Microhardness of feldspar single crystals (Or98 and An58) as a function of temperature

The Vickers hardness VH was measured as a function of temperature T from 20 to 900 °C for single crystals of orthoclase (Or98) and labradorite (An58) using the indentation technique with a square-based pyramidal diamond. A load dependence of the measurements was found. The values obtained with the load of 100 g are considered here because they are more representative of plastic properties in the feldspar case. For both feldspars, the hardness decreases whith increasing temperature. Two regimes have been found. At low temperature T < Tc, thermal softening is small and the hardnesses of orthoclase and labradorite are equal within the accuracy of the measurements : VH is about 9 GPa at room temperature and 8 GPa at Tc. At high temperature, thermal softening is more important. The phenomenon is more pronounced for orthoclase than for labradorite : at 900 °C, VH is 5 GPa for orthoclase and 6.5 GPa for labradorite. The results hold for measurements on the (010) or (001) crystal face. Tc is considered as the brittle-ductile transition which is around 600 °C.