EXPERIMENTS ON THE STABILITY OF FeOOH ON THE SURFACE OF THE MOON

The FeOOH phase in Apollo 16 rocks is akaganéite. Other polymorphs of FeOOH, i.e. geothite and lepidocrocite, may exist but have not been identified by X-ray diffraction. Experiments performed during this study using akaganéite, geothite, and lepidocrocite placed at 135°C and under a pumping vacuum of 10^?2 torr, demonstrate that these forms of FeOOH are not stable on the surface of the Moon, where daytime temperatures are about 140°C and pressures are about 10^?7 torr. The effect of the vacuum is to strip off the water, and the temperature speeds up this dehydration process. These data lend support to the contention of Taylor et al. (1973) that the FeOOH in lunar rocks formed by the oxyhydration of lawrencite, FeCl₂, due to contamination by terrestrial water vapor.