Polymorphs of Silicon Dioxide

Silicon dioxide surrounds us on every hand. The soil on which we walk contains silicon dioxide sand. The windows that adorn our houses and public buildings are made of glass composed of silicon dioxide

Several different crystalline forms of silicon dioxide occur in nature. We refer to each of these crystalline forms as a polymorph of silicon dioxide. The word “polymorph” comes from Greek words meaning “many” and “form.”

Quartz

Quartz is the most common form of crystalline silicon dioxide. In quartz, silicon and oxygen atoms unite to form a lattice. Its basic unit is a tetrahedron in which four oxygen atoms are attached symmetrically to a single silicon atom. However, each oxygen atom is a constituent part of two different tetrahedra, so that all the tetrahedra are joined together to form a unit.

The standard chemical formula for quartz is SiO2; but as you can see from the foregoing, quartz is not composed of discrete SiO2 units. It is one large molecule.

Ideally, quartz should form neat-looking hexagonal crystals with a six-sided pyramid at two opposite ends. However, natural quartz exhibits imperfections. This is partly due to that fact that bonds between the atoms in the crystal lattice are inevitably warped to an extent at room temperature. Moreover, normal crystal growth may be hindered by environmental factors, and impurities may intrude themselves into the composition of quartz. As a result, quartz crystals are jagged and irregular. They sometimes look like a statue that a sculptor has left half-finished.

There are two different quartz polymorphs: alpha-quartz and beta-quartz. Beta-quartz is stable at higher temperatures. When it cools down, it becomes alpha-quartz. Beta-quartz forms better crystals. The best alpha-quartz crystals are cooled beta-quartz crystals that managed to retain a semblance of their original shape.

Cristobalite

Cristobalite is a polymorph of quartz that formed at high temperatures. It was discovered in Cerro San Cristobal, a high hill in the state of Hidalgo, Mexico.

Cristobalite is a silica polymorph that forms at extremely high temperatures. For this reason, it is often found in volcanic rock.

Like quartz, cristobalite consists of tetrahedrons linked together to form a lattice. However, the cristobalite tetrahedrons are linked together in a different way from those of quartz. At high temperatures, cristobalite tetrahedra sit at the corners of a cube, but the cubic arrangement is usually lost as the temperature cools.

Cristobalite crystals are small and often microscopic. Some appear to have an octahedral form, while others are little balls called spherulites. The spherulites seem to be cristobalite that has lost some of its silica.

Tridymite

In tridymite, the basic unit is the tetrahedron. These tetrahedra are linked together to form rings. The rings are linked together to form sheets. Finally, the sheets are bonded together here and there to complete the lattice.

Coesite

Coesite owes its name to Loring Coes, the man who first synthesized it. It was subsequently discovered in Meteor Crater, Arizona.

Coesite also has tetrahedra as its basic unit. However, in coesite, these tetrahedra are linked together in such a way that they form small rings, and the resultant rings are linked together to form chains. Because of this loose structure, coesite is somewhat compressible.

Coesite needs high pressures for its formation. It occurs in metamorphic rocks.

Stishovite

Sergey Stishov synthesized stishovite in the laboratory before it was discovered in nature. The tremendous pressure generated by sizable meteors is sufficient to trigger its formation. Like Coesite, it was first discovered in nature in the Meteor Crater of Arizona.

The basic structural unit of stishovite is not a tetrahedron, but an octahedron. The stishovite crystals are small, and the octahedra are poorly formed, but the mineral is much harder and denser than quartz.

References

The Free Dictionary by Farlex: Polymorph

http://www.thefreedictionary.com/polymorph

Wikipedia: Quartz

http://en.wikipedia.org/wiki/Quartz

The Quartz Page: SiO4 Tetrahedron

http://www.quartzpage.de/gen_struct.html

Galleries: The Quartz Group of Minerals

http://www.galleries.com/minerals/silicate/quartz.htm

Web Mineral: Cristobalite Mineral Data

http://webmineral.com/data/Cristobalite.shtml

Galleries: The Mineral Cristobalite

http://www.galleries.com/minerals/silicate/cristoba/cristoba.htm

Wikipedia: Coesite

http://en.wikipedia.org/wiki/Coesite

Mineralogical Society of America: High-Pressure Crystal Structure and Compressibility of Coesite

http://www.minsocam.org/ammin/AM66/AM66_324.pdf

Seattle Community Network: Stishovite

http://www.scn.org/~bh162/stishovite.html

Auburn University: Tridymite and Cristobalite ‘” SiO2

http://www.auburn.edu/~hameswe/Tridy_Cristobpage.html