Desilication of Halloysite and Its Relation to Gibbsite Formation'

ABSTRACT: The evidence presented points to the alteration of halloysite to gibb-site. Mineralogical data, as determined by X-ray and differential analysis, verify identification of halloysite and gibbsite. Chemical data confirm the expected lower silica and higher alumina content for samples which are predominantly gibbsitic. It is reasonable to assume from petrographic evidence that gibbsite develops by desilica-rion of halloysite. Halloysite amygdules undergo desilication along the outer peri-pheries, where acid silica-deficient waters pass, attacking the halloysite by dissolving silica. Halloysite is stable only if it is protected from such solutions, or if the solution passing by is saturated with silica. Whereas alteration of feldspar to halloysite involves a gain in volume, a loss in volume follows desilication of halloysite. This loss in volume is exemplified by the surface cracks clearly visible in the desilicated halloysite. IN AN EARLIER PAPER Sherman and Ikawa (1959) described the occurrence of gibbsite amygdules in the Haiku bauxite area of Maui