Sunday, May 23, 2010

Postperovskite, a high-density material named after the mineral perovskite

Perovskite, CaTiO3, is a naturally occurring mineral with a hardness of 5.5 on Mohs scale and a specific gravity of 4.0 [1]. It was first found in the Ural mountains. This mineral is named after the Russian mineralogist L. A. Perovski (1792-1856) [2]. The name perovskite also stands for the class of compounds which have the same type of crystal structure as perovskite. This perovskite structure is, for example, found in magnesium silicate (MgSiO3), which makes up 70 percent (by weight) of the perovskite layer in earth's lower mantle [3].

Postperovskite is a synthetic material or material phase observed in the laboratory under high temperature and very high pressure simulating conditions at the deep-earth boundary between the lower mantle and the outer core. Kei Hirose tells the story of its discovery and name [3]. Here, an excerpt about its name and relation to known crystalline materials is selected:
We decided to name the new phase postperovskite. (Strictly speaking, it is not a mineral, because it has yet to be found in nature.) As it turns out, its structure is essentially identical to that of two known crystals, uranium ferrous sulfate (UFeS3) and calcium iridiate (CaIrO3), which are stable under ambient conditions. And our direct measurements have shown that the density of postperovskite is indeed higher than that of perovskite, by 1 to 1.5 percent.
A postperovskite layer is assumed to exist in the inner earth at a depth of 2,600 to 2,900 km, as a phase that is formed by transformation of perovskite. Interestingly, this layer at the lowermost mantle could not have been formed in the “early earth” (2.3 billion years ago and before) when temperatures were too hot.

Keywords: earth sciences, mineralogy, earth mantle, crystal structure, high pressure, calcium titanium oxide, magnesium silicate

Dictionary of Geology & Mineralogy. McGraw Hill. Second Edition. 2003.
[3] Kei Hirose: The Earth's Missing Ingredient. Scientific American June 2010, Volume 302, Number 6, pp. 76-83.