Weathering Studies. 1. New Data on Vermiculite

A hydrothermal study in the range up to 800° C. and 2,000 atm. water pressure shows the successive "equilibrium" reactions in vermiculite on heating. Under 10,000 psi water pressure, the first dehydration reaction occurs at 550° C, while above 650° C. only non-expanding structures are formed. The most important change in vermiculite occurs gradually, becoming quite marked above 300°; this change leads to a redistribution of the mica-layer Mg²⁺ and the formation of a "chlorite-like" phase of the vermiculite composition. It is therefore clear that no primary vermiculite could have crystallized under even mild hydro-thermal conditions. The rarity of vermiculites with low Fe-contents supports the postulation that the mechanism of its formation is by the low-temperature leaching of K⁺ coupled with oxidation of Fe²⁺ to balance the charge initially. The dry dehydration behavior of clay-sized vermiculite shows its resemblance to the montmorillonoids. The relationship to other members of the montmorillonoid group is discussed.