Mullite
Encyclopedia
Mullite or porcelainite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge balancing cations present. As a result, there are three different Al sites: two distorted tetrahedral Al sites and one Al other site which adopts a higher co-ordinate octahedral state.
Mullite was first described in 1924 for an occurrence on the Isle of Mull
, Scotland
. It occurs as argillaceous inclusions in volcanic rock
s in the Isle of Mull, inclusions in sillimanite
within a tonalite
at Val Sissone, Italy
and with emery
like rocks in Sithean Sluaigh, Scotland.
.
It is produced artificially during various melting and firing processes, and is used as a refractory material, due to its high melting point of 1840°C.
In 2006 researchers at University College London
and Cardiff University
discovered that potters in the Hesse
region of Germany since the late Middle Ages
had used mullite in the manufacture of a type of crucible
(known as Hessian crucible
s), that were renowned for enabling alchemists to heat their crucibles to very high temperatures.
The formula for making it (using kaolinitic clay and then firing it at temperatures above 1100 °C) was kept a closely guarded secret.
Mullite morphology also is important for its application. in this case, there are two common morphologies for mullite. One is a platelet shape with low aspect ratio and the second is a needle shape with high aspect ratio. If the needle shape mullite can form in a ceramic body during sintering, it has an effect on both the mechanical and physical properties by increasing the mechanical strength and thermal shock resistance. The most important condition relates to ceramic chemical composition. If the silica and alumina ratio with low basic materials such as sodium and calcium is adjusted, the needle shape mullite forms at about 1400 °C and the needles will interlock. This mechanical interlocking causes the porcelain to have high mechanical strength.
Mullite was first described in 1924 for an occurrence on the Isle of Mull
Isle of Mull
The Isle of Mull or simply Mull is the second largest island of the Inner Hebrides, off the west coast of Scotland in the council area of Argyll and Bute....
, Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
. It occurs as argillaceous inclusions in volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
s in the Isle of Mull, inclusions in sillimanite
Sillimanite
Sillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....
within a tonalite
Tonalite
Tonalite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.In older references tonalite is...
at Val Sissone, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
and with emery
Emery (mineral)
Emery is a very hard rock type used to make abrasive powder. It largely consists of the mineral corundum , mixed with other species such as the iron-bearing spinels hercynite and magnetite, and also rutile...
like rocks in Sithean Sluaigh, Scotland.
Use in porcelain
Mullite is present in the form of needles in porcelainPorcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...
.
It is produced artificially during various melting and firing processes, and is used as a refractory material, due to its high melting point of 1840°C.
In 2006 researchers at University College London
University College London
University College London is a public research university located in London, United Kingdom and the oldest and largest constituent college of the federal University of London...
and Cardiff University
Cardiff University
Cardiff University is a leading research university located in the Cathays Park area of Cardiff, Wales, United Kingdom. It received its Royal charter in 1883 and is a member of the Russell Group of Universities. The university is consistently recognised as providing high quality research-based...
discovered that potters in the Hesse
Hesse
Hesse or Hessia is both a cultural region of Germany and the name of an individual German state.* The cultural region of Hesse includes both the State of Hesse and the area known as Rhenish Hesse in the neighbouring Rhineland-Palatinate state...
region of Germany since the late Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...
had used mullite in the manufacture of a type of crucible
Crucible
A crucible is a container used for metal, glass, and pigment production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents...
(known as Hessian crucible
Hessian crucible
A Hessian crucible is a type of ceramic crucible that was manufactured in the Hesse region of Germany from the late Middle Ages through the Renaissance period. They were renowned for their ability to withstand very high temperatures, rapid changes in temperature, and strong reagents. These...
s), that were renowned for enabling alchemists to heat their crucibles to very high temperatures.
The formula for making it (using kaolinitic clay and then firing it at temperatures above 1100 °C) was kept a closely guarded secret.
Mullite morphology also is important for its application. in this case, there are two common morphologies for mullite. One is a platelet shape with low aspect ratio and the second is a needle shape with high aspect ratio. If the needle shape mullite can form in a ceramic body during sintering, it has an effect on both the mechanical and physical properties by increasing the mechanical strength and thermal shock resistance. The most important condition relates to ceramic chemical composition. If the silica and alumina ratio with low basic materials such as sodium and calcium is adjusted, the needle shape mullite forms at about 1400 °C and the needles will interlock. This mechanical interlocking causes the porcelain to have high mechanical strength.