Abstract: PEEM allows ‘real-time’ observations to be made of solid-state transformations and other high-temperature processes taking place during vacuum-heating up to c. 2000°C The solid state transformations of an amphibole-rich hornblendite specimen have been observed in the temperature range of 750–1000°C (± 50°C Between c. 970–990°C a rapid change in orientation contrast was observed, indicating a structural rearrangement from an oxyhornblende crystal lattice to a clinopyroxene structure. This phase retains the original amphibole shape and texture (including two 120°C intersecting cleavage traces), but possesses a clinopyroxene crystal structure. At higher temperatures this phase is seen to decompose, forming iron oxides and other fine-grained products. PEEM has provided useful information on both the nature and rates of transformation of natural amphiboles which has proved invaluable in our understanding of the mineralogically-controlled mechanisms of argon release during ⁴⁰Ar/³⁹Ar dating of amphibole samples.