Abstract: Five illitic clays were treated with various inorganic cations and heated to over 1400°C in a high-temperature X-ray diffraction furnace.
The illite lattice undergoes dehydroxylation from 475–600°C, and the resulting anhydrides begin to disappear at 850°C. The major high-temperature phases formed from untreated illites β-quartz (1000–1300°C), spinel (1000–1400°C), and mullite (1150–1400 + °C). Minor amounts of cordierite, kalsilite, feldspar, corundum, iron oxide and forsterite may also occur with various adsorbed cations at temperatures above 1000°C. β-cristobalite was never observed in any of the runs, contrasting strongly with its prominent development among the high-temperature phases of montmorillonite.
Certain adsorbed cations act as repressors of high-temperature phase development, while others seem to have the opposite effect. The order of decreasing repressive effect—K+> Na+> Li+> Ca+2> Mg+2> Be+2> Sn+2,4—also represents the sequence of increasing electronegativity and ionic potential of these cations. It is postulated that the degree of electronegativity and ionic potential of the adsorbed cations is proportional to the rapidity and degree of mullite crystallization, because of the influence of these factors in ion transfer and reaction product removal in the critical liquid phase at tho clay mineral surface.