What is mullite?

Is mullite produced in 3Al? O? ·2SiO? The crystalline phase is the main component of refractory raw material. Mullite is divided into natural mullite and synthetic mullite. Natural mullite is rare and is generally synthetic.
The chemical composition of mullite is Al? O? 71.8%, SiO? 28.2%. The mineral structure is rhombic crystal system, and the crystals are arranged in long columns, needles, and chains. Needle mullite intersperses with each other in the products to form a solid skeleton. There are three types of mullite: α-mullite, which is equivalent to pure 3Al? O? ·2SiO? Type 3:2 for short; β-mullite, solid solution with excess aluminum oxide, lattice slightly expanded, referred to as 2:1 type; Gamma-mullite, solid solution with small amounts of titanium oxide and iron trioxide. Mullite is chemically stable and insoluble in hydrofluoric acid. Its density is 3.03g/cm3, Mohs hardness 6-7, melting point 1870℃, thermal conductivity (1000℃) is 13.8W/(m.K), linear expansion coefficient (20-1000℃) is 5.3*10?? /℃, elastic modulus 1.47*10?? Pa.
Because mullite has good high temperature mechanical and high temperature thermal properties, synthetic mullite and its products have the advantages of high density and purity, high temperature structural strength, low high temperature creep rate, low thermal expansion rate, strong chemical erosion resistance, thermal shock resistance and so on.
The key indexes to evaluate the quality level of mullite are the phase composition and density of mullite.