Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or . The ancient origin of the name is uncertain; it may be from the similarity of its texture or color to snake skin.
Serpentinite has been called serpentine or serpentine rock, particularly in older geological texts and in wider cultural settings.[California Government Code § 425.2; see ]
Most of the chemical reactions necessary to synthesize acetyl-CoA, essential to basic biochemical pathways of life, take place during serpentinization. Serpentinite thermal vents are therefore considered a candidate for the origin of life on Earth.
Formation and mineralogy
Serpentinite is formed by near to complete
serpentinization of
mafic or
.
Serpentinite is formed from mafic rock that is hydrated by
carbon dioxide-deficient
sea water that is pressed into the rock at great depths below the ocean floor. This occurs at
mid-ocean ridges and in the
forearc mantle of
.
The final mineral composition of serpentinite is usually dominated by antigorite, lizardite, chrysotile (minerals of the serpentine subgroup), and magnetite (), with brucite () less commonly present. Lizardite, chrysotile, and antigorite all have approximately the formula or , but differ in minor components and in form.
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:
- 3 Fe2SiO4 + 2 H2O -> 2 Fe3O4 + 3 SiO2 + 2 H2
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 .
- 3 Fe(OH)2 -> Fe3O4 + 2 H2O + H2
In the Schikorr reaction, the two reduced into are these from two , 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
generally resemble
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
Alkalinity, 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 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 .
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 Mariana Trench subduction zone hosts large serpentinite , 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.
Potential 'cradle of life'
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
resemble sulfide minerals formed during serpentinization.
Ecology
Soil cover over serpentinite
bedrock tends to be thin or absent.
Soil with serpentine is poor in
calcium and other major plant
, but rich in elements toxic to plants such as
chromium and
nickel.
[ "CVO Website - Serpentine and serpentinite" , USGS/NPS Geology in the Parks Website, September 2001, accessed 27 February 2011.] Some species of plants, such as
Clarkia franciscana and certain species of
manzanita, are adapted to living on serpentinite
. 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.
are widely distributed on Earth, in part mirroring the distribution of and other serpentine bearing rocks.
Occurrences
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.
Another occurrence of serpentinite is in Chester County, Pennsylvania.
Uses
Decorative stone in architecture and art
Serpentine group minerals have a
Mohs hardness of 2.5 to 3.5, so serpentinite is easily
Carving.
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.
[Ashurst, John. Dimes, Francis G. Conservation of building and decorative stone. Elsevier Butterworth-Heinemann, 1990, p. 51.]
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.
[Eva Maria Hoyer: Sächsischer Serpentin: ein Stein und seine Verwendung. Edition Leipzig, Leipzig 1996, pp. 20–22.]
By the Inuit
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.
File:Serpentinite_Walrus_2012.jpg|Magnetic serpentine walrus
File:Qulliq_1999-04-01.jpg|Inuit Elder tending the Qulliq, a ceremonial oil lamp made of serpentinite.
As an ovenstone
A variety of
Chlorite group talc schist associated with Alpine serpentinite is found in Val d'Anniviers,
Switzerland and was used for making "ovenstones" (), a carved stone base beneath a
cast iron stove.
[ Talcose-schist from Canton Valais. By Thomags Bonney, (Geol. Mag., 1897, N.S., [iv], 4, 110--116) abstract]
Neutron shield in nuclear reactors
Serpentinite has a significant amount of
bound water, hence it contains abundant
hydrogen atoms able to slow down
by elastic collision (neutron
thermalization process). Because of this, serpentinite can be used as dry filler inside
steel jackets in some designs of
. 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 () 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 up the reaction, serpentinite may be reacted with carbon dioxide at elevated temperature in carbonation reactors. Carbon dioxide may also be reacted with
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 CO
2 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.
See also
External links
