Moissanite

• Silicon carbide

Moissanite, chemically known as carborundum or silicon carbide, is a rarely occurring mineral from the mineral class of elements. It crystallizes in the hexagonal crystal system with the chemical composition of SiC and forms flat, rounded, hexagonal crystals of up to five millimeters in size. The mineral is colorless in the pure state, indicated by traces of other elements such as nitrogen, boron or aluminum, however, a wide range of colors from green ( nitrogen ) through blue to black (aluminum, boron).

Special Features

Moissanite is one of the hardest known naturally occurring substances, only diamond is harder. Moissanite is like diamond optically transparent, but is contrary to this birefringent.

Etymology and history

Moissanite was first in a mineral sample of the Canyon Diablo meteorite, which was found near the Barringer Crater, detected in 1904 by Henri Moissan. Its composition was first investigated in 1892 by François Ernest Mallard and 1893 by Georges Friedel; while they realized that it contained a very hard material that is inert to hydrochloric acid, and thought this was initially for diamond. 1904 Moissan could examine a larger amount of the meteorite he realized that the typical hexagonal crystals that the meteorite contains silicon carbide. The new mineral was named after the explorer Moissanite.

The artificial production of silicon carbide succeeded for the first time in 1891 by Edward Goodrich Acheson (patented 1893), moissanite gem quality could be shown for the first time in 1997.

Classification

In the classification by Strunz Moissanite is counted among the non-metals. After the 8th edition forms together with Chaoit, diamond, fullerite, graphite and lonsdaleite a group of semi - and non-metals. In the 9th edition is the only representative of non-metal carbides, a subset of non -metallic carbon and nitrogen compounds.

In the classification by Dana, it forms a separate subgroup of the semi - and non-metals.

Modifications and varieties

Moissanite is found in different polymorphic forms. Among them are several hexagonal, rhombohedral and cubic modifications. Most often you will find the hexagonal moissanite -6H- modification whose structure corresponds to that of wurtzite. Rarely, the cubic β - modification ( Mosang -3C) corresponding to the zinc blende structure, in front. She was found in the U.S. state of Wyoming. Of the 74 known in artificially produced silicon carbide modifications are known eight from nature.

Education and Locations

Silicon carbide is formed at high temperatures that occur on the earth in the mantle or when struck by meteorites. It first forms the α - modification at temperatures 1900-2000 ° C. The formation conditions are similar to those of diamond, so that the two minerals sometimes, as in Fuxian in the People's Republic of China, socialized occur in kimberlite. Is the α -SiC at elevated temperatures in contact with elemental silicon, and carbon dioxide present, in addition, the silicon may react with the carbon dioxide to β -SiC, which anneals to the α - SiC. Other minerals except diamond, moissanite is associated with which, iron ( in meteorites ), quartz, garnet, clinopyroxene, coesite, rutile, graphite, pyrrhotite and pyrite cobalt ( in kimberlite ).

Locations are different meteorites, as Indarch meteorite in Azerbaijan, the Krymka meteorite in Ukraine and the Canyon Diablo meteorite in Arizona; Impact craters such as the Ries; Volcanoes like Tolbachik on the Kamchatka peninsula (Russia) and diamond mines, such as Sakha ( Russia) and Kimberley in Western Australia.

Morphology

Moissanite typically crystallizes in hexagonal, tabular crystals. These are flattened along the plane, and rounded on the corners.

Crystal structure

In the most common α - modification crystallizes in the hexagonal crystal system moissanite in the space group P63mc ( Raumgruppen-Nr. 186) with the lattice parameters a = 3.073 Å and c = 15.08 Å and six formula units per unit cell. This corresponds to the wurtzite structure.

Use

Of course, due to the rareness occurring moissanite is not used economically. Silicon carbide but is synthesized from silica and carbon in large amounts. It is as carborundum an important abrasive, but is also used as a ceramic insulator and due to its semiconductor properties of light-emitting diodes, transistors, and varistors.

High purity moissanite crystals can be used on the basis of comparable properties as a diamond substitute. Although Mosang has a slightly lower hardness than diamond, but is thermally stable in air (up to 1127 ° C, only diamond to 837 ° C) and much cheaper to manufacture. It is therefore used in experiments at high pressure and high temperature.