Serpentinite
Serpentinite is a metamorphic rock comprised of an admixture of
serpentine minerals. Minerals in this group are formed by serpentinization, a
metamorphic chemical alteration of the Earth's
mantle occurring at the sea floor at
tectonic plate boundaries. It is the state rock of
California,
USA.
Encyclopedia
Serpentinite is a metamorphic rock comprised of an admixture of
serpentine minerals. Minerals in this group are formed by serpentinization, a
metamorphic chemical alteration of the Earth's
mantle occurring at the sea floor at
tectonic plate boundaries. It is the state rock of
California,
USA.
Formation
Serpentinization is a geological
metamorphic process involving heat and water in which low-silica mafic and
ultramafic rocks are
oxidized and
hydrolyzed with water into serpentinite. The
peridotites and dunites of the seafloor are converted to
serpentine and
magnetite. 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/cm
3 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
hydrogen gas.
Sulfates and carbonates are reduced and form
methane and
hydrogen sulfide. The hydrogen, methane, and hydrogen sulfide provide energy sources for deep sea
chemotroph microorganisms.
Serpentinite reactions
Serpentinite is formed from
olivine via two reactions which are complementary. Olivine is composed of two parts of a
solid solution between magnesium-rich forsterite and iron rich 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.
Reaction 1a:
Fayalite + water ? Magnetite + aquaeous silica + hydrogenReaction 1b:
Forsterite + aqueous silica ? SerpentineA similar suite of reactions occurs with
pyroxene minerals, though less readily and with complication of the end-products due to the wider composition of pyroxene. Commonly,
talc, magnesian
chlorite,
chrysotile and
amphibolite are formed from pyroxene-rich rocks.
In the presence of carbon dioxide, however, serpentinitization may form either magnesite or generate methane . 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 Reaction 2b:
Olivine + Water + Carbonic acid ? Serpentine + Magnetite + Magnesite + Silica The degree to which a mass of mafic or ultramafic rock undergoes serpentinisation is heavily dependent on the composition of the olivines and pyroxene. The ideal composition is an olivine with 50% forsterite and 50% fayalite, at which point the olivine can completely metamorphose to serpentine and magnetite with water only as a catalyst. A heavily forsteritic olivine composition or a heavily fayalitic olivine composition will not react completely due to lack of silica and iron for products in the serpentinite reaction.
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, masses of forsteritic olivine are usually less severely deformed as they cannot usually be supplied with enough silica and water to drive the reaction in the absence of fayalite. In these cases, the peridotite may be well preserved and act in a rigid fashion.
Carbon sequestration
Serpentinite has been proposed as an efficient reagent for
carbon sequestration using 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 or Ofenstein . 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 is releasing a film that describes the quarrying of this stone and its use in the construction of stoves.
See also
- Lost City
- Olivine
- Hydrothermal and metasomatism
External links
References
