Transformation of Ordered Albite into Kaolinite: Implication for the "booklet" Morphology

Kaolinization of feldspars is widespread in various geological environments including weathering and hydrothermal environments. However, the mechanisms of such mineral transformation and kaolinite formation remain obscure. In this study, the experiments of the transformation of ordered albite into kaolinite were carried out under hydrothermal conditions to reveal the transformation mechanism involved and the formation mechanism of kaolinite aggregates with a "booklet"morphology. The obtained results demonstrated that a complete dissolution-recrystallization mechanism is responsible for the transformation process since the configurations and linkages of Al-O4 tetrahedrons in ordered albite are significantly different from those of the Al-O2(OH)4 octahedral sheets in kaolinite. During the dissolution of precursor albite, Si might be released from the structure of albite prior to Al. Subsequently, four-coordinated Al (AlIV) would be progressively converted into six-coordinated Al (AlVI) through hydration, which is beneficial to the formation of kaolinite. For albite with a large particle size, the alteration products were kaolinite aggregates with a "booklet"morphology, which were formed by the attachment of generated intermediate components of Al and Si during the dissolution of albite and subsequent crystallization of kaolinite platelets or layers stacking up to a micron height most likely along the albite cleavages. However, the crystallinity and layer stacking order of the "booklet"kaolinite aggregates were low. These findings provide insights for a better understanding of the transformation of feldspars into kaolinite and the formation of kaolinite aggregates with a "booklet"morphology under both natural and experimental conditions.