Serpentinite is a rock composed of one or more
serpentine minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramaficrock from the Earth's mantle. The alteration is particularly important at the sea floor at tectonic plate boundaries. It is the state rock of California, USA.
Serpentinization is a geological low-temperature metamorphic process involving heat and water in which low-
silica maficand ultramaficrocks are oxidized and hydrolyzedwith water into serpentinite. Peridotite, including dunite, at and near the seafloor and in mountain belts is converted to serpentine, brucite, magnetite, and other minerals -- some rare, such as awaruite (Ni3Fe), and even native iron. In the process large amounts of water are absorbed into the rock increasing the volume and destroying the structure.
The density changes from 3.3 to 2.7 g/cm3 with a concurrent volume increase of about 40%. The reaction is exothermic and large amounts of heat energy are produced in the process.
Rock temperatures can be raised by about 260 oC, providing an energy source for formation of non-volcanic
hydrothermal vents. The magnetite-forming chemical reactions produce hydrogengas. Sulfates and carbonates are reduced and form methaneand hydrogen sulfide. The hydrogen, methane, and hydrogen sulfide provide energy sources for deep sea chemotroph microorganisms.
Serpentinite is formed from
olivinevia several reactions, some of which are complementary. Olivine is a solid solutionbetween the magnesium-endmember forsteriteand the iron-endmember fayalite. Serpentinite reactions 1a and 1b, below, exchange silica between forsterite and fayalite to form serpentine-group minerals and magnetite. These are highly exothermic reactions.
"Fayalite + water → Magnetite + aqueous silica + hydrogen":
"Forsterite + aqueous silica → Serpentine":
Other possibilities include the reaction of olivine plus water to yield serpentine plus Mg(OH)2 (
brucite), and reactions in which magnesium and silica are transported out of the serpentinizing volume.
A similar suite of reactions involves
pyroxene-group minerals, though less readily and with complication of the additional end-products due to the wider compositions of pyroxene and pyroxene-olivine mixes. Talcand magnesian chloriteare possible products, together with the serpentine minerals antigorite, lizardite, and chrysotile. The final mineralogy depends both on rock and fluid compositions, temperature, and pressure. Antigorite forms in reactions at temperatures that can exceeed 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. Fluids involved in serpentinite formation commonly are highly reactive and may transport calciumand other elements into surrounding rocks; fluid reaction with these rocks may create metasomatic reaction zones enriched in calcium and called rodingites.
In the presence of carbon dioxide, however, serpentinitization may form either magnesite (MgCO3) or generate methane (CH4). It is thought that some hydrocarbon gases may be produced by serpentinite reactions within the oceanic crust, and the serpentinite reaction is a key argument for the theory of
abiogenic petroleum origin.
Reaction 2a::"Olivine + Water + Carbonic acid → Serpentine + Magnetite + Methane "
or, in balanced form: →
Reaction 2b::"Olivine + Water + Carbonic acid → Serpentine + Magnetite + Magnesite + Silica "
Reaction 2a is favored if the serpentinite is Mg-poor or if there isn't enough carbon dioxide to promote
talcformation. Reaction 2b is favored in highly magnesian compositions and low partial pressure of carbon dioxide. (Something is missing from Reaction 2b because Fe is oxidized and nothing gets reduced.)
The degree to which a mass of ultramafic rock undergoes serpentinisation depends on the starting rock composition and on whether or not fluids transport
calcium, magnesiumand other elements away during the process. If an olivine composition contains sufficient fayalite, then olivine plus water can completely metamorphose to serpentine and magnetite in a closed system. In most ultramafic rocks formed in the Earth's mantle, however, the olivine is about 90% forsterite endmember, and for that olivine to react completely to serpentine, magnesium must be transported out of the reacting volume.
Serpentinitization of a mass of peridotite usually destroys all previous textural evidence because the serpentine minerals are weak and behave in a very ductile fashion. However, some masses of serpentinite are less severely deformed, as evidenced by the apparent preservation of textures inherited from the peridotite, and the serpentinites may have behaved in a rigid fashion.
Serpentinite has been proposed as an efficient reagent for
carbon sequestrationusing the magnesite reaction, above, or a variation where serpentine is reacted with carbon dioxde and hydrogen to form magnesite, magnetite, silica. The ideal composition of olivine or serpentinite for this process is thus highly magnesian, to favor production of magnesite and the fixation of carbon.
Bleistein or Ofenstein
A lamelled variety of serpentinite is found in South Tyrol and is locally called Bleistein (Leadstone) or Ofenstein (Ovenstone). It was used primarily for building wood burning indoor heating stoves as it has the capacity of storing and dispersing heat very well along the veins. A New York educational documentary film production company [http://www.vinestreetworks.com] is releasing a film that describes the quarrying of this stone and its use in the construction of stoves.
Lost City (hydrothermal field)
Life on Mars
* [http://www.zeca.org/process_overview/process_overview.html ZECA Corporation serpentinite carbon sequestration process]
* [http://www.lostcity.washington.edu/science/chemistry/serpentinization.html Serpentinization: The Heat Engine at Lost City and Sponge of the Oceanic Crust]
* [http://www.pnas.org/cgi/content/full/101/35/12818 H2-rich fluids from serpentinization: Geochemical and biotic implications]
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