Reduction of hematite to magnetite under natural and laboratory conditions.

Thermomagnetic analysis of natural and synthetic samples containing hematite have confirmed SCHWARZ'S (1969) observations that 1) extensive reduction of hematite occurs when the experiment is perfumed in vacuum but not when it is performed in air 2) the reduction is arrested when the sample is heated in air before the vacuum run. In addition, we have found that the reduction is controlled by backstreaming vacuum pump oils, which buffer the atmosphere in the furnace region, but is not dependent upon reducing agents within the samples. Heating of the sample creates a surface layer which is not permeable to the reducing atmosphere and which thus prevents reduction during subsequent beatings. X-ray diffraction data from reduced synthetic hematite shows that the reduced phase is magnetite. Reducing agents within samples can contribute to reduction, and in some cases are so important that reduction may occur in air. Reduction produces undesirable effects during thermal demagnetization procedures and paleointensity determinations on samples containing hematite, but these effects can be eliminated or controlled if certain precautions are followed.