Serpentinite is a rock composed predominantly of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Serpentinite has been called serpentine or serpentine rock, particularly in older geological texts and in wider cultural settings. Formation and mineralogy Serpentinite is formed by near to complete serpentinization of mafic to ultramafic rocks. Serpentinite can be formed wherever ultramafic rock is infiltrated by water poor in carbon dioxide. This occurs at mid-ocean ridges and in the forearc mantle of subduction zones. The final mineral composition of serpentinite is usually dominated by lizardite, chrysotile (two minerals of the serpentine subgroup), and magnetite (). Brucite () and antigorite are less commonly present. Lizardite, chrysotile, and antigorite all have approximately the formula or , but differ in minor components and in form. Accessory minerals, present in small quantities, include awaruite, other native metal minerals, and sulfide minerals. Hydrogen production The serpentinization reaction involving the transformation of fayalite (Fe-end member of olivine) by water into magnetite and quartz also produces molecular hydrogen according to the following reaction: This reaction closely resembles the Schikorr reaction also producing hydrogen gas by oxidation of Fe ions into Fe ions by the protons of water. Two are then reduced into . In the Schikorr reaction, the two reduced into are these from two anions, then transformed into two oxide anions () directly incorporated into the magnetite crystal lattice while the water in excess is liberated as a reaction by-product. Hydrogen produced by the serpentinization reaction is important because it can fuel microbial activity in the deep subsurface environment. Hydrothermal vents and mud volcanoes Deep sea hydrothermal vents located on serpentinite close to the axis of mid-ocean ridges generally resemble black smokers located on basalt, but emit complex hydrocarbon molecules. The Rainbow field of the Mid-Atlantic Ridge is an example of such hydrothermal vents. Serpentinization alone cannot provide the heat supply for these vents, which must be driven mostly by magmatism. However, the Lost City Hydrothermal Field, located off the axis of the Mid-Atlantic Ridge, may be driven solely by heat of serpentinization. Its vents are unlike black smokers, emitting relatively cool fluids () that are highly alkaline, high in magnesium, and low in hydrogen sulfide. The vents build up very large chimneys, up to in height, composed of carbonate minerals and brucite. Lush microbial