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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. Minerals in this group, which are rich in magnesium and water, light to dark green, greasy looking and slippery feeling, are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle. The mineral alteration is particularly important at the sea floor at tectonic plate boundaries. Serpentinite has been called serpentine or serpentine rock, particularly in older geological texts and in wider cultural settings.

Formation and petrology
Serpentinization is a form of low-temperature metamorphism of ultramafic rocks, such as dunite, harzburgite, or lherzolite. These are rocks low in silica and composed mostly of olivine ((Mg(2+), Fe(2+))2SiO4), pyroxene (XY(Si,Al)2O6), and chromite (approximately FeCr2O4). Serpentinization is driven largely by hydration and oxidation of olivine and pyroxene to serpentine minerals, brucite (Mg(OH)2), and magnetite (Fe3O4). Under the unusual chemical conditions accompanying serpentinization, water is the oxidizing agent, and is itself reduced to hydrogen,. This may lead to further reactions that produce rare iron group native element minerals, such as awaruite 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 with a concurrent volume increase on the order of 30-40%. The reaction is highly exothermic and rock temperatures can be raised by about 260 Celsius, 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.

The final mineral composition of serpentinite is usually dominated by lizardite, chrysotile, and magnetite. Brucite and antigorite are less commonly present. Lizardite, chrysotile, and antigorite all have approximately the formula Mg3(Si2O5)(OH)4 or (Mg(2+), Fe(2+))3Si2O5(OH)4, but differ in minor components and in form "crystal habit". Accessory minerals, present in small quantities, include awaruite, other native metal minerals, and sulfide minerals.

Formation of serpentine minerals
Olivine is a solid solution of forsterite, the magnesium endmember of (Mg(2+), Fe(2+))2SiO4, and fayalite, the iron endmember, with forsterite typically making up about 90% of the olivine in ultramafic rocks. Serpentinite 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.

The final mineralogy depends both on rock and fluid compositions, temperature, and pressure. Antigorite forms in reactions at temperatures that can exceed 600 C during metamorphism, and it is the serpentine group mineral stable at the highest temperatures. Lizardite and chrysotile can form at low temperatures very near the Earth's surface.

Breakdown of diopside and formation of rodingites
Ultramafic rocks often contain calcium-rich pyroxene (diopside), which breaks down according to the reaction

This raises both the pH, often to very high values, and the calcium content of the fluids involved in serpentinization. These fluids are highly reactive and may transport calcium and other elements into surrounding mafic rocks. Fluid reaction with these rocks may create metasomatic reaction zones enriched in calcium and depleted in silica, called rodingites.

Formation of magnetite and hydrogen
In most crustal rock, the chemical activity of oxygen is prevented from dropping to very low values by the fayalite-magnetite-quartz (FMQ) buffer. The very low chemical activity of silica during serpentinization eliminates this buffer, allowing serpentinization to produce highly reducing conditions. Under these conditions, water is capable of oxidizing ferrous ions in fayalite. The process is of interest because it generates hydrogen gas:

However, studies of serpentinites suggest that iron minerals are first converted to ferroan brucite, that is, brucite containing Fe(OH)2, which then undergoes the Schikorr reaction in the anaerobic conditions of serpentinization:

Maximum reducing conditions, and the maximum rate of production of hydrogen, occur when the temperature of serpentinization is between 200 and 315 C. If the original ultramafic rock (the protolith) is peridotite, which is rich in olivine, considerable magnetite and hydrogen are produced. When the protolith is pyroxenite, which contains more pyroxene than olivine, iron-rich talc is produced with no magnetite and only modest hydrogen production. Infiltration of silica-bearing fluids during serpentinization can suppress both the formation of brucite and the subsequent production of hydrogen.

Chromite present in the protolith will be altered to chromium-rich magnetite at lower serpentinization temperatures. At higher temperatures, it will be altered to iron-rich chromite (ferrit-chromite). During serpentinization, the rock is enriched in chlorine, boron, fluorine, and sulfur. Sulfur will be reduce to hydrogen sulfide and sulfide minerals, though significant quantities are incorporated into serpentine minerals, and some may later be reoxidized to sulfate minerals such as anhydrite. The sulfides produced include nickel-rich sulfides, such as mackinawite.

Methane and other hydrocarbons
Laboratory experiments have confirmed that at a temperature of 300 C and pressure of 500 bars, olivine serpentinizes with release of hydrogen gas. In addition, methane and complex hydrocarbons are formed through reduction of carbon dioxide. The process may be catalyzed by magnetite formed during serpentinization. One reaction pathway is:

Metamorphism at higher pressure and temperature
Lizardite and chrysotile are stable at low temperatures and pressures, while antigorite is stable at higher temperatures and pressure. Its presence in a serpentinite indicates either that serpentinization took place at unusually high pressure and temperature or that the rock experienced higher grade metamorphism after serpentinization was complete.

