Serpentinization is a hydration and metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the serpentine group minerals (antigorite, lizardite, chrysotile), brucite, talc, Ni-Fe alloys, and magnetite. The mineral alteration is particularly important at the sea floor at tectonic plate boundaries.

Serpentinization is a form of low-temperature (0 to ~600 °C) metamorphism of ferromagnesian minerals in mafic and ultramafic rocks, such as dunite, harzburgite, or lherzolite. These are rocks low in silica and composed mostly of olivine ((Mg²⁺, Fe²⁺)₂SiO₄), pyroxene (XY(Si,Al)₂O₆), and chromite (approximately FeCr₂O₄). Serpentinization is driven largely by hydration and oxidation of olivine and pyroxene to serpentine group minerals (antigorite, lizardite, and chrysotile), brucite (Mg(OH)₂), talc (Mg₃Si₄O₁₀(OH)₂), and magnetite (Fe₃O₄). Under the unusual chemical conditions accompanying serpentinization, water is the oxidizing agent, and is itself reduced to hydrogen, H
₂. This leads to further reactions that produce rare iron group native element minerals, such as awaruite (Ni
₃Fe) and native iron; methane and other hydrocarbon compounds; and hydrogen sulfide.

During serpentinization, large amounts of water are absorbed into the rock, increasing the volume, reducing the density and destroying the original structure. The density changes from 3.3 to 2.5 g/cm³ (0.119 to 0.090 lb/cu in) with a concurrent volume increase on the order of 30-40%. The reaction is highly exothermic, releasing up to 40 kilojoules (9.6 kcal) per mole of water reacting with the rock, and rock temperatures can be raised by about 260 °C (500 °F), providing an energy source for formation of non-volcanic hydrothermal vents. The hydrogen, methane, and hydrogen sulfide produced during serpentinization are released at these vents and provide energy sources for deep sea chemotroph microorganisms.

Olivine is a solid solution of forsterite, the magnesium endmember of (Mg²⁺, Fe²⁺)₂SiO₄, and fayalite, the iron endmember, with forsterite typically making up about 90% of the olivine in ultramafic rocks. Serpentine can form from olivine via several reactions:

Reaction 1a tightly binds silica, lowering its chemical activity to the lowest values seen in common rocks of the Earth's crust. Serpentinization then continues through the hydration of olivine to yield serpentine and brucite (Reaction 1b). The mixture of brucite and serpentine formed by Reaction 1b has the lowest silica activity in the serpentinite, so that the brucite phase is very important in understanding serpentinization. However, the brucite is often blended in with the serpentine such that it is difficult to identify except with X-ray diffraction, and it is easily altered under surface weathering conditions.

A similar suite of reactions involves pyroxene-group minerals:

Reaction 2a quickly comes to a halt as silica becomes unavailable, and Reaction 2b takes over. When olivine is abundant, silica activity drops low enough that talc begins to react with olivine:

This reaction requires higher temperatures than those at which brucite forms.