Difference between revisions of "Silicon"
From apm
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(added section == Stishovite and Seifertite ==) |
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* [https://en.wikipedia.org/wiki/Quartz Quartz] (Mohs 7 by definition | density 2.65 g/cm<sup>3</sup>) <br> α-quartz: trigonal -- β-quartz: hexagonal | * [https://en.wikipedia.org/wiki/Quartz Quartz] (Mohs 7 by definition | density 2.65 g/cm<sup>3</sup>) <br> α-quartz: trigonal -- β-quartz: hexagonal | ||
---- | ---- | ||
| − | * [https://en.wikipedia.org/wiki/Stishovite Stishovite] ('''Mohs 9-9.5 | density 4.287 g/cm<sup>3</sup>''' | tetragonal - | + | * [https://en.wikipedia.org/wiki/Stishovite Stishovite] ('''Mohs 9-9.5 | density 4.287 g/cm<sup>3</sup>''' | tetragonal - rutile group) |
* [https://en.wikipedia.org/wiki/Seifertite Seifertite] ('''Mohs ~9 | density 4.294 g/cm<sup>3</sup>''' | orthorhombic - resembles scrutinyite α-PbO<sub>2</sub>) | * [https://en.wikipedia.org/wiki/Seifertite Seifertite] ('''Mohs ~9 | density 4.294 g/cm<sup>3</sup>''' | orthorhombic - resembles scrutinyite α-PbO<sub>2</sub>) | ||
---- | ---- | ||
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Related: [[Binary diamondoid compound]] | Related: [[Binary diamondoid compound]] | ||
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| + | === 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 toxygen with coordination number three instead of the normal two. | ||
| + | This causes their unusually high density and hardness. <br> | ||
| + | They are metastable at unpressurized conditions. | ||
| + | And surprisingly more chemically stable than plain quartz. | ||
| + | |||
| + | To increase stability at the cost of hardness checkerboard [[neo-polymorph]]s 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): | ||
| + | * '''[https://en.wikipedia.org/wiki/Rutile Rutile] TiO<sub>2</sub> (Mohs 6-6.5 | titanium is very abundant)''' | ||
| + | * [https://en.wikipedia.org/wiki/Pyrolusite Pyrolusite] MnO<sub>2</sub> (Mohs 6-6.5 | Mn is not too rare but todays mining is environmentally destructive) | ||
| + | * NbO<sub>2</sub> [https://en.wikipedia.org/wiki/Niobium_dioxide] | ||
| + | * [https://en.wikipedia.org/wiki/Cassiterite Cassierite] SnO<sub>2</sub> (Mohs 6-7) | ||
| + | * [https://en.wikipedia.org/wiki/Plattnerite Plattnerite] β-PbO<sub>2</sub> (Mohs 5.5 unusually hard for a lead compound | '''useful high density of ~9 g/cm<sup>3</sup>''') | ||
| + | * Note that lead Pb tin Sn and niobium Nb are similar in their not too high abundance. | ||
| + | ---- | ||
| + | * [https://en.wikipedia.org/wiki/Argutite Argutite] GeO<sub>2</sub> (Mohs 6-7 | germanium is pretty rare) | ||
| + | * [https://en.wikipedia.org/wiki/Tripuhyite Tripuhyite] FeSbO<sub>4</sub> (Mohs 6-7 | Antimony Sb is a little more rare than germanium Ge.<br> The much more common phosphorus analog FePO<sub>4</sub> forms a soft hydroxide [https://en.wikipedia.org/wiki/Strengite Strengite] with different crystal structure) | ||
| + | * [https://de.wikipedia.org/wiki/Paratellurit Paratellurit (de)] TeO<sub>2</sub> (soft compound and tellurium is extremely rare) | ||
| + | |||
| + | Related: [[Hypervalency]] [https://en.wikipedia.org/wiki/Hypervalent_molecule]; Three center four electron bond [https://en.wikipedia.org/wiki/Three-center_four-electron_bond] formerly thought to be sp<sup>3</sup>d<sup>2</sup> hybridisation | ||
Revision as of 16:15, 13 December 2016
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 uncomman in that they have:
- hypervalent silicon with a coordination number of six instead the normal four and
- hypervalent toxygen 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.
- 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
