Cristobalite
| Cristobalite | |
|---|---|
 size: 5.9 cm × 3.8 cm × 3.8 cm |
|
| General and classification | |
| chemical formula | SiO 2 |
|
Mineral class (and possibly department) |
Oxides |
|
System no. to Strunz and to Dana |
4.DA.15 ( 8th edition : IV / D.01) 75.01.01.01 |
| Similar minerals | Quartz , tridymite , coesite |
| Crystallographic Data | |
| Crystal system |
tetragonal (α-cristobalite) cubic (β-cristobalite) |
| Crystal class ; symbol | tetragonal-trapezoidal; 422 cubic hexakisoctahedral; 4 / m 3 2 / m |
| Frequent crystal faces | {111} |
| Twinning | often, interpenetrating twins according to the spinel law; Adhesion plane (111) |
| Physical Properties | |
| Mohs hardness | 6.5 |
| Density (g / cm 3 ) | 2.33 |
| Cleavage | no |
| Break ; Tenacity | shell-like |
| colour | colorless, white, partly yellowish; colorless in thin section |
| Line color | White |
| transparency | translucent to opaque |
| shine | Glass gloss |
| Crystal optics | |
| Refractive index | n = 1.485 to 1.487 |
| Birefringence | δ = 0.003 |
| Optical character | negative (α-cristobalite) |
| Axis angle | 2V = k. A. |
| Pleochroism | not known |
| Other properties | |
| Chemical behavior | insoluble in water and acids , exception: hydrofluoric acid (HF) well soluble in sodium carbonate melts (Na 2 CO 3 ) |
| Special features | no |
Cristobalite is a rather rare mineral and a naturally occurring high-temperature modification of silicon dioxide (SiO 2 ). From a chemical point of view, it is therefore a crystalline form of the anhydride of silica . Cristobalite itself exists in two modifications, tetragonal α-cristobalite ( deep cristobalite ) and cubic β-cristobalite ( high cristobalite ), which are structurally very closely related. The latter crystallizes in a structure derived from diamond and is therefore also comparable to the crystal structure of the zinc blende (ZnS).
Etymology and history
The first description of cristobalite goes back to Gerhard vom Rath from 1884. The name is derived from the type locality near San Cristóbal ( Chiapas , Mexico ). The first structure determination of cristobalite with the help of X-ray diffraction took place in 1925.
classification
According to Strunz 's system , which classifies the minerals according to their chemical composition, cristobalite is classified in the mineral class of oxides with a molar ratio of metal: oxygen = 1: 2 and is there together with coesite , melanophlogite , mogánite , opal , quartz , stishovite and tridymite member of the quartz group .
Dana's systematics classifies the minerals according to their crystal structure and since both cristolbalite modifications are made up of [SiO 4 ] tetrahedra linked over all four corners , with a coordination number of 4 for silicon and 2 for oxygen , these are followed assigned to the scaffold silicates in the Dana system.
Crystal structure
Cristobalite forms at temperatures above 1470 ° C and is present as metastable α-cristobalite under standard conditions . The low-temperature form α-cristobalite ( Tiefcristobalit ) converts at temperatures of about 240-275 ° C (depending on purity) in the high temperature form of β-cristobalite ( Hochcristobalit ) around. The higher symmetry of the β-cristobalite comes about through a coupled torsional vibration of the [SiO 4 ] tetrahedra, whereby the bond angle of the Si-O-Si bonds changes from 147 ° in the tetragonal α-cristobalite to 180 °. Strictly speaking, the higher symmetry is only a snapshot of the continuous oscillations of the tetrahedra, which, averaged, results in the cubic symmetry. Below approx. 240 ° C the vibrations freeze and the lower tetragonal symmetry can be observed.
Crystal structure of the tetragonal α-cristobalite viewed along the a-axis
Crystal structure of the cubic β-cristobalite. The [SiO 4 ] tetrahedra are twisted differently to one another compared to α-cristobalite, which results in the higher symmetry.
Idealized unit cell of β-cristobalite. The [SiO 4 ] tetrahedra actually carry out continuous vibrations.
properties
The melting point of cristobalite is 1713 ° C.
Modifications and varieties
Cristobalite exists in two modifications, the tetragonal α-cristobalite ( deep cristobalite ) and the cubic β-cristobalite ( high cristobalite ). Both structures consist of corner-linked [SiO 4 ] tetrahedra , which differ only in that the tetrahedra are twisted slightly differently from one another. The crystallographic data for both modifications are given in the table.
| Crystallographic data for cristobalite | ||
|---|---|---|
| modification | α-cristobalite | β-cristobalite |
| Alternative name | Deep cristobalite | High cristobalite |
| Crystal system | tetragonal | cubic |
| Crystal class | tetragonal-trapezoidal; 422 | cubic hexakisoctahedral; 4 / m 3 2 / m |
| Space group | P 4 1 2 1 2 (No. 92) | Fd 3 m (No. 227) |
| Lattice parameters |
a = 4.98 Å c = 6.95 Å |
a = 7.17 Å |
| Number of formula units ( Z ) | 4th | 8th |
The only known variety is lussatite , which is tuberous, grape-like or fibrous .
Education and Locations
372 locations are known of cristobalite.
