Weathering of Viamão granodiorite, South Brazil: Part 1 – Clay minerals formation and increase in total porosity.

Chemiographic projections of the chemical compositions of the minerals obtained with the SEM EDS probe in the 4Si – M+ – R2+ ternary diagram. The trend lines depict the weathering sequences of the minerals. Chloritization of some biotite of the un-weathered rock due to post-magmatic alteration. Weathering of biotite into smectite in a first step in the saprock and into kaolinite in a second step in the saprolite. Weathering of feldspars into smectite in the saprock and then into kaolinite in the saprolite. The trend lines for feldspars and biotites are distinct due to different chemical compositions of the secondary minerals, magnesian for biotite weathering and aluminous for feldspars weathering. [Display omitted] • Weathering intensity measured by chemical losses and fine-sized particles formation. • Weathering of biotites by expansion form vermiculites and subsequently kaolinite. • Weathering of felspars in dissolution pits forms kaolinite. • Incipient weathering of biotite creates porosity allowing fluids percolation. • Total porosity increases during weathering of granodiorite in South Brazil. 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 analyses, and porosity measurements. The studied granodiorite profile presented a saprock of ∼2 m thickness with spheroidal weathering, a ∼15 m thick saprolite of and soil of ∼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, Δ4Si = 29.31 %) and increased in the topsoil (CIA = 85.73, Δ4Si = 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 weathering 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. [ABSTRACT FROM AUTHOR]