Thermal expansion of the Al₂SiO₅ polymorphs, kyanite, andalusite and sillimanite, between 10 and 1573 K determined using time-of-flight neutron powder diffraction

Abstract

Powder and single-crystal diffraction data measured from the three geologically important Al₂SiO₅ polymorphs, kyanite, andalusite and sillimanite, over the last six decades have given varying accounts of each polymorph’s thermal expansion, the scatter between and even within experimental datasets often being quite large. Furthermore, there are no lattice parameter determinations below 273 K, where the thermodynamic functions vary substantially, and few measurements above 1200 K. Accurate and precise lattice parameters of natural kyanite, andalusite and sillimanite have therefore been obtained in the range 10–1573 K using the high-resolution powder diffractometer (HRPD) at the ISIS neutron spallation source. Accuracy is ensured by the use of an internal standard (NIST silicon SRM640c) and use of a bulk probe (neutrons) to avoid the specimen-displacement corrections required by typical back-reflection X-ray diffraction methods. Precision is ensured by use of the time-of-flight method on one of the longest primary neutron flightpath instruments in the world. For kyanite, the improved precision reveals the true temperature dependence of the three inter-axial angles for the first time, permitting derivation of accurate thermal expansion tensor coefficients. For both andalusite and sillimanite, the measurements reveal hitherto unknown regions of substantial negative linear expansivity below room temperature, along the c-axis in andalusite and along the a-axis in sillimanite. Above 1200 K, sillimanite exhibits an anomalous increase in thermal expansion that may be due to a change in the Al/Si tetrahedral site ordering.