Halloysite in Argentinian deposits: origin and textural constraints

Abstract

AB ST R ACT : In Argentina, the only halloysite deposits found so far are located in the southwest of the province of Rio Negro. The areas are named Mamil Choique and Buitrera, separated by 50 km in a W-E direction. The deposits are located in pyroclastic rocks, rhyolitic tuffs and ignimbrites.The aim of this paper is to determine the origin of the alteration, any relationship between the processes in Mamil Choique and Buitrera, and the factors which controlled the halloysite formation. The work was based on field work, mineralogical and geochemical studies, form and distribution of the alteration. Fresh as well as altered rocks were collected. The first were studied by petrographic and chemical methods. In altered samples, the clay mineralogy was determined by XRD and textures by SEM and TEM. Chemical analyses on fresh and altered samples were used to characterize the alteration process and the rock composition.It was concluded that halloysite has been formed by weathering in a time range from Middle Eocene to Middle Oligocene and extended along 50 km in a W-E direction, independent of the source rock. Particle morphology depends on the original texture of the rock. Spheroidal halloysite is related to rocks with low porosity and tubular particles are related to rocks with open spaces.

Spheroidal halloysites from Patagonia, Argentina: Some aspects of their formation and applications

“…The latter paper also concluded that there is an increase in the substitution of Al by Fe as the spheroidal halloysite transforms into tubular and platy forms. Cravero et al (2012) concluded that in Mamil Choique and Buitrera deposits, halloysite is formed from the dissolution of volcanic glass and that the particle morphology depends on the original texture of the rock; spheroidal halloysite is related to rocks with low porosity, and tubular particles are related to rocks with open spaces similar to what was found by Adamo et al (2001). Halloysite coming from different deposits has low iron content, and there are no differences associated with different morphologies.…”

“…Cravero et al (2012) indicated that probably waters circulating towards small basins filled with these rocks (thus giving the current pocket shape of the deposits) came into contact with them for long periods of time, and halloysite formed after the dissolution of glass. In Urquiza, where kaolinite predominates, the parent rock is acid plutonic (granite, granodiorite).…”

“…In Buitrera, halloysite, along with cristobalite and tridymite, is the main component (Cravero et al, , 2012(Cravero et al, , 2014a.…”

“…In Argentina, small and low grade deposits are located in the area of Mamil Choique and Buitrera, Patagonia, Argentina (Cravero et al, , 2012. In Mamil Choique, the deposits are not small but spheroids predominate over tubes, whereas in Buitrera, the deposit is small and tubes predominate over spheroids.…”

“…In between, there are several localities where the rocks are also altered to halloysite. The extension of the alteration, the high halloysite/rock ratio, not common in this type of deposit, and the formation of halloysite rather than kaolinite in different rocks at the same time and with different morphologies make these areas of interest to contribute to the understanding of the formation conditions of halloysite (Cravero et al, 2012).…”