The 14 Å phase developed in heated dickites

Abstract

Dickites with well-ordered crystal structures and with relatively high dehydroxylation temperatures develop a 14 Å phase when heated to temperatures in the range 550–800°C. The actual spacing is close to 13·7–13·8 Å. Dickites with less well-ordered structures and with lower dehydration temperatures give less development of this phase or none at all. Other kaolinite group minerals do not produce a similar phase. Thermal weight-change and differential thermal analysis data show that the 14 Å phase develops to a maximum as hydroxyl ions are eliminated from the dickite structure and infrared spectroscopic data are consistent with this result. AIKα fluorescence wavelength measurements indicate that the Al ions are in four-fold coordination in the 14 Å phase. Absence of other diffraction data makes structure analysis impossible. The description ‘chlorite-like’ for the 14 Å phase is of questionable validity.