Mullite or porcelainite is a rare

silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consi ...

formed during contact metamorphism of

clay minerals Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay mineral ...

. It can form two

stoichiometric Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...

forms: 3 Al₂ O₃2 SiO₂ or 2Al₂O₃ SiO₂. Unusually, mullite has no charge-balancing

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s present. As a result, there are three different aluminium sites: two distorted tetrahedral and one octahedral. Mullite was first described in 1924 for an occurrence on the Isle of Mull,

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to th ...

. It occurs as argillaceous inclusions in

volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcan ...

s in the Isle of Mull, inclusions in sillimanite within a tonalite at Val Sissone,

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...

and with emerylike rocks in Argyllshire, Scotland.

Porcellanite

Mullite (porcelainite) can be found as a constituent mineral in a type of thermally-metamorphosed rock called porcellanite.

Use in porcelain

Mullite is present in the form of needles in

porcelain Porcelain () is a ceramic material made by heating substances, generally including materials such as kaolinite, in a kiln to temperatures between . The strength and translucence of porcelain, relative to other types of pottery, arises main ...

. It is produced during various melting and firing processes, and is used as a refractory material,H. Schneider & S. Komarneni (2005) Mullite. Wiley, VCH, 509 pp, because of its high melting point of 1840 °C. In 2006 researchers at

University College London , mottoeng = Let all come who by merit deserve the most reward , established = , type = Public research university , endowment = £143 million (2020) , budget = � ...

and

Cardiff University , latin_name = , image_name = Shield of the University of Cardiff.svg , image_size = 150px , caption = Coat of arms of Cardiff University , motto = cy, Gwirionedd, Undod a Chytgord , mottoeng = Truth, Unity and Concord , established = 1 ...

discovered that potters in the

Hesse Hesse (, , ) or Hessia (, ; german: Hessen ), officially the State of Hessen (german: links=no, Land Hessen), is a state in Germany. Its capital city is Wiesbaden, and the largest urban area is Frankfurt. Two other major historic cities are Da ...

region of Germany since the late

Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire ...

had used mullite in the manufacture of a type of crucible (known as Hessian crucibles), 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 is also 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 Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

, 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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is o ...

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 contributes to the high mechanical strength of porcelain.

Use as a catalyst

Recent research indicates that a synthetic analogue of mullite can be an effective replacement for platinum in diesel engines for exhaust management.

Notes

* * * {{Authority control Nesosilicates Orthorhombic minerals Minerals in space group 58 Minerals in space group 62