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 causes their unusually high density and hardness.
They are metastable at unpressurized conditions. And surprisingly more chemically stable than plain quartz. It is not attacked by hydrofluoric acid HF.

To increase stability at the cost of hardness checkerboard neo-polymorphs can be created by replacing some of the silicon with other suitable elements that crystallize in the same crystal structure. namely the rutile group crystal structure.

Stishovite can form a continuum to the minerals of the rutile group (here ordered by elemental abundance in earths crust):

  • Rutile TiO2 (Mohs 6-6.5 | titanium is very abundant)
  • Pyrolusite MnO2 (Mohs 6-6.5 | Mn is not too rare but todays mining is environmentally destructive)
  • NbO2 [1]
  • Cassierite SnO2 (Mohs 6-7)
  • Plattnerite β-PbO2 (Mohs 5.5 unusually hard for a lead compound | useful high density of ~9 g/cm3)
  • Note that lead Pb tin Sn and niobium Nb are similar in their not too high abundance.
  • Argutite GeO2 (Mohs 6-7 | germanium is pretty rare)
  • Tripuhyite FeSbO4 (Mohs 6-7 | Antimony Sb is a little more rare than germanium Ge.
    The much more common phosphorus analog FePO4 forms a soft hydroxide Strengite with different crystal structure)
  • Paratellurit (de) TeO2 (soft compound and tellurium is extremely rare)

Related: Hypervalency [2]; Three center four electron bond [3] formerly thought to be sp3d2 hybridisation

Related

Some options for an elemental storage medium for silicon:

  • 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]
  • ...

misc

Geopolymers: [14]

External Links

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