Serpentinite

Serpentinite is a rock composed 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.[1] 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.[2]

A sample of serpentinite rock, partially made up of chrysotile, from Slovakia
A rock of serpentinite from the Maurienne valley, Savoie, French Alps
Sample of serpentinite from the Golden Gate National Recreation Area, California, United States
Chromitic serpentinite (7.9 cm across), Styria Province, Austria. Protolith was a Proterozoic-Early Paleozoic upper mantle dunite peridotite that has been multiply metamorphosed during the Devonian, Permian, and Mesozoic.
Tightly folded serpentinite from the Tux Alps, Austria. Closeup view about 30cm × 20cm.

Formation and petrology

Serpentinization is a geological low-temperature metamorphic process involving heat and water in which low-silica mafic and ultramafic rocks are oxidized (anaerobic oxidation of Fe2+ by the protons of water leading to the formation of H2) and hydrolyzed with 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, reducing the density and destroying the structure.[3]

The density changes from 3.3 to 2.7 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 °C (500 °F),[3] providing an energy source for formation of non-volcanic hydrothermal vents. The magnetite-forming chemical reactions produce hydrogen gas under anaerobic conditions prevailing deep in the mantle, far from the Earth's atmosphere. Carbonates and sulfates are subsequently reduced by hydrogen and form methane and hydrogen sulfide. The hydrogen, methane, and hydrogen sulfide provide energy sources for deep sea chemotroph microorganisms.[3]

Formation of serpentinite

Serpentinite can form from olivine via several reactions. Olivine is a solid solution of forsterite, the magnesium-endmember, and fayalite, the iron-endmember.

Forsterite3 Mg2SiO4 + silicon dioxideSiO2 + 4 H2O → serpentine2 Mg3Si2O5(OH)4

 

 

 

 

(Reaction 1b)

Forsterite2 Mg2SiO4 + water3 H2O → serpentineMg3Si2O5(OH)4 + bruciteMg(OH)2

 

 

 

 

(Reaction 1c)

Reaction 1c describes the hydration of olivine to yield serpentine and Mg(OH)2 (brucite).[4] Serpentine is stable at high pH in the presence of brucite like calcium silicate hydrate, (C-S-H) phases formed along with portlandite (Ca(OH)2) in hardened Portland cement paste after the hydration of belite (Ca2SiO4), the artificial calcium equivalent of forsterite.

Analogy of reaction 1c with belite hydration in ordinary Portland cement:

Belite2 Ca2SiO4 + water4 H2O → C-S-H phase3 CaO · 2 SiO2 · 3 H2O + portlanditeCa(OH)2

 

 

 

 

(Reaction 1d)

After reaction, the poorly soluble reaction products (aqueous silica or dissolved magnesium ions) can be transported in solution out of the serpentinized zone by diffusion or advection.

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. Talc and magnesian chlorite are 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 exceed 600 °C (1,112 °F) 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 calcium and 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.

Olivine(Fe,Mg)2SiO4 + watern·H2O + carbon dioxideCO2 → serpentineMg3Si2O5(OH)4 + magnetiteFe3O4 + methaneCH4

 

 

 

 

(Reaction 2a)

or, in balanced form:[5]

18 Mg2SiO4 + 6 Fe2SiO4 + 26 H2O + CO2 → 12 Mg3Si2O5(OH)4 + 4 Fe3O4 + CH4

 

 

 

 

(Reaction 2a')

Olivine(Fe,Mg)2SiO4 + watern·H2O + carbon dioxideCO2 → serpentineMg3Si2O5(OH)4 + magnetiteFe3O4 + magnesiteMgCO3 + silicaSiO2

 

 

 

 

(Reaction 2b)

Reaction 2a is favored if the serpentinite is Mg-poor or if there isn't enough carbon dioxide to promote talc formation. Reaction 2b is favored in highly magnesian compositions and low partial pressure of carbon dioxide.

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, magnesium and 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.

Hydrogen production by anaerobic oxidation of fayalite ferrous ions

Serpentine is the product of the reaction between water and fayalite's ferrous (Fe2+) ions. The process is of interest because it generates hydrogen gas:[6][7]

Fayalite3 Fe2SiO4 + water2 H2O → magnetite2 Fe3O4 + silicon dioxide3 SiO2 + hydrogen2 H2

 

 

 

 

(Reaction 1a)

The reaction can be viewed simplistically as follows:[5][8]

6 Fe(OH)2ferrous hydroxide → 2 Fe3O4magnetite + 4 H2Owater + 2 H2hydrogen

 

 

 

 

(Reaction 3e)

This reaction resembles the Schikorr reaction observed in the anaerobic oxidation of the ferrous hydroxide in contact with water:

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.[9][10]


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.[11] The high pH is interpreted to be a key consequence of serpentinization of chondritic rock, that leads to the generation of H2, a geochemical source of energy that can support both abiotic and biological synthesis of organic molecules.[11][12]

Impact on agriculture

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.[13]

Uses

Decorative stone in architecture

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.[14]

Carvingstone tools, oil lamp-known as the Qulliq and Inuit sculpture

Inuit and 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. Inuit made tools and more recently carvings of animals for commerce.

