Green-blue Fe(II)Fe(III)-and Fe(II)Al(III)-hydroxycarbonates with a pyroaurite structure were prepared by Taylor & McKenzie (1980) and Taylor (1980). They were thought to be related to the unstable mineral phases which occur naturally in the dark green-blue mottles of wet, anaerobic horizons of gleyed soils. These synthetic compounds are unstable to oxidation, suffering structural and colour changes, with the ultimate products depending on the environmental conditions. This instability makes it difficult to identify them and study their characteristics by X-ray diffraction and other methods.

The technique described in this note has been found to stabilize the synthetic green-blue compounds satisfactorily. It involves handling and storage of the compounds in a CO2 atmosphere, such as might occur under natural anaerobic conditions in a soil below the ground-water level. The same principles can be applied to collect field samples and maintain them in a stable state.