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.
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