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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)
  • Transparent
  • 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 [1]).
  • Magnetite

and sulfides like

  • Pyrite

are all fulfilling the wishlist rather poorly.

Fayalite as end member of the olivine/peridot group

At the mantle core boundary of Earth (and other rocky planets / dwarf planets / giant moons) iron gets so abundant that is can no longer be bond to silicates. It goes into metallic state. This picture shows how this material probably looks like. It shows a piece of a metallic meteorite that may stem from the core of a later broken up protoplanet (one or many is unknown ?) between Mars and Jupiter. The region that is now our solar systems main asteroid belt.

In nature there is a continuous mixing series where iron can be substituted by very common magnesium or common manganese.

  • 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
  • ...