PHOTOEMISSION ELECTRON-MICROSCOPY (PEEM) HEATING INVESTIGATIONS OF A NATURAL AMPHIBOLE SAMPLE

PEEM allows 'real-time' observations to be made of solid-state transfo rmations and other high-temperature processes taking place during vacu um-heating up to c. 2000 degrees C. The solid state transformations of an amphibole-rich hornblendite specimen have been observed in the tem perature range of 750-1000 degrees C (+/- 50 degrees C). Between c. 97 0-990 degrees C a rapid change in orientation contrast was observed, i ndicating a structural rearrangement from an oxyhornblende crystal lat tice to a clinopyroxene structure. This phase retains the original amp hibole shape and texture (including two 120 degrees intersecting cleav age traces), but possesses a clinopyroxene crystal structure. At highe r temperatures this phase is seen to decompose, forming iron oxides an d other fine-grained products. PEEM has provided useful information on both the nature and rates of transformation of natural amphiboles whi ch has proved invaluable in our understanding of the mineralogically-c ontrolled mechanisms of argon release during Ar-40/Ar-39 dating of amp hibole samples.