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.

中文翻译：

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使用飞行时间中子粉末衍射测定的 Al2SiO5 多晶型物、蓝晶石、红柱石和硅线石的热膨胀在 10 到 1573 K 之间

在过去的 60 年中，从三种重要的地质 Al2SiO5 多晶型（蓝晶石、红柱石和硅线石）测量的粉末和单晶衍射数据对每种多晶型的热膨胀给出了不同的解释，实验数据集之间甚至内部的分散通常非常大。此外，没有确定低于 273 K 的热力学函数显着变化的晶格参数，并且在 1200 K 以上的测量很少。 ISIS 中子散裂源处的高分辨率粉末衍射仪 (HRPD)。通过使用内标（NIST 硅 SRM640c）和使用体探针（中子）来确保精度，以避免典型背反射 X 射线衍射方法所需的样品位移校正。通过在世界上最长的初级中子飞行路径仪器之一上使用飞行时间方法来确保精度。对于蓝晶石，精度的提高首次揭示了三个轴间角的真实温度依赖性，从而可以推导出准确的热膨胀张量系数。对于红柱石和硅线石，测量结果揭示了在室温以下沿 c 轴和硅线石沿 a 轴在室温以下具有显着负线性膨胀率的未知区域。1200 K 以上，