AN INVESTIGATION OF TWINNING PHENOMENA IN GIBBSITE

The quality of gibbsite with respect to calcination is determined by the strength, size and purity of gibbsite agglomerates. Agglomerates are typically made up of hexagonal or diamond shaped crystallites, which are commonly twinned. Twinning plays a significant role in the formation of hexagonal gibbsite crystals, hence an investigation into this phenomenon was conducted with the use of spectroscopic and microscopic techniques.

Polarised light microscopy identified six-fold rotational twinning in large (>5μm) hexagonal gibbsite crystals. The occurrence of six-fold twins was observed to increase with time. The technique also identified gibbsite which did not exhibit twinning. These crystals were smaller in size (<5μm) with diamond or hexagonal morphology. Polarised Raman spectroscopy of the OH stretching region and the associated Al-O region indicated differences in bond orientation due to twinning. Crystallographic axes of each twin domain were determined. The directions of orientation of the twin boundaries were used to determine the mode of twinning.

Twinning was not observed during early stages of growth, and a number of possible causes for subsequent twinning have been investigated, including localised changes in liquor concentration at nucleation, transformation due to pressure or temperature changes during growth, or the nucleation of a metastable structure which is only sustainable as a single crystal at small dimensions, beyond which stabilisation is provided via twinning.

The occurrence and nature of twinning within the individual components of agglomerates may be a relevant consideration in determining their strength during calcination.