The object of this study is magnesian clay minerals present in carbonate rocks of the post-rift phase of the pre-salt in the Santos Basin. These rocks developed in an Aptian-age alkaline lacustrine environment. This study summarizes the formation of clay minerals associated with different lithotypes in a range of 19 m and a depth of more than 5100 m. They were characterized from petrographic analysis by optical microscopy, X-ray diffraction (total sample and clay fraction), and modeling by Newmod^®; and examined and analyzed by scanning electron microscopy. An approach based on identifying lithotypes and characterization of microsites allowed us to understand the occurrence of different clay minerals. Kerolite was the most abundant mineral in the sampled range. It occurs in lamellar aggregates under greater preservation of the original rock lamination and in association with spherulites and shrubs. The