We show that a quantum mechanical version of the virtual crystal approximation can be used to study Al/Si-disorder in silicates. Full geometry optimizations show that this approach reproduces disordered crystal structures with an accuracy equal to that of density functional calculations for fully ordered structures. As this approach is based on density functional theory in conjunction with a plane wave basis set and ultrasoft pseudopotentials, it is possible to study large and complex crystal structures. As first examples we present calculations for hollandite-type KAlSi3O8, where Si is octahedrally coordinated, and gehlenite, Ca2Al2SiO7, with tetrahedrally coordinated Al and Si-atoms. (C) 2002 Elsevier Science B.V. All rights reserved.