Infiltration of CO2-bearing fluids into serpentinite causes distinctive talc-carbonate alteration. Brucite rapidly converts to magnesite and serpentine minerals (other than antigorite) are converted to talc. The presence of pseudomorphs of the original serpentinite minerals shows that this alteration takes place after serpentinization.

Serpentinite may contain chlorite (a phyllosilicate mineral), tremolite (Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2), and metamorphic olivine and diopside (calcium-rich pyroxene). This indicates that the serpentinite has been subject to more intense metamorphism, reaching the upper greenschist or amphibolite metamorphic facies.

Above about 450 C, antigorite begins to break down. Thus serpentinite does not exist at higher metamorphic facies.

Extraterrestrial production of methane by serpentinization
The presence of traces of methane in the atmosphere of Mars has been hypothesized to be a possible evidence for life on Mars if methane was produced by bacterial activity. Serpentinization has been proposed as an alternative non-biological source for the observed methane traces. In 2022 it was reported that microscopic examination of the ALH 84001 meteorite, which came from Mars, shows that indeed the organic matter it contains was formed by serpentinization, not by life processes.

Using data from the Cassini probe flybys obtained in 2010–12, scientists were able to confirm that Saturn's moon Enceladus likely has a liquid water ocean beneath its frozen surface. A model suggests that the ocean on Enceladus has an alkaline pH of 11–12. The high pH is interpreted to be a key consequence of serpentinization of chondritic rock, that leads to the generation of, a geochemical source of energy that can support both abiotic and biological synthesis of organic molecules.

Occurrence
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.

Conditions are highly favorable for serpentinization at slow to ultraslow spreading mid-ocean ridges. Here the rate of crustal extension is high compared with the volume of magmatism, bringing ultramafic mantle rock very close to the surface where fracturing allows seawater to infiltrate the rock.

Serpentinized ultramafic rock is found in many ophiolites. Ophiolites are fragments of oceanic lithosphere that has been thrust onto continents, a process called obduction. They typically consist of a layer of serpentinized harzburgite (sometimes called alpine peridotite in older writings), a layer of hydrothermally altered diabases and pillow basalts, and a layer of deep water sediments containing radiolarian ribbon chert.

Serpentinization is nearly total in the forearc mantle of subduction zones. Here mantle rock is cooled by the subducting slab to temperatures at which serpentinite is stable, and fluids are released from the subducting slab in great quantities into the ultramafic mantle rock. Direct evidence that serpentinization is taking place in the Mariana Islands island arc is provided by the activity of serpentinite mud volcanoes. Xenoliths of harzburgite and (less commonly) dunite are occasionally erupted by the mud volcanoes, giving clues to the nature of the protolith.

Because serpentinization increases the volume and lowers the density of the original rock, serpentinitization may lead to uplift that creates coastal ranges above mantle forearcs. Further uplift can bring serpentinite to the surface when subduction ceases, as has taken place with the serpentinite exposed at the Presidio of San Francisco.

Seismic wave studies can detect the presence of large bodies of serpentinite in the crust and upper mantle, since serpentinization lowers seismic wave velocities. This is particularly true of S waves, due to the high value of Poisson's ratio for serpentinites. Seismic measurements confirm that serpentinization is pervasive in forearc mantle. The serpentization can produce an inverted Moho discontinuity, in which seismic velocity abruptly decreases across the crust-mantle boundary, which is the opposite of the usual behavior. The serpentinite is highly deformable, creating an aseismic zone in the forearc, and the presence of serpentinite may limit the maximum depth of megathrust earthquakes. Serpentinization at slow spreading mid-ocean ridges can cause the seismic Moho discontinuity to be placed at the serpentinization front, rather than the base of the crust as defined by normal petrological criteria. The Lanzo Massif of the Italian Alps shows a sharp serpentinization front that may be a relict seismic Moho.

Notable occurrences of serpentinite are found at Thetford Mines, Quebec; Lake Valhalla, New Jersey; Gila County, Arizona; Lizard complex, Lizard Point, Cornwall; and in localities in Greece, Italy, and other parts of Europe. Notable ophiolites containing serpentinite include the Semail Ophiolite of Oman, the Troodos Ophiolite of Cyprus, the Newfoundland ophiolites, and the Main Ophiolite Belt of New Guinea.

Hydrothermal vents and mud volcanoes
The formation of serpentinite is highly exothermic, releasing up to 40 kJ per mole of water reacting with the rock. This corresponds to a release of about 660 MJ/m3 and can raise the rock temperature by about 260 Celsius, 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.

