Silicon

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

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• 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)

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• 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 uncomman 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.

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. Thy 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.

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• 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