Mg2Al4Si5O18

Mg2Al4Si5O18 is a magnesium aluminum silicate mineral and the magnesium end-member of cordierite, whose general formula is (Mg,Fe)2Al4Si5O18. In natural samples, iron commonly substitutes for magnesium, giving a range of compositions around the magnesium-rich end-member Mg2Al4Si5O18.

Crystal structure and properties: Cordierite is an orthorhombic framework silicate. Its structure consists of SiO4 tetrahedra linked to Al in tetrahedral and octahedral sites, forming a three-dimensional network with channels parallel to one crystallographic axis. This arrangement contributes to its relatively low thermal expansion and stability at high temperatures. Physical properties typical of the mineral include a hardness near 7, a specific gravity around 2.62, and a vitreous to waxy luster. Coloring is usually colorless to pale gray or blue-gray, and cleavage is imperfect.

Occurrence and formation: Mg-rich cordierite forms in high-temperature metamorphic rocks, particularly aluminous granulites and migmatites, under conditions of high temperature and relatively low water activity. It can also occur in some granitic and pegmatitic rocks. In metamorphic assemblages, cordierite commonly associates with quartz, sillimanite, mullite, and other aluminous silicates.

Uses and significance: As a mineral, cordierite is mainly of interest to mineralogists and collectors. The magnesium end-member, and especially synthetic cordierite, are valued for their very low thermal expansion and high thermal stability, properties exploited in ceramic materials and refractories. Cordierite-based ceramics find applications in components requiring resistance to thermal shock, such as certain heating elements and industrial kiln furniture.