Cristobalite
Encyclopedia
The mineral
cristobalite is a high-temperature polymorph
of silica, meaning that it has the same chemical formula, SiO2
, but a distinct crystal structure. Both quartz
and cristobalite are polymorphs with all the members of the quartz group which also include coesite
, tridymite
and stishovite
. Cristobalite occurs as white octahedra or spherullites in acidic volcanic rocks and in converted diatomaceous deposits in the Monterey Formation of the US state of California and similar areas. Cristobalite is stable only above 1470 °C, but can crystallize and persist metastably at lower temperatures.
The persistence of cristobalite outside of its thermodynamic stability range occurs because the transition from cristobalite to quartz
or tridymite
is "reconstructive", requiring the breaking up and reforming of the silica framework. These frameworks are composed of Si
O
4 tetrahedra in which every oxygen atom is shared with a neighbouring tetrahedron, so that the chemical formula
of silica is Si
O
2. The breaking of these bonds required to convert cristobalite to tridymite and quartz requires considerable activation energy
and may not happen on a human time frame. Framework silicates are also known as tectosilicates
.
There is more than one form of the cristobalite framework. At high temperatures, the structure is cubic
, Fdm, No.227, Pearson symbol
cF104. A tetragonal
form of cristobalite (P41212, No. 92, Pearson symbol tP12) occurs on cooling below ca. 250 °C at ambient pressure, and is related to the cubic form by a static tilting of the silica tetrahedra in the framework. This transition is variously called the low-high or
transition. It may be termed "displacive"; i.e., it is not generally possible to prevent the cubic β-form from becoming tetragonal by rapid cooling. Under rare circumstances the cubic form may be preserved if the crystal grain is pinned in a matrix that does not allow for the considerable spontaneous strain that is involved in the transition, which causes a change in shape of the crystal. This transition is highly discontinuous. The exact transition temperature depends on the crystallinity of the cristobalite sample, which itself depends on factors such as how long it has been annealed at a particular temperature.
The cubic β-phase consists of dynamically disordered silica tetrahedra. The tetrahedra remain fairly regular and are displaced from their ideal static orientations due to the action of a class of low-frequency phonons called rigid unit modes. It is the "freezing" of one of these rigid unit modes that is the soft mode for the α–β transition.
In the α–β phase transition only one of the three degenerate cubic crystallographic axes retains a fourfold rotational axis in the tetragonal form. The choice of axis is arbitrary, so that various twins
can form within the same grain. These different twin orientations coupled with the discontinuous nature of the transition can cause considerable mechanical damage to materials in which cristobalite is present and that pass repeatedly through the transition temperature, such as refractory bricks.
When devitrifying silica, cristobalite is usually the first phase to form, even when well outside of its thermodynamic stability range. The dynamically disordered nature of the β-phase is partly responsible for the low enthalpy of fusion of silica.
The micrometre-scale spheres that make up precious opal
are made of cristobalite, crystallized metastably at low temperature.
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
cristobalite is a high-temperature polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
of silica, meaning that it has the same chemical formula, SiO2
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
, but a distinct crystal structure. Both quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and cristobalite are polymorphs with all the members of the quartz group which also include coesite
Coesite
Coesite[p] is a form of silicon dioxide SiO2 that is formed when very high pressure , and moderately high temperature , are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953. In 1960, coesite was found by Edward C. T...
, tridymite
Tridymite
Tridymite is a high-temperature polymorph of quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal triclinic crystals, or scales, in cavities in acidic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in 1868 and the type location is in Hidalgo,...
and stishovite
Stishovite
Stishovite is an extremely hard, dense tetragonal form of silicon dioxide. It was long considered the hardest known oxide; however, boron suboxide has recently been discovered to be much harder...
. Cristobalite occurs as white octahedra or spherullites in acidic volcanic rocks and in converted diatomaceous deposits in the Monterey Formation of the US state of California and similar areas. Cristobalite is stable only above 1470 °C, but can crystallize and persist metastably at lower temperatures.
The persistence of cristobalite outside of its thermodynamic stability range occurs because the transition from cristobalite to quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
or tridymite
Tridymite
Tridymite is a high-temperature polymorph of quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal triclinic crystals, or scales, in cavities in acidic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in 1868 and the type location is in Hidalgo,...
is "reconstructive", requiring the breaking up and reforming of the silica framework. These frameworks are composed of Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
4 tetrahedra in which every oxygen atom is shared with a neighbouring tetrahedron, so that the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
of silica is Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
2. The breaking of these bonds required to convert cristobalite to tridymite and quartz requires considerable activation energy
Activation energy
In chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction...
and may not happen on a human time frame. Framework silicates are also known as tectosilicates
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
.
There is more than one form of the cristobalite framework. At high temperatures, the structure is cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
, Fdm, No.227, Pearson symbol
Pearson symbol
The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W.B. Pearson. The symbol is made up of two letters followed by a number. For example:* Diamond structure, cF8...
cF104. A tetragonal
Tetragonal crystal system
In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
form of cristobalite (P41212, No. 92, Pearson symbol tP12) occurs on cooling below ca. 250 °C at ambient pressure, and is related to the cubic form by a static tilting of the silica tetrahedra in the framework. This transition is variously called the low-high or
transition. It may be termed "displacive"; i.e., it is not generally possible to prevent the cubic β-form from becoming tetragonal by rapid cooling. Under rare circumstances the cubic form may be preserved if the crystal grain is pinned in a matrix that does not allow for the considerable spontaneous strain that is involved in the transition, which causes a change in shape of the crystal. This transition is highly discontinuous. The exact transition temperature depends on the crystallinity of the cristobalite sample, which itself depends on factors such as how long it has been annealed at a particular temperature.The cubic β-phase consists of dynamically disordered silica tetrahedra. The tetrahedra remain fairly regular and are displaced from their ideal static orientations due to the action of a class of low-frequency phonons called rigid unit modes. It is the "freezing" of one of these rigid unit modes that is the soft mode for the α–β transition.
In the α–β phase transition only one of the three degenerate cubic crystallographic axes retains a fourfold rotational axis in the tetragonal form. The choice of axis is arbitrary, so that various twins
TWINS
Two Wide-Angle Imaging Neutral-Atom Spectrometers are a pair of NASA instruments aboard two United States National Reconnaissance Office satellites in Molniya orbits. TWINS was designed to provide stereo images of the Earth's ring current. The first instrument, TWINS-1, was launched aboard USA-184...
can form within the same grain. These different twin orientations coupled with the discontinuous nature of the transition can cause considerable mechanical damage to materials in which cristobalite is present and that pass repeatedly through the transition temperature, such as refractory bricks.
When devitrifying silica, cristobalite is usually the first phase to form, even when well outside of its thermodynamic stability range. The dynamically disordered nature of the β-phase is partly responsible for the low enthalpy of fusion of silica.
The micrometre-scale spheres that make up precious opal
Opal
Opal is an amorphous form of silica related to quartz, a mineraloid form, not a mineral. 3% to 21% of the total weight is water, but the content is usually between 6% to 10%. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most...
are made of cristobalite, crystallized metastably at low temperature.
Further reading
- American Geological Institute Dictionary of Geological Terms.
- Durham, D.L., "Monterey Formation:Diagenesis. In: Uranium in the Monterey Formation of California". US Geological Survey Bulletin 1581-A, 1987.
- Reviews in Mineralogy and Geochemistry, vol. 29., Silica: behavior, geochemistry and physical applications. Mineralogical Society of America, 1994.
- R.B. Sosman, The Phases of Silica. (Rutgers University Press, 1965)