Andalusite (Al2O3·SiO2) is a natural mineral with low thermal conductivity, stable volume, high mechanical strength, excellent creep resistance and CO erosion resistance. It can be directly used to make refractory materials without calcination. It is used in metallurgy and building materials industries. Compared with high alumina refractory raw materials such as scorch gemstone, high alumina bauxite, sintered mullite and brown corundum, andalusite transforms into a unique composite structure at high temperature: net-like mullite with capillary pores and filled Silica-rich glass phase. It has excellent slag resistance and thermal shock resistance. Studies have shown that the mullite reaction of andalusite is affected by factors such as chemical composition, temperature and particle size, which largely determines the performance of andalusite products. This article describes the composition and structure of andalusite, the changes during heating, and typical applications.
Composition and structure of andalusite
Andalusite is usually columnar crystals with a cross section close to a square. The content of andalusite phase is generally 90% to 95%, and the impurity components are arranged in the crystal orientation and are cross-shaped on the cross section. It is called hollow spar, usually carbonaceous, iron oxide, mica (sericite and black Mica), ilmenite and quartz, some impurities also appear in the crystal lattice of andalusite. Worldwide, andalusite is rich in reserves, the main producing areas are South Africa and France, and the content is usually 8% to 25% (w). The particle size of commercially available andalusite ranges from 2μm to 8mm, and the content of Al2O3 is between 55% and 61% (w).
The crystal structure of andalusite belongs to the orthorhombic system, a=0.778nm, b=0.792nm, c=0.557nm, z=4. Al-O octahedrons are connected in a common edge manner and are connected in a chain along the c-axis direction. The chains are connected with Al and Si-O tetrahedra with a coordination number of 5. The anion has two coordination situations: one is to participate in the Si-O tetrahedron, which is connected with 1 Si and 2 Al; the other is to not participate in the Si-O tetrahedron, and only connects with 3 Al. Andalusite crystals are rhombohedral columns, which are consistent with the extension direction of the Al-O octahedral chain. The Al3+ in the Al-O octahedron is replaced by Fe3+ and turns red, and it is replaced by Mn3+ and turns green.
2. Heating changes of andalusite
Andalusite has obvious anisotropy. M.Ghassemi et al. took samples along different crystalline directions of andalusite (see Figure 1), and measured and compared its expansion behavior below 900°C. At this time, the sample did not undergo a phase change and was still in the andalusite phase. The results are as follows Figure 2 shows: the thermal expansion coefficient in the a-axis direction is the largest, and the thermal expansion in the c-axis direction is the smallest.