In Germany a number of localities is known. Most of them are located in Rhineland-Palatinate in the Eifel Mountains, other sites in Rhineland-Palatinate are the Königsberg near Wolfstein in the North Palatinate Bergland or near Braubach in the Rhein - Lahn district . In Bavaria there is a site in the Fichtel Mountains , in Swabia and in the Upper Palatinate . There are also three sites in Hessen . In North Rhine-Westphalia there are two sites near Aachen and one in the Sauerland . In Saxony , there is a locality in Königshain in Görlitz in Upper Lusatia . In Thuringia there is a site near Gera .
There are six sites in Austria . In Burgenland there is one site on Pauliberg near Kobersdorf in the Oberpullendorf district and one in Kohfidisch in the Oberwart district . In Carinthia there is a site in the municipality of St. Paul im Lavanttal in the Wolfsberg district . There are three sites in Styria : one in Bad Gleichenberg , one in Bad Radkersburg and one in Graz .
In Switzerland only one locality has been discovered, namely in Breno in the municipality Alto Malcantone in county Breno in Ticino .
There are other sites in Algeria , Antarctica , Argentina , Australia , Azerbaijan , Bolivia , Brazil , Chile , China , Costa Rica , Denmark , Ecuador , Fiji , France , Georgia , Greece , Iceland , India , in the Indian Ocean , Indonesia , Israel , Italy , Japan , Canada , Kazakhstan , Mexico , Mozambique , Namibia , New Zealand , Nigeria , the Pacific Ocean , Pakistan , Papua New Guinea , Peru , Poland , Portugal , Romania , Russia , Saint Lucia , Slovakia , Spain , Tunisia , the Turkey , Hungary , Ukraine , the United Kingdom and the United States of America .
Cristobalite could also be detected outside the earth. On the one hand, it was found in rock samples from Gale crater on Mars and, on the other hand, in various rock samples on the moon near the landing sites of the Apollo 11 , Apollo 14 , Apollo 16 and Apollo 17 lunar missions .
Synthetic manufacture
Cristobalite is technically made 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 it is far more luminous.
It is mainly used in different grain sizes as a pigment and filler , fine meal <0.1 mm and fine meal 8–12 µm in colorants , coarse meal and grain size for wall plaster . Cristobalite flours are still used in dental ceramics and sealants .
Obsidians (volcanic glass) with enclosed cristobalite crystals are often used as gemstones or in the form of small sculptures under the trade name snowflake obsidian .
literature
- G. von Rath: Ueber cristobalit vom Cerro S. Cristóbal near Pachuca (Mexico) In: New Yearbook for Mineralogy, Geology and Palaontology , 1887, pp. 198–199 ( PDF )
- WA Dollase: Reinvestigation of the structure of low cristobalite In: Zeitschrift für Kristallographie , Volume 121, 1965, pp. 369–377, doi: 10.1524 / zkri.1965.121.16.369 ( PDF )
- WA Deer, RA Howie, J. Zussman: An Introduction to the Rock Forming Minerals . Prentice Hall, Harlow 1992, ISBN 0-582-30094-0 , pp. 457 f. (English)
- M. Okrusch, S. Matthes: Mineralogy . Springer, Berlin 2005, ISBN 3-540-23812-3
- U. Müller: Inorganic Structural Chemistry . 5th edition, Teubner, Stuttgart 2006, ISBN 3-8351-0107-2 , p. 184 f.
Web links
- Mineral Atlas: Cristobalite (Wiki)
- Cristobalite on webmineral.com
- Silica chemistry: stuffed cristobalite
- Database-of-Raman-spectroscopy - cristobalite (database, among other things with further literature)
- American-Mineralogist-Crystal-Structure-Database - Cristobalit (crystal structure database )
Individual evidence
- ^ Fillers ( Memento of September 21, 2008 in the Internet Archive ), Kremer Pigments
- ↑ RWG Wyckoff: The crystal structure of the high temperature form of cristobalite . In: American Journal of Science . Series 5, No. 9, 1925, pp. 448-459.
- ↑ William Alexander Deer, Robert Andrew Howie, J. Zussman: An Introduction to the Rock Forming Minerals Longman Scientific & Technical, 1966, pp. 340-355, ISBN 978-0-582-44210-8
- ^ WA Dollase: Reinvestigation of the structure of low cristobalite. In: Journal for crystallography, crystal geometry, crystal physics, crystal chemistry. No. 121, 1965, pp. 369–377, doi: 10.1524 / zkri.1965.121.16.369 ( PDF 410.9 kB )
- ^ DR Peacor: High-temperature single-crystal study of the cristobalite inversion. In: Journal for crystallography, crystal geometry, crystal physics, crystal chemistry. No. 138, 1973, pp. 127-220, doi: 10.1524 / zkri.1973.138.jg.274 .
- ↑ 2.2.3.10 β-cristobalite structure. In: R. Alsfasser, C. Janiak, TM Klapötke, H.-J. Meyer: Modern Inorganic Chemistry . Ed .: Erwin Riedel. 3. Edition. De Gruyter, Berlin [a. a.] 2007, ISBN 978-3-11-019060-1 , pp. 196 . (accessed via De Gruyter Online)
- ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 5th completely revised and supplemented edition. Weise, Munich 2008, ISBN 978-3-921656-70-9 .
- ↑ Mindat - Cristobalite
- ↑ List of locations for cristobalite in the Mineralienatlas and Mindat
- ↑ Mindat - Gale Crater, Aeolis quadrangle, Mars
- ↑ Mindat - Locations for cristobalite on the moon at the landing sites of the missions Apollo 11 , Apollo 14 , Apollo 16 and Apollo 17