Abstract: Hydrothermal studies have failed to yield synthetic attapulgite or sepiolites in the system MgO-Al2O3-SiO2-H2O. However, the natural minerals can be decomposed to yield montmorillonoids by mild hydrothermal treatment as low as 200°C and probably as low as 100°C. These data indicate that they are metastable and probably could not have formed at these temperatures.
Dry dehydration at 400° and 750°C gave no evidence for the presence of “zeolitic water” in these minerals and failed to establish the existence of “anhydride” phases. The field study of relationships in one sepiolite locality suggests the importance of structural control in the formation of these minerals.