Cristobalite

Cristobalite is a rarely occurring mineral and a naturally occurring high-temperature modification of the silica ( SiO2). Chemically, it is thus a crystalline form of the anhydride of silicic acid. Cristobalite itself exists in two modifications, tetragonal α -cristobalite ( Tiefcristobalit ) and cubic β -cristobalite ( Hochcristobalit ), which are structurally very closely related. The latter derived from the crystallized in a diamond structure, and is thus similar to the crystal structure of zinc blende (ZnS).

Etymology and history

The first description of cristobalite goes back to Gerhard vom Rath in 1884. The name is derived from the type locality, near San Cristóbal ( Chiapas, Mexico), from. The first structure determination of cristobalite by X-ray diffraction was carried out in 1925.

Classification

After Strunz'schen system that arranges the minerals according to their chemical composition, cristobalite is in the mineral class of oxides with a molar ratio of metal: oxygen = 1:2 classified and is there together with coesite, melanophlogite, Moganite, opal, quartz, stishovite and tridymite member of the quartz group.

The systematics of Dana arranges the minerals according to their crystal structure and since both Cristolbalit modifications from all four corners -sharing [ SiO4 ] tetrahedra are constructed in the results for silicon has a coordination number of 4 and the oxygen of 2, they will be after the systematics of Dana assigned to the framework silicates.

Modifications and varieties

Cristobalite exists in two modifications, the tetragonal α -cristobalite ( Tiefcristobalit ) and the cubic β -cristobalite ( Hochcristobalit ). Both structures are composed of corner-sharing [ SiO4 ] tetrahedra, which differ only in that the tetrahedra are twisted slightly different from each other. The crystallographic data of both modifications are given in the table.

The only previously known variety of the bulbous - traubig developed to fibrous Lussatit is called.

Crystal structure

Cristobalite is formed at temperatures above 1470 ° C, and is at standard conditions before metastable α - cristobalite. The low-temperature form α -cristobalite ( Tiefcristobalit ) converts at temperatures of about 240-275 ° C (depending on purity) in the high temperature form β -cristobalite ( Hochcristobalit ) to. The higher symmetry of the β -cristobalite it comes about through a coupled torsional vibration of the [ SiO4 ] tetrahedra, which changes the bond angle of Si -O -Si bonds by 147 ° in the tetragonal α -cristobalite at 180 °. Strictly speaking, the higher symmetry is only a snapshot of the continuous vibrations of the tetrahedron, the average cubic symmetry results. Below about 240 ° C, freeze the vibrations and the lower symmetry tetragonal observable.

Crystal structure of the cubic β - cristobalite. The [ SiO4 ] tetrahedra are compared to α -cristobalite otherwise twisted with each other, whereby the higher symmetry is produced.

Idealized unit cell of β - cristobalite. The [ SiO4 ] tetrahedra lead in reality of ongoing oscillations.

Synthetic production

Cristobalite is prepared industrially from pure quartz sand at high temperatures.

Use

Cristobalite is characterized by its high whiteness. Due to the strong reflection, it has a very high color saturation as a pigment. It is not as pure white as titanium dioxide ( titanium white ), but much more luminous.

It is mainly used in various particle sizes as pigment and filler, fine flour < 0.1 mm and Feinstmehle 8-12 microns in colorants, coarse flour and grain for wall plaster. Cristobalite shall continue in dental ceramics and sealants use.

Obsidian (volcanic glass) with included cristobalite crystals commonly found under the trade name Schneeflockenobsidian as gemstones or in the form of small sculptures using.