Silicate perovskite

Silicate perovskite is the term given to (Mg,Fe)SiO3 (also known as bridgmanite[1]) and CaSiO3 (calcium silicate) when arranged in a perovskite structure. Silicate perovskites are mainly found in the lower part of Earth's mantle, between about 670 and 2,700 kilometres (420 and 1,680 mi). They are thought to form the main mineral phases, together with ferropericlase.

Natural silicate perovskite was discovered in a heavily shocked meteorite.[2] [3] In 2014, the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) approved the name bridgmanite for perovskite-structured (Mg,Fe)SiO3,[1] in honor of physicist Percy Bridgman, who won the Nobel Prize in Physics in 1946 for his high-pressure research.[4]

Abundance

Silicate perovskite may form up to 93% of the lower mantle[5] and the magnesium iron form is considered to be the most abundant mineral in the Earth, making up 38% of its volume.[6][7]

Under the very high pressures of the lowermost mantle, below about 2700 km, the silicate perovskites are replaced by post-perovskite.[8]

The physical properties of silicate perovskites under lower mantle conditions, such as seismic velocity, are studied experimentally using laser-heated diamond anvil cells. Naturally occurring silicate perovskites cannot be studied as they are unstable at the Earth's surface.[9]

Structure

The perovskite structure (first identified in the mineral perovskite) occurs in substances with the general formula ABX3, where A is a metal that forms large cations, B is another metal that forms smaller cations and X is typically oxygen. The structure may be cubic, but only if the relative sizes of the ions meet strict criteria. Typically, substances with the perovskite structure show lower symmetry, owing to the distortion of the crystal lattice and silicate perovskites in the orthorhombic crystal system.[10]

Occurrence

Upper limit of stability

The existence of silicate perovskite in the mantle was first suggested in 1962, and both MgSiO3 and CaSiO3 had been synthesised experimentally before 1975.[9] By the late 1970s, it had been proposed that the discontinuity at about 650 km in the mantle represented a change from spinel structure minerals with an olivine composition to silicate perovskite with ferropericlase.

Lower limit of stability

In 2004 it was proposed that silicate perovskites experience a further change in structure below about 2700 km to post-perovskite. This change is thought to explain the presence of the D" layer in the lowermost mantle.[11]

Chemistry

The partitioning of Fe between magnesium perovskite and ferropericlase under lower mantle conditions has been extensively studied experimentally. The effects of varying the amount of Al in the silicate perovskite structure have also been studied.[12]

Abundance

Silicate perovskite is thought to be the main constituent of the lower mantle,[6] possibly reaching up to 93% by volume.[5] Magnesium silicate perovskite is probably the most abundant mineral phase in the Earth.[6] The highest proposed abundances of silicate perovskites suggest that the lower mantle is richer in silica than the upper mantle and are consistent with the overall chondritic composition of the Earth.[5]

Deformation

Experimental deformation of polycrystalline MgSiO3 under the conditions of the uppermost part of the lower mantle suggests that silicate perovskite deforms by a dislocation creep mechanism. This may help explain the observed seismic anisotropy in the mantle.[13]

See also

References

  1. 1 2 "Bridgmanite". Mindat.org.
  2. ↑ Tomioka, N.; Fujino K. (1997). "Natural (Mg,Fe)SiO3-ilmenite and -perovskite in the Tenham meteorite". Science 277: 1084–1086. doi:10.1126/science.277.5329.1084. PMID 9262473.
  3. ↑ Tschauner, O.; Ma C., Beckett, J. R., Prescher, C., Prakapenka, V. B., Rossman, G. R. (2014). "Discovery of bridgmanite, the most abundant mineral in Earth, in a shocked meteorite". Science 346: 1100–1102. doi:10.1126/science.1259369. Cite uses deprecated parameter |coauthors= (help)
  4. ↑ Eos, Transactions American Geophysical Union, Volume 95, Issue 23, Article first published online: 10 June 2014 PDF
  5. 1 2 3 Murakami, M.; Ohishi Y.; Hirao N.; Hirose K. (2012). "A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data". Nature 485 (7396): 90–94. Bibcode:2012Natur.485...90M. doi:10.1038/nature11004. Retrieved 3 June 2012.
  6. 1 2 3 Murakami, M.; Sinogeikiin S.V.; Hellwig H.; Bass J.D.; Li J. (2007). "Sound velocity of MgSiO3 perovskite to Mbar pressure" (PDF). Earth and Planetary Science Letters (Elsevier) 256: 47–54. Bibcode:2007E&PSL.256...47M. doi:10.1016/j.epsl.2007.01.011. Retrieved 7 June 2012.
  7. ↑ Sharp, T. (27 November 2014). "Bridgmanite--named at last". Science 346 (6213): 1057–1058. doi:10.1126/science.1261887.
  8. ↑ Murakami M., Hirose K., Kawamura K., Sata N. & Ohishi Y. (2004). "Post-Perovskite Phase Transition in MgSiO3" (PDF). Science 304: 855–858. Bibcode:2004Sci...304..855M. doi:10.1126/science.1095932. PMID 15073323.
  9. 1 2 Ross, N.L.; Hazen R.M. (1990). "High-Pressure Crystal Chemistry of MgSiO3 Perovskite". Physics and Chemistry of Minerals 17: 228–237. Bibcode:1990PCM....17..228R. doi:10.1007/BF00201454. Retrieved 3 June 2012.
  10. ↑ Hemley, R.J.; Cohen R.E. (1992). "Silicate Perovskite" (PDF). Annual Review of Earth and Planetary Sciences 20: 553–600. Bibcode:1992AREPS..20..553H. doi:10.1146/annurev.ea.20.050192.003005. Retrieved 3 June 2012.
  11. ↑ Auzende, A.-L.; Badro J.; Ryerson F.J.; Weber P.K.; Fallon S.J.; Addad A.; Siebert J.; Fiquet G. (2008). "Element partitioning between magnesium silicate perovskite and ferropericlase: New insights into bulk lower-mantle chemistry" (PDF). Earth and Planetary Science Letters (Elsevier) 269: 164–174. Bibcode:2008E&PSL.269..164A. doi:10.1016/j.epsl.2008.02.001. Retrieved 3 June 2012.
  12. ↑ Vanpeteghem, C.B.; Angel R.J.; Ross N.L.; Jacobsen S.D.; Dobson D.P.; Litasov K.D.; Ohtani E. (2006). "Al, Fe substitution in the MgSiO3 perovskite structure: A single-crystal X-ray diffraction study" (PDF). Physics of the Earth and Planetary Interiors (Elsevier) 155: 96–103. Bibcode:2006PEPI..155...96V. doi:10.1016/j.pepi.2005.10.003. Retrieved 7 June 2012.
  13. ↑ Cordier, P.; Ungár T.; Zsoldos L.; Tichy G. (2004). "Dislocation creep in MgSiO3 perovskite at conditions of the Earth's uppermost lower mantle". Nature 428 (6985): 837–840. Bibcode:2004Natur.428..837C. doi:10.1038/nature02472. Retrieved 7 June 2012.

External links

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