Andalusite is an aluminum-rich silicate mineral. Andalusite is a
common mineral in aluminum-bearing metamorphic rocks. It forms
at low to medium temperatures and pressures. Andalusite is
trimorphous with sillimanite and kyanite. It means that these
three minerals have exactly the same chemical composition but
they have different crystalline structure and therefore quite
different appearance. The chemical composition of these three
minerals are often expressed the following way: Al2SiO5 but not
always. Sometimes it is written as AlAlOSiO4 or Al2OSiO4 to show
that they are orthosilicates. Orthosilicates are silicate
minerals which possess isolated silica tetrahedra (SiO4) in
their crystal structure. These tetrahedra are like three-
dimensional islands surrounded by other elements. Other well-
known orthosilicates are zircon, olivine, garnet, topaz,
titanite, etc. These are the least siliceous minerals among the
silicate minerals and their chemical formula is usually written
in the way which clearly shows isolates