Tridymite

• Asmanit

Tridymite (also Asmanit ) is a high temperature orthorhombic modification of quartz, a crystalline form of silicon dioxide. Only since the 1960s tridymite has been widely recognized as a stable phase of SiO2. Another high-temperature modification of quartz cristobalite.

Etymology and history

In July 1867 Gerhard vom Rath received two samples of trachytic porphyry, by the Cerro San Cristóbal near Pachuca de Soto ( Hidalgo, Mexico ) were obtained. In these samples, he discovered in addition to the already known minerals hematite ( hematite) and hornblende also an unknown, colorless mineral of unusual crystal shape. This turned out to be characteristic Drillingsverwachsung basis of which Gerhard vom Rath described the mineral as tridymite after the Greek word τρίδυμο [ tridymo ] for Drilling.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz tridymite belonged to the mineral class of " oxides and hydroxides " and then to the Department of " Oxides with metal: oxygen = 1: 2", where he, together with coesite, cristobalite, melanophlogite, Moganite, opal, quartz and stishovite, the " quartz group " with the system no. IV/D.01 formed.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the tridymite also in the class of " oxides and hydroxides " and there in the department of " Metal: Oxygen = 1: 2 and comparable " one. This division, however, is further divided according to the size of the involved cations and the crystal structure or membership of a related mineral family, so that the mineral "With small cations: Silica family," according to its composition in the subdivision is to be found where there are only still forms the unnamed group 4.DA.10 along with opal.

In contrast to the Strunz 's Mineral classification predominantly common in English-speaking classification of minerals according to Dana assigns the tridymite in the class of " silicates and Germanates " and there in the department of " framework silicate minerals " one. Here he is the only member of the unnamed group 75.01.02 within the subdivision " framework silicates: tetrahedral Si lattice, with coordinated Si - SiO2 " to find.

Modifications and varieties

Tridymite is stable up to 3 kbar and transforms under atmospheric pressure at 870 ° C in high-quartz and cristobalite at 1470 ° C in order.

Education and Locations

As a rather rare mineral tridymite formation at different sites can indeed be abundant in some cases, overall it is not very common. So far (as of 2012) are considered to be known around 300 localities. Apart from its type locality Cerro San Cristóbal Hidalgo the mineral still occurred in Mexico in the " La Esperanza mine ," the " Barranca Mine" and " Remedios Mine" in Durango, the " Santín Mine" in Guanajuato and the " Tocho Mine" in San Luis Potosí to light.

In Germany the mineral appeared primarily in Rhineland -Palatinate, where it could be found at many sites in the Eifel, so among other things at Andernach, down, Ettringen and Mendig. In addition, tridymite but also from Baden- Württemberg, Bavaria, Hesse, North Rhine -Westphalia, Saxony and Thuringia is known.

In Austria, tridymite found among others in the basalt quarry at Pauliberg in Burgenland and the quarry " Schlarbaum " Klausen ( Bad Gleichenberg ) on Stradner Kogel and basalt quarry near Klöch in Styria.

Other localities lie among other things in Antarctica, Argentina, Azerbaijan, Australia, Bolivia, Brazil, Burundi, China, Ecuador, France, Greece, Iceland, India, Indonesia, Israel, Italy, Japan, Canada, Madagascar, Namibia, the Netherlands, in New Zealand, Oman, Papua New Guinea, Peru, Romania, Russia, Slovakia, Spain, South Africa, Tajikistan, Czech Republic, Turkey, Hungary, the United Kingdom ( UK) and the United States of America (USA).

Furthermore, tridymite still found in the basalt of the lunar sea " Sea of ​​Tranquility " from which the crew of the first moon landing mission Apollo 11 some rock samples brought back as well as in the basalt of Fra -Mauro highlands, which was attended by the crew of the Apollo 14 mission.

Crystal structure

Tridymite is at room temperature of monoclinic symmetry, but there are a large number of temperature-dependent structural modifications, whose complexity increases with decreasing temperature. The determination of these phases was completed in the 1980s. With decreasing temperature, these phases are:

The most important stage is HP- tridymite, it is the ideal high-temperature phase of tridymite. It is composed of the same layers of SiO2 tetrahedra, which are arranged in hexagonal rings. These layers are stacked in an ABAB sequence and can thus freely through the tunnel.

With decreasing temperature in OC Tridymite first performs a tilting of the tetrahedra to form a Verscherung behind the other layers. The remaining modifications in addition, the hexagonal rings are deformed to ditrigonalen and oval configurations, which alternate in a way characteristic of the modification superstructure.