| 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.
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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