We demonstrate that neutron diffraction measurements make it possible to quantify elastic strains within the interior of solid samples, and thus have great potential for addressing a wide range of problems connected with the characterization of the mechanical properties of geological materials. We use the time-of-flight neutron diffraction technique, in combination with acoustic emission monitoring, to study the evolution of thermal strain within the interior of samples of a pure quartzite during slow heating, and the onset of the associated thermal cracking. Thermal cracking commences around 180 ¿C when the thermal strain deficit along the a-axes of quartz grains induces a thermal stress that is close to the bulk tensile strength of the rock. ¿ 2001 American Geophysical Union