi). The Mg2+ is an essential catalyst for many biochemical reactions. It is well known that Mg2+ is required for the stabilization of the diphosphate group of ADP and the triphosphate group of ATP. The reason is that Mg2+ forms six-membered rings with the oxygen and phosphorus atoms of ADP and ATP.