Difference between revisions of "Aluminium"

Revision as of 17:48, 13 June 2021

Mining

Today (2016) Aluminium is still only economically extractable from minerals that do not contain silicon. Silicon is the second most common element in earth's crust just after oxygen.

As waste copious amounts or red mud [1] are produced. Red mud is mostly composed out of iron oxides, titanium oxides and alkaline silicic acid compounds. The problematic part is that red mud also contains ~1% soluble heavy metal salts (V Cr As) which makes it rather toxic. Advanced atomically precise technology should make it possible to fully extract those (potentially valuable) heavy metals directly by efficient chemomechanic processing and indirectly by the availability of very cheap energy. This is a form of atomically precise disassembly which is harder than mechanosynthesis and thus not to expect early on.

Simple oxides

The most obvious form to use aluminium in advanced atomically precise technology is in the form of the naturally occuring gemstone leuco-sapphire (aka colorless gem grade corundum) other strongly metastable polymorphs though may be of equal or even higher interest due to their different crystal structure.

Hexagonal polymorphs are:

• α-Al₂O₃ leuco-sapphire (Mohs 9 | Trigonal).
  
• χ-Al₂O₃ ( Mohs ?? | hexagonal)

Cubic polymorphs (may be of special interest due to their high symmetry):

• γ-Al₂O₃ (known to be strongly metastable) (Mohs 8)
  (Is there no natural mineral with that structure?)
• η-Al₂O₃

Orthorhombic:

• κ-Al₂O₃
• δ-Al₂O₃

Monoclinic (maybe less interesting due to very low symmetry):

• θ-Al₂O₃

(TODO: Identify the exact structure types of the polymorphs with Wyckoff points such that related compounds with other elements replacing aluminium can be identified. Those can then be used to explore the corresponding pseudo phase diagrams and to induce isostructural bending)

Limitedness of stable binary compounds

Most other nonmetal compounds of aluminium beside Al₂O₃ strongly react with water and thus may not be considered as useful structural building materials.

• aluminium nitride AlN (surface hydrolysis creates a passivation layer on macroscopic chunks) this may still be useful
• aluminium carbide Al₄C₃ (reacts with water to aluminium hydroxide and methane - irritant)
• aluminium fluoride AlF₃ (water soluble; interestingly low toxicity)
• aluminium sulfide Al₂S₃ (toxic due to hydrogen sulfide H₂S generation when contacting water)
• aluminium phosphide AlP (highly toxic due to generation of phosphine PH₃ when contacting water)
• aluminium chloride AlCl₃ (water soluble; toxic neurotoxine)

Mechanosynthetisized nonthermodynamic checkerboard pattern compounds like the following may be stable:

• aluminium sulfoxide Al₄O₃</sub>S₃
• aluminium phosphonitride Al₂NP

Aluminum carbides

Aluminium carbide Al₄C₃ is interesting because:

• silicon and aluminium behave similar with oxygen
  forming quartz and sapphire respectively - both hard transparent water insoluble
• silicon and aluminium behave different with carbon
  forming moissanite and a nameless (since water soluble) gem
  

In thermodynamic equilibrium aluminum carbide has complex crystal structure(s), but via mechanosynthesis more regular strongly metastable compounds may be possible (maybe more water stable but probably unlikely). There's a good chance that in solid crystal form (instead of the known powder) it might have good mechanical thermal and other properties (will it be transparent like SiC or not?)

Also the compound is interesting since the partner carbon is enriched and thus extremely abundant in the biosphere. From a mechanical material standpoint the one big disadvantage of aluminum carbides is their reactivity with water. It's always possible to seal against water in the inside of products. Maybe preferably in a micro-packaging way. If seals break it spills and reacts with water then aluminum hydroxide (and methane) these are not exactly the best compounds to release into the biosphere when in bigger quantities and localized in space and time. Aluminum salts are rare and thus life is not adapted for high quantities see below. But there are compounds that compose into far far worse stuff like e.g. AlP.

More aluminium based gemstone materials

A good point to start are always the natural aluminium minerals [2]. Since they need to be insoluble to persist on the surface (except in very dry environments).

There are many aluminium silicate minerals [3] like:

• Al₂SiO₅ Andalusite [4] & Sillimanite [5] (Mohs 6.5-7)
• Al₆Si₂O₁₃ Mullite [6] (Mohs 6-7)
• Al₂SiO₅ Kyanite [7] (Mohs 4.5-7 anisotropic)

Related: Ternary and higher gem-like compounds

Interesting facts

Salts of aluminium usually have low solubility in water. Albeit aluminiums extreme abundance in our environment (most abundant metal in earth's crust) it does not play a known role in human biology. Elevated intake of aluminium salts (rare in nature) is suspected of having detrimental effects on health.