Iron will be used to a great deal in the gemstone Fayalite
Iron is one of the most abundant elements on earth and in space. There are pure iron asteroids, remnants of the never formed core of the dwarf-planet Ceres that couldn't become to a proper planet due to the gravitational disturbances caused by Jupiters proximity. There's so much iron everywhere because irons nuclear core has the highest binding energy. This is putting iron at the end of the stellar fusion process where it piles up (along with nickel).
Since iron is of so much use today either mostly in its raw form or alloyed to steels one might assume that in advanced atomically precise technology it will retain its important role.
Both the problem of diffusion (exhibited by almost all metals) and the problem of oxidation can be solved by oxidizing iron with suitable nonmetals to make gemstones. (The usual method.)
So how can iron be used in advanced atomically precise technology (with focus on usage as structural building material)? The answer is that there are surprisingly few options. One of the best gemstones for usage as structural building material is probably Fayalite Fe2SiO4
The problem - View hard iron gemstones
But there is a problem. Unlike other highly abundant metals like magnesium aluminum and titanium irons oxides and other nonmetal compounds usually have far less hardness and are almost all non-transparent and often electrically conductive.
There are only very view iron compounds that fulfill the following conditions simultaneously:
- High in iron content (ratio of atoms)
- Hard (at least 5 on the Mohs scale but better 7)
- Electrically isolating
The one abundant element compound best fulfilling these is maybe:
- Fayalite Fe2SiO4 (spinell with iron instead of magnesium).
Others abundant element compounds which are a little low in iron content are:
- Almandin Fe3Al2(Si2O4)3 (Garnet)
- Andardite Ca3Fe2(Si2O4)3 (Garnet)
Simple iron oxides like
- Hematite (beside the usual form α-Fe2O3 there is also a barely known second form β-Fe2O3 ).
and sulfides like
are all fulfilling the wishlist rather poorly.
Fayalite as end member of the olivine/peridot group
In nature there is a continuous mixing series where iron can be substituted by very common magnesium or common manganese.
- Fayalite Fe2SiO4 (Mohs 6.5-7)
High pressure crystal structure γ-Fe2SiO4 is called Ahrensite
- Forsterite Mg2SiO4 (Mohs 7)
Mid pressure crystal structure is called: Wadsleyite
High pressure crystal structure is called: Ringwoodite
- Tephroite Mn2SiO4 (Mohs 6)
- Liebenbergite (de) Ni2SiO4 (Mohs 6-6.5)
- Larnite β-Ca2SiO4 (Mohs 6)
- Calcio-Olivine γ-Ca2SiO4 (Mohs 4.5)
As always with advanced mechanosynthesis highly ordered checkerboard neo-polymorphs can be made.
While (according to wikipedia 2016) an iron analog to Wadsleyite is not synthesizable by thermodynamic means is very likely mechanosynthesizable.
Other remotely interesting iron compounds
There are some peculiar compounds: iron borides iron silicides and possibly toxic iron phosphides.
- There is Goethite which is pretty hard for a hydroxide.
- Hercynite FeAl2O4
- Cuprospinell CuFe2O4