Mullite

Mullite is a rarely occurring mineral from the mineral class of silicates, more precisely an island silicate tetrahedron with foreign anions. It crystallizes in the orthorhombic crystal system with the chemical formula - 2xO10 xSi2 Al2Al2 2 -x where x = oxygen vacancies per unit cell, and usually develops small, prismatic crystals in white, yellow, red, or violet color. Even colorless mullite crystals are known.

Special Features

The thermal expansion of mullite with a high degree of purity is at about 1100 ° C a change in the coefficient of thermal expansion that is attributable to a phase transition, and the annealing of defects. Mullite is a refractory structure -destroying share, resulting in clay during firing of refractory bricks from the binder content due to this thermal expansion.

At temperatures above 250 to 300 ° C mullite has a higher microhardness than corundum.

Commercially available mullite has a glass phase fraction ( in SEM images by fine lines in a 120 ° angle to each other recognizable), through which the melting point is lowered. By the production method of the composition is called synthetic mullite 3Al2O3 2SiO2 • as sintered mullite and mullite composition of 2Al2O3 1SiO2 • a fused mullite.

Mullite to sillimanite chemically similar in all physical and optical properties is very similar.

Etymology and history

Further distinctions in the names are related to the conditions of formation of the mullite. Thus, in the solid phase formed mullite grain is called Schuppenmullit while forming in the presence of a melt mullite needles. At low temperatures, formed mullite is also called Primärmullit and referred to from this newly formed at high temperatures and recrystallized mullite as Sekundärmullit.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the mullite belonged to the mineral class of " silicates and Germanates " and then to the Department of " island silicates with tetrahedral foreign anions ( Neso - Subsilikate ) ", where he, together with andalusite, Kanonait, kyanite and sillimanite, topaz and Yoderit was the unnamed group VIII/B.02.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the mullite also in the class " silicates and Germanates " and then in the Department of the " island silicates ( nesosilicates ) ". This division, however, is further subdivided according to the possible presence of other anions and the coordination of the cations involved, so that the mineral according to its composition in the subdivision of " island silicates with additional anions; Cations iner, he is and / or to find nurer coordination ", where it forms the unnamed group 9.AF.20 together with Krieselit

The commonly used in English-speaking classification of minerals according to Dana assigns the mullite also in the class of silicates, there, however, in the department of " island silicates with SiO 4 Groups and O, OH, F, and H 2 O with cations in and > coordination " where he together with sillimanite and Boromullit the " Al2SiO5 ( sillimanite - subgroup ) " with the system no. 52.2.2a forms.

Modifications and varieties

Of mullite, it is a metastable pseudotetragonale modification which is attributed to the formation of domains, and / or twinning. This road leads into the orthorhombic modification above 1000 ° C.

Education and Locations

Mullite formed by metamorphosis of kaolinite at about 1200 ° C or as a decomposition product of sillimanite at about 1000 ° C ( → sillimanite, mullite SiO2).

Locations include Pechbrunn, Wiesau and the Eifel in Germany, Le Puy -en- Velay in France, Marrubiu in Italy, Isle of Mull in Scotland, Radvanice ( Okres Trutnov ) in the Czech Republic, Celldömölk in Hungary, as well as in New Jersey in the USA.

Crystal structure

Mullite crystallizes in the orthorhombic crystal system, space group Pbam with the lattice parameters a = 7.5785 Å, b = 7.6817 Å and c = 2.8864 Å, and one formula unit per unit cell.

Use

By starting mineral kaolinite mullite is formed as an integral part in the production of porcelain and bricks and fireclay bricks.