Al2SiO5 forms three naturally-occurring polymorphs, kyanite, andalusite and sillimanite. These minerals are produced by varying degrees of metamorphism of sedimentary rocks, the polymorph (or mixture of polymorphs) produced being diagnostic of the P/T path followed. Use of these key petrological indicators is dependent on a firm thermodynamic description of their properties and behaviour. Characterisation of the thermal expansion is a central part of this thermodynamic description.The thermal expansion of kyanite, andalusite and sillimanite have only been measured above room-T and largely using X-ray powder diffraction methods. This limits the accuracy and precision of the thermal expansion tensor coefficients and prevents us from being able to parameterise the thermal behaviour in terms of fundamental thermodynamic quantities. The aim of this proposal is to correct these deficiencies.