The first stage of dehydration of magnesian chlorites is shown to involve the removal of water from the brucite layer only of the structure. Movement of the magnesium atoms from their original positions is indicated by the results of Fourier analysis. The second stage of dehydration is followed by the formation of olivine. The orientation of the olivine relative to the chlorite structure and the movements of the atoms in the transformation are discussed. These reactions proceed at somewhat higher temperatures as we go from penninite to clinochlore to sheridanite. Spinel and probably enstatite appear when the heating is continued to still higher temperatures. Curves obtained by the method of differential thermal analysis show endothermic peaks corresponding to the dehydration processes and an exothermic peak corresponding to the formation of olivine. They exhibit, however, a greater variety of results than the X-ray measurements, and the latter provide no immediate explanation of certain features of ~hese curves.

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