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Recently, steinhardite was accepted as a new mineral by the International Mineralogical Association. It's quite an interesting story in its own right. There's an interesting article in New Scientist about it, but it's paywalled.

The formula for steinhardite is $\ce{Al63Cu24Fe13}$. It's an ultra-rare quasicrystal, at least partially because of the mixture of aluminum without oxygen, as I understand it.

One of the critics (as quoted in the article)

... who promptly declared the possibility of the sample being natural as ‘‘impossible’’. His concern was not the degree of perfection but, rather, the baffling presence of metallic Al in cupalite, khatyrkite and in the icosahedral quasicrystal phase. Metallic Al has a remarkably strong affinity for O, such that it could not possibly form naturally on the surface of the Earth, he argued ... he amended the conclusion to allow for the improbable possibility that it could have formed under the intense heat and pressure that exist near the core mantle boundary or in a high impact collision of meteors in space. [1]

The core/mantle boundary is obviously rife with lots of chemical interactions, but what is it exactly about the conditions that prevents aluminum from oxidizing? Is it just physical pressure or the presence of other gases?

There's the obvious fact that the aluminum isn't exposed to the atmosphere, but my (limited) understanding is that oxygen is plentiful in combination with other elements like silicon.

Reference:

  1. Bindi, L. & Steinhardt, P.J. (2014) The quest for forbidden crystals. Mineralogical Magazine, 78(2), 467–482. [DOI] [PDF] (seems to be freely available)
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  • $\begingroup$ I think the key point is that there isn't that much oxygen in there. Especially when it's combined with silicon in a very stable way. $\endgroup$ – Gimelist Oct 8 '14 at 11:22
  • $\begingroup$ @Michael If you add a few more details, I think that's definitely a good answer. $\endgroup$ – jonsca Oct 9 '14 at 5:25
  • $\begingroup$ I'm sorry, but I do not know much about mantle-core boundary conditions. I guess I could look it up with the references on the article but that's too much for me now. $\endgroup$ – Gimelist Oct 9 '14 at 6:20
  • $\begingroup$ @Michael Not a problem, the question was just for curiosity's sake. I was just offering you the opportunity to answer with the info from that comment if you wanted to. $\endgroup$ – jonsca Oct 9 '14 at 6:40
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A reason found experimentally is that the pressure and temperature conditions at the mantle-core boundary result in the reduction of aluminium oxides. According to the Nature article Chemical interaction of $\ce{Fe}$ and $\ce{Al2O3}$ as a source of heterogeneity at the Earth's core–mantle boundary (Dubrovinsky et al. 2001) through their experiments simulating the thermochemical conditions at the boundary, that one of the main factors in the core-mantle boundary region is

that iron is able to reduce aluminium out of oxides at core–mantle boundary conditions, which could provide an additional source of light elements in the Earth's core and produce significant heterogeneity at the core–mantle boundary.

Further, from the ESRF Highlights 2001 webpage Chemical Interaction of Iron and Corundum at High Pressure and Temperature: Implication for the Earth's Deep Interior, experimentally found that the following reaction, forming iron-aluminium compounds (amongst other alloys) occurred at temperatures that are believed to exist at the mantle-core boundary:

$$\ce{($2+3x$)Fe + $x$Al2O3 -> 2FeAl_{$x$} + $3x$FeO}$$

Where $x$ varied between 0.02-0.03 to 0.25

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