Crystal structure of SiO₂
Polymorphs of Quartz

Silicon and oxygen combine to form the second most abundant molecule on the earth: silica (or silicon dioxide -- SiO₂). There are many ways silicon dioxide molecules can be arranged to form a mineral. This leads to different minerals that can be formed from silica alone. Depending on temperature and pressure during crystallization of the silica molecules, different minerals are formed. The table below lists all silica variables together with their crystal group and crystal classification.

In the table below, Quartz, Tridymite, and Cristobalite all have a beta counterpart. When these three minerals form, they form in intense heat, and form in the crystals described in their beta representation (see below). When they cool below a particular temperature (a different temperature for each mineral), they transform into the alpha representation. The alpha representation is the type that is stable at normal temperature. Although the alpha and beta forms of the minerals may appear the same, they are structurally different.

Note:
1) When referring to Quartz, Tridymite, and Cristobalite without specifying the alpha or beta type, it always refers to the alpha type
2) Low and high are sometimes synonymous with alpha and beta (i.e. Beta Quartz is High Quartz)

Quartz and Beta Quartz

Quartz is the most stable and most common form of silica. Beta Quartz is only stable at temperatures above 1063º F (573º C). Thus, all Quartz specimens we see are alpha Quartz. Once a sample of beta Quartz is lowered below the above-mentioned temperature, it automatically transforms into alpha Quartz. However, it preserves the original shape, but decreases in symmetry and adds some trigonal faces. Occasionally, in specific circumstances, the beta Quartz transforms into alpha Quartz without losing symmetry, and therefore does not add the trigonal faces. Such specimens are shaped as bipyramidal hexagons, and are sold by dealers as "Beta Quartz". They obviously cannot be beta Quartz at the current temperature, but are alpha Quartz paramorphs of beta Quartz.
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Tridymite and Beta Tridymite

Tridymite, another form of silica, is stable at temperatures below 1598º F (870º C). At temperatures between 1598º F and 2678º F (870º C and 1470º C) it is Beta Tridymite, with a different crystal structure. Tridymite specimens are all paramorphs of Beta Tridymite, and although orthorhombic (or monoclinic?), they retain Beta Tridymite's hexagonal shape. Most Tridymite specimens alter to Quartz while retaining their original, distinctive shape. Thus most "Tridymite" specimens are really Quartz paramorphs after Tridymite.
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Cristobalite and Beta Cristobalite

Cristobalite is yet another natural form of silica. It is stable at temperatures below 514º F (268º C), and at temperatures between 3142º F and 2678º F (1470º C and 1470º C) it is Beta Cristobalite, in a stable form (with a different crystal structure). At temperatures between 2678º F and 514º F (1470º C and 268º C), it is in a metastable form. Cristobalite specimens are all paramorphs of Beta Cristobalite, and, although tetragonal, retain Beta Cristobalite's original isometric shape.
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Coesite

Coesite is a rare form of silica formed under intense heat and great pressure. There are only two environments where Coesite is found: Crater impact sites and Diamond Kimberlite pipes. Both these environments provide the the great heat and pressure this mineral needs to form. One famous locality is the Barringer Crater (also known as Meteor Crater) in Coconino Co., Arizona, a site where a major meteorite hit the earth thousands of years ago.
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Stishovite

Stishovite, like Coesite, is another rare form of silica formed under intense heat and extreme pressure. It was synthesized like Keatite before it was recently discovered in the Barringer Crater (also known as Meteor Crater) in Coconino Co., Arizona. It was formed when the large meteorite struck the earth, causing a tremendous amount of heat and pressure, enough to suit the formation of Stishovite. The Barringer Crater is currently the only locality where this mineral occurs.
Stishovite is isomorphous with Rutile.
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Lechatelierite

Lechatelierite is a very rare, natural form of silica that lacks a chemical structure. It is amorphous, and considered a glass. It falls under the same method of classification as Opal. Since it has a definite chemical composition, it can be classified as a mineral, but since it lacks a crystal structure, it is classified as a mineraloid.


Keatite

Keatite is a synthetic form of silica; it does not exist in nature. Therefore, it cannot be classified as a mineral. However, it is a possible distinct form of silica and deserves to be mentioned. It is conceivable, although unlikely, that a natural example of Keatite will be found in the future, and thus will become classified as a mineral.

Other minerals in Silica group

Chalcedony

Opal

Melanophlogite