What makes the conditions at the core/mantle boundary ideal for aluminum to combine with other elements besides oxygen? 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:
  
  
  Bindi, L. &amp; Steinhardt, P.J. (2014) The quest for forbidden crystals. Mineralogical Magazine, 78(2), 467–482. [DOI] [PDF] (seems to be freely available)
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:

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

Where $x$ varied between 0.02-0.03 to 0.25