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Sample of serpentinite from the Golden Gate National Recreation Area, California, USA


Formation and petrology[edit]



Serpentinite reactions[edit]

Serpentinite is formed from olivine via several reactions, some of which are complementary. Olivine is a solid solution between the magnesium-endmember forsterite and 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.

Reaction 1a:
Fayalite + water → magnetite + aqueous silica + hydrogen

3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2

Reaction 1b:
Forsterite + aqueous silica → serpentine

3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5(OH)4

Reaction 1c:
Forsterite + water → serpentine + brucite



Analogy of reaction 1c with belite hydration in ordinary Portland cement:
Belite + water → C-S-H phase + portlandite





Reaction 2a:

Olivine + water + carbonic acid → serpentine + magnetite + methane
\mathrm{(Fe,Mg)_2SiO_4 + n H_2O + CO_2}\mathrm{Mg_3Si_2O_5(OH)_4 + Fe_3O_4 + CH_4}

or, in balanced form:

\mathrm{18 Mg_2SiO_4 + 6 Fe_2SiO_4 + 26 H_2O + CO_2}\mathrm{12 Mg_3Si_2O_5(OH)_4 + 4 Fe_3O_4 + CH_4}

Reaction 2b:

Olivine + water + carbonic acid → serpentine + magnetite + magnesite + silica
\mathrm{(Fe,Mg)_2SiO_4 + n H_2O + CO_2}\mathrm{Mg_3Si_2O_5(OH)_4 + Fe_3O_4 + MgCO_3 + SiO_2}




Hydrogen production by anaerobic oxidation of fayalite ferrous ions[edit]

In the absence of atmospheric oxygen (O2), in deep geological conditions prevailing far away from Earth atmosphere, hydrogen (H2) is produced by the anaerobic oxidation of ferrous ions (Fe2+) present in the crystal lattice of the iron-endmember fayalite by the protons (H+) of water.[2][3]

Considering three formula units of fayalite (Fe2(SiO4)) for the purpose of stoechiometry and reaction mass balance, four ferrous ions will undergo oxidation by water protons while the two remaining will stay unoxidised. Neglecting the orthosilicate anions not involved in the redox process, it is then possible to schematically write the two half-redox reactions as follows:

4 (Fe2+ → Fe3+ + e) (oxidation of ferrous ions)
2 (H2O + 2 e → O2– + H2) (reduction of protons into hydrogen)

This leads to the global redox reaction involving ferrous ions oxidation by water:

4 Fe2+ + 2 H2O → 4 Fe3+ + 2 O2– + 2 H2

The two unoxidised ferrous (Fe2+) ions still available in the three formula units of fayalite finally combine with the four ferric (Fe3+) cations and oxide anions (O2–) to form two formula units of magnetite (Fe3O4).

Finally, considering the required rearrangement of the orthosilicate anions into free silica (SiO2) and free oxide anions (O2–), it is possible to write the complete reaction of anaerobic oxidation and hydrolysis of fayalite according to the following mass balance:

3 Fe2SiO4 + 2 H2O → 2 Fe3O4 + 3 SiO2 + 3 H2
fayalite + water → magnetite + quartz + hydrogen


3 Fe(OH)2 → Fe3O4 + 2 H2O + H2
ferrous hydroxide → magnetite + water + hydrogen

Abiotic methane production on Mars by serpentinization[edit]


Impact on agriculture[edit]


Uses for serpentinite[edit]

Decorative stone in architecture[edit]

Grades of serpentinite higher in calcite, along with the breccia form of serpentinite, have historically been used as decorative stones for their marble-like qualities. Popular sources in Europe before contact with the Americas were the mountainous Piedmont region of Italy and Larissa, Greece.[7]

Carvingstone Tools, Oil lamp-known as the Qulliq and Inuit Sculpture[edit]


Swiss ovenstone[edit]

A variety of chlorite talc schist associated with Alpine serpentinite is found in Val d’Anniviers, Switzerland and was used as ovenstone in stove construction.[8]

Neutron shield in nuclear reactors[edit]

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.[9] 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.[10][11]

Cultural references[edit]


See also[edit]


  2. ^ "Methane and hydrogen formation from rocks – Energy sources for life". Retrieved 2011-11-06. 
  3. ^ 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. Retrieved 2011-11-06. 
  4. ^ "Life on Mars?". American Scientist. March–April 2006. Retrieved 1 June 2009. 
  5. ^ "Methane: Evidence of life on Mars?". 15 January 2009. Retrieved 1 June 2009. 
  6. ^ "CVO Website - Serpentine and serpentinite", USGS/NPS Geology in the Parks Website, September 2001, accessed 27 February 2011.
  9. ^ Lithuanian Energy Institute (2011-05-28). "Design of structures, components, equipments and systems". Ignalina Source Book. Retrieved 2011-05-28. 
  10. ^ 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". ICENES 2007: International Conference on Emerging Nuclear Energy Systems. Session 12B: Radiation effects. Istanbul, Turkey. p. 7. 
  11. ^ Abulfaraj, Waleed H.; Salah M. Kamal. "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. 
  12. ^ California Government Code § 425.2; see

External links[edit]