Silicon

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Silicon dioxide SiO2

Beside the commonly known quartz and amorphous glass there are many polymorphs of silicon dioxide (tectosilicates).

  • Quartz (Mohs 7 by definition | density 2.65 g/cm3)
    α-quartz: trigonal -- β-quartz: hexagonal
  • Stishovite (Mohs 9-9.5 | density 4.287 g/cm3 | tetragonal - rutile group)
  • Seifertite (Mohs ~9 | density 4.294 g/cm3 | orthorhombic - resembles scrutinyite α-PbO2)
  • Coesite (Mohs 7.5 | density 2.92 g/cm3 (calculated) | monoclinic)
  • Cristobalite (Mohs 6-7 | density 2.32–2.36 g/cm3)
    α-Cristobalite tetragonal
    β-Cristobalite cubic - resembles diamond / ZnS
  • Tridymite (Mohs 7 | density 2.25–2.28 g/cm3) there are seven crystal phases of tridymite
    two hexagonal: HP (β) & LHP -- three orthorhombic: OC (α) & OS & OP -- two monoclinic: MC & MX
  • Moganite (not MoRganite!) (Mohs 6 | 2.52-2.58 g/cm3 | monoclinic)

Related: Binary diamondoid compound

Stishovite and Seifertite

They are both uncommon in that they have:

  • hypervalent silicon with a coordination number of six instead the normal four and
  • hypervalent oxygen with coordination number three instead of the normal two.

This is probably what causes the unusually high density and hardness.
Related: Hypervalency [1]; Three center four electron bond [2] formerly thought to be sp3d2 hybridisation.

Stishovite and Seifertite are metastable at unpressurized conditions.
And surprisingly more chemically stable than plain quartz. It is not attacked by hydrofluoric acid HF.

To increase high temperature stability at the cost of hardness, it may be possible to substitute some of the silicon atoms <be> with atoms of other suitable elements that would make in case of a 100% substitution termodynamically more stable minerals in the same crysral structure (the rutile structure).
Such a substitution in a checkerboard patterned way done via force applying mechanosynthesis can make makes neo-polymorph materials.

That is: stishovite can form a continuum to the minerals with rutile structure.
See "rutile structure" for a list of elements that are potentially compatible for atom substitution in stishovite without too much crystal structure disturbance.

Misc

Silicon is the second most abundant element on Earth (crust and surface). Right after oxygen.

Maybe remotely related to silicon: Geopolymers: [3]

An elemental storage medium for silicon (think: nanofactory cartridges for silicon) might be a bit of a challenge because of its strong tenency of polymerization and generally low solubility in benign solvents like water. Here's a list of some obvious options:

  • Silicic acid would be great due to its non-toxicity. But silicic acid [4] has a strong tendency to self polymerize. This is undesirable for standardized processing.
  • Silanes [5] (the silicon analogues to hydrocarbons) do not self polymerize but they are toxic and explosive.
  • Disiloxane [6] ?
  • Tetramethylsilane [7] is also toxic but not explosive. It may carry too much carbon for many applications (e.g. in case moissanite SiC with a stoichiometry Si:C of 1:1 is mechanosynthesitzed).
  • Trimethylsilane [8]; Dimethylsilane [9]; Methylsilane [10] -- (better Si:C ratio & maybe falling toxicity ??)
  • There's also trimethylsilanol [11]
  • hydrogen silsesquioxane H8Si8O12 ?? [12]
  • Trimethylsilylacetylene [13]
  • ...

Related

  • Chemical element
  • Elements in the same group: Carbon, Silicon, Germanium, Tin, Lead
    Tin and lead are actually surprisingly abundant for their high period. Lead thanks to being at the end of many decay chains. Germanium is quite rare.
  • Mining

External Links

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