Weathering of Viama tilde o granodiorite, South Brazil: Part 1-Clay minerals formation and increase in total porosity

Weathering transforms fresh un-weathered rock into saprolite and soils, porous materials that may hold available water for plants and nutrients. The conjoint characterization of mineral weathering and development of porosity is helpful in understanding rock weathering and the development of soil. The objective of this study was to investigate mineralogical transformations and the increase in total porosity during granodiorite weathering in South Brazil by combining petrographical observations, mineralogical ana -lyses, and porosity measurements. The studied granodiorite profile presented a saprock of similar to 2 m thickness with spheroidal weathering, a-15 m thick saprolite of and soil of similar to 2 m thickness. The intensity of weathering was estimated using chemical indices (CIA and WIS) and by the production of fines particles of silt and clay sizes. As weathering increased from saprock to saprolite and soils total porosity increased from 1.73 % in the un-weathered rock, between 3.57 and 10.5 % in the saprock and 11.02 % in the saprolite. The chemical losses were limited in saprock and saprolite (CIA = 68, & UDelta;4Si = 29.31 %) and increased in the topsoil (CIA = 85.73, delta 4Si = 56.83 %), indicating a moderate weathering intensity. Petrographic observations by optical microscope and SEM, chemical composition using SEM-EDS and X-ray diffraction results showed principal mineral weath-ering reactions were the transformations of biotite into vermiculite (with biotite/vermiculite mixed layers as intermediate weathering stages) and weathering of plagioclases and potassic felspars into kaolinite. The multi-mineral composition of the saprolite indicated a progressive transformation of primary minerals with increasing weathering intensity. The results show that the opening of the porosity is likely to control the development of the weathering and should deserve more detailed characterisation.

Automated summary

This study investigated the mineralogical transformations and the increase in total porosity during granodiorite weathering in South Brazil by combining petrographical observations, mineralogical analyses, and porosity measurements. The results show that the increase in total porosity is likely controlled by the opening of the porosity.