Bridgmanite – Silicate Perovskite

In the issue from the 28th of November the magazine Science reported on the successfull characterization of a natural sample of the ‘most abundant’ solid phase of the earth.

It is: magnesium iron silicate (Mg,Fe)SiO3, which crystallizes in the fameous perovskite-analogue structure of CaTiO3:

Bridgmanite_b

 

So, we have tilted and twisted corner-connected SiO6 octahedra (orange) and in the centers of each 8 such octahedra the cation Mg or Fe is located with occupancies of 70 and 30 %, respectively. The space group is Pnma (#62, setting 1, orthorhombic crystal system) and the cell constants are a = 5.02 Å, b = 6.90 Å, and c = 4.81 Å.

But what (the hell) is the reason that this rather unspectacular structure is worth a story in Science?

Well, although being the most abundant phase of the earth it is kind of concealed, namely in the lower earth mantle, which begins in a depth of 660 km under the ground and where pressures of 250,000 atm exist. Nobody is able to dig a hole of such depth. But some tiny chunks of Bridgmanite came into existence also on the earth crust, when in 1879 a meteorite crashed with a huge amount of energy the earth ground in Australia. Only now, the team of Oliver Tschauner of the University of Nevada was able to fully structurally characterize this mineral.

The structure itself was not a surprise as high-pressure experiments with artificial silicate perovskites already lead to the assumption that this mineral has this structure. But now it is proved!

And only when a mineral is fully characterized from an natural occuring sample it can be named! The scientist named it Bridgmanite after the inventor of high-pressure crystallography Percy W. Bridgman (1882 to 1961).

Read the full story here:

http://www.sciencemag.org/content/346/6213/1057

 

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