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 (40 to 75 C) that are highly alkaline, high in magnesium, and low in hydrogen sulfide. The vents build up very large chimneys, up to 60 m in height, composed of carbonate minerals and brucite. Lush microbial communities are associated with the vents. Though the vents themselves are not composed of serpentinite, they are hosted in serpentinite estimated to have formed at a temperature of about 200 C. Sepiolite deposits on mid-ocean ridges may have formed through serpentinite-driven hydrothermal activity. However, geologists continue to debate whether serpentinization alone can account for the heat flux from the Lost City field.

The forearc of the Marianas subduction zone hosts large serpentinite mud volcanoes, which erupt serpentinite mud that rises through faults from the underlying serpentinized forearc mantle. Study of these mud volcanoes gives insights into subduction processes, and the high pH fluids emitted at the volcanoes support a microbial community.

Serpentinite thermal vents are a candidate for the environment in which life on Earth originated. Most of the chemical reactions necessary to synthesize acetyl-CoA, essential to basic biochemical pathways of life, take place during serpentinization. The sulfide-metal clusters that activate many enzymes resemble sulfide minerals formed during serpentinization.

Experimental drilling into the gabbro layer of oceanic crust near mid-ocean ridges has demonstrated the presence of a sparse population of hydrocarbon-degrading bacteria. These may feed on hydrocarbons produced by serpentinization of the underlying ultramafic rock.

Ecology


Soil cover over serpentinite bedrock tends to be thin or absent. Soil with serpentine is poor in calcium and other major plant nutrients, but rich in elements toxic to plants such as chromium and nickel. Some species of plants, such as Clarkia franciscana and certain species of manzanita, are adapted to living on serpentinite outcrops. However, because serpentinite outcrops are few and isolated, their plant communities are ecological islands and these distinctive species are often highly endangered. On the other hand, plant communities adapted to living on the serpentine outcrops of New Caledonia resist displacement by introduced species that are poorly adapted to this environment.

Serpentine soils are widely distributed on Earth, in part mirroring the distribution of ophiolites. There are outcroppings of serpentine soils in the Balkan Peninsula, Turkey, the island of Cyprus, the Alps, Cuba, and New Caledonia. In North America, serpentine soils also are present in small but widely distributed areas on the eastern slope of the Appalachian Mountains in the eastern United States, and in the Pacific Ranges of Oregon and California.

Decorative stone in architecture and art


Serpentine group minerals have a Mohs hardness of 2.5 to 3.5, so serpentinite is easily carved. Grades of serpentinite higher in calcite, along with the verd antique (breccia form of serpentinite), have historically been used as decorative stones for their marble-like qualities. College Hall at the University of Pennsylvania, for example, is constructed out of serpentine. Popular sources in Europe before contact with the Americas were the mountainous Piedmont region of Italy and Larissa, Greece. Serpentinites are used in many ways in the arts and crafts. For example, the rock has been turned in Zöblitz in Saxony for several hundred years.

Carving stone tools, oil lamp-known as the Qulliq and Inuit sculpture
The Inuit and other indigenous people of the Arctic areas and less so of southern areas used the carved bowl shaped serpentinite qulliq or kudlik lamp with wick, to burn oil or fat to heat, make light and cook with. The Inuit made tools and more recently carvings of animals for commerce.

Swiss ovenstone
A variety of chlorite talc schist associated with Alpine serpentinite is found in Val d'Anniviers, Switzerland and was used for making "ovenstones" (Ofenstein), a carved stone base beneath a cast iron stove.

Neutron shield in nuclear reactors
Serpentinite has a significant amount of bound water, hence it contains abundant hydrogen atoms able to slow down neutrons by elastic collision (neutron thermalization process). Because of this serpentinite can be used as dry filler inside steel jackets in some designs of nuclear reactors. For example, in RBMK series, as at Chernobyl, it was used for top radiation shielding to protect operators from escaping neutrons. Serpentine can also be added as aggregate to special concrete used in nuclear reactor shielding to increase the concrete density (2.6 g/cm3) and its neutron capture cross section.

CO2 sequestration
Because it readily absorbs carbon dioxide, serpentinite may be of use for sequestering atmospheric carbon dioxide. To speed the reaction, the serpentinite may be reacted with carbon dioxide at elevated temperature in carbonation reactors. Carbon dioxide may also be reacted with alkaline mine waste from serpentine deposits, or carbon dioxide may be injected directly into underground serpentinite formations. Serpentinite may also be used as a source of magnesium in conjunction with electrolytic cells for CO2 scrubbing.

Cultural references
It is the state rock of California, USA and the California Legislature specified that serpentine was "the official State Rock and lithologic emblem." In 2010, a bill was introduced which would have removed serpentine's special status as state rock due to it potentially containing chrysotile asbestos. The bill met with resistance from some California geologists, who noted that the chrysotile present is not hazardous unless it is mobilized in the air as dust.