Minerals & Gemstone 480x104
Minerals & Gemstone 480x104
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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 several 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. 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. (In general, when referring to Quartz, Tridymite, and Cristobalite without specifying the alpha or beta type, it always refers to the alpha type.)

Another name sometimes used for alpha and beta is Low and High (i.e. Beta Quartz is High Quartz and Alpha Quartz is Low Quartz).

The chart below describes all the forms of Silica.
MINERAL CRYSTAL SYSTEM CLASS
Quartz
(alpha Quartz)
trigonal 3 2
Tridymite
(alpha Tridymite)
triclinic 2 / m 2 / m 2 / m or 2 / m
Cristobalite
(alpha Cristobalite)
tetragonal 4 2 2
Beta Quartz hexagonal 6 2 2
Beta Tridymite hexagonal 6 / m 2 / m 2 / m
Beta Cristobalite isometric 4 / m -3- 2 / m
Coesite monoclinic 2 / m
Stishovite tetragonal 4 / m 2 / m 2 / m
Lechatelierite amorphous ---
Keatite tetragonal 4 2 2


Quartz and Beta Quartz
Quartz is the most common as well as the most stable form of silica. Beta Quartz is only stable at temperatures above 1063º F (573º C). Thus, all Quartz specimens are alpha Quartz. However, when beta Quartz forms at hight temperatures and then cools down, it transforms to alpha Quartz but preserves the original beta Quartz shape, though it decreases in symmetry and adds some trigonal crystal faces. There are specific circumstances where 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 sometimes called "Beta Quartz" by collectors and dealers. They obviously cannot be beta Quartz at the current temperature, but are in reality alpha Quartz paramorphs of beta Quartz.


Tridymite and Beta Tridymite
Tridymite is stable at temperatures below 1598º F (870º C). At higher temperatures it is Beta Tridymite, with a different crystal structure. Tridymite specimens are all paramorphs of Beta Tridymite, and retain Beta Tridymite's original hexagonal shape. Most Tridymite specimens alter to Quartz while retaining their original, distinctive shape. Thus most "Tridymite" specimens are really Quartz paramorphs after Tridymite.


Cristobalite and Beta Cristobalite
Cristobalite is stable at temperatures below 514º F (268º C), At higher temperatures it is Beta Cristobalite, in a stable form, with a different crystal structure. Cristobalite specimens are all paramorphs of Beta Cristobalite, and, although tetragonal, retain Beta Cristobalite's original isometric shape.


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.


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 (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.


Lechatelierite
Lechatelierite is a very rare, natural form of silica that lacks a definitive crystal structure. It is amorphous and considered a natural glass, and is scientifically 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. 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


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