  • Magnetic Serpentine Walrus
  • Inuit Elder tending the Qulliq, a ceremonial oil lamp made of serpentinite.

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" (Ger. Ofenstein), a carved stone base beneath a cast iron stove.[15]

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 it was used for top radiation shielding to protect operators from escaping neutrons.[16] 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.[17][18]

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."[19] In 2010, a bill was introduced which would have removed serpentine's special status as state rock due to it potentially containing chrysotile asbestos.[20] 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.[21]

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See also

References
  1. Schoenherr, Allan A. (11 July 2017). A Natural History of California: Second Edition. Univ of California Press. pp. 35–. ISBN 9780520295117. Retrieved 6 May 2017.
  2. "Serpentine definition". Dictionary of Geology. Retrieved 23 October 2018.
  3. Serpentinization: The heat engine at Lost City and sponge of the oceanic crust
  4. Coleman, Robert G. (1977). Ophiolites. Springer-Verlag. pp. 100–101. ISBN 978-3540082767.
  5. Russell, M. J.; Hall, A. J.; Martin, W. (2010). "Serpentinization as a source of energy at the origin of life". Geobiology. 8 (5): 355–371. doi:10.1111/j.1472-4669.2010.00249.x. PMID 20572872.
  6. "Methane and hydrogen formation from rocks – Energy sources for life". Retrieved 6 November 2011.
  7. Sleep, N.H.; A. Meibom, Th. Fridriksson, R.G. Coleman, D.K. Bird (2004). "H2-rich fluids from serpentinization: Geochemical and biotic implications". Proceedings of the National Academy of Sciences of the United States of America. 101 (35): 12818–12823. Bibcode:2004PNAS..10112818S. doi:10.1073/pnas.0405289101. PMC 516479. PMID 15326313.CS1 maint: multiple names: authors list (link)
  8. Schrenk, M. O.; Brazelton, W. J.; Lang, S. Q. (2013). "Serpentinization, Carbon, and Deep Life". Reviews in Mineralogy and Geochemistry. 75 (1): 575–606. Bibcode:2013RvMG...75..575S. doi:10.2138/rmg.2013.75.18.
  9. Baucom, Martin (March–April 2006). "Life on Mars?". American Scientist. 94 (2): 119–120. doi:10.1511/2006.58.119. JSTOR 27858733.
  10. esa. "The methane mystery". European Space Agency. Retrieved 22 April 2019.
  11. R. Glein, Christopher; Baross, John A.; Waite, Hunter (16 April 2015). "The pH of Enceladus' ocean". Geochimica et Cosmochimica Acta. 162: 202–219. arXiv:1502.01946. Bibcode:2015GeCoA.162..202G. doi:10.1016/j.gca.2015.04.017.
  12. Wall, Mike (7 May 2015). "Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life". Space.com. Retrieved 8 May 2015.
  13. "CVO Website - Serpentine and serpentinite" Archived 19 October 2011 at the Wayback Machine, USGS/NPS Geology in the Parks Website, September 2001, accessed 27 February 2011.
  14. Ashurst, John. Dimes, Francis G. Conservation of building and decorative stone. Elsevier Butterworth-Heinemann, 1990, p. 51.
  15. Talcose-schist from Canton Valais. By Thomags Bonney, (Geol. Mag., 1897, N.S., [iv], 4, 110--116) abstract
  16. Lithuanian Energy Institute (28 May 2011). "Design of structures, components, equipments and systems". Ignalina Source Book. Retrieved 28 May 2011.
  17. Aminian, A.; Nematollahi, M.R.; Haddad, K.; Mehdizadeh, S. (3–8 June 2007). Determination of shielding parameters for different types of concretes by Monte Carlo methods (PDF). ICENES 2007: International Conference on Emerging Nuclear Energy Systems. Session 12B: Radiation effects. Istanbul, Turkey. p. 7.
  18. Abulfaraj, Waleed H.; Salah M. Kamal (1994). "Evaluation of ilmenite serpentine concrete and ordinary concrete as nuclear reactor shielding". Radiation Physics and Chemistry. 44 (1–2): 139–148. Bibcode:1994RaPC...44..139A. doi:10.1016/0969-806X(94)90120-1. ISSN 0969-806X.
  19. California Government Code § 425.2; see "Archived copy". Archived from the original on 28 June 2009. Retrieved 24 December 2009.CS1 maint: archived copy as title (link)
  20. Fimrite, Peter (16 July 2010). "Geologists protest bill to remove state rock". SFGate. Retrieved 17 April 2018.
  21. Frazell, Julie; Elkins, Rachel; O'Geen, Anthony; Reynolds, Robert; Meyers, James. "Facts about Serpentine Rock and Soil Containing Asbestos in California" (PDF). ANR Catalog. University of California Division of Agriculture and Natural Resources. Retrieved 17 April 2018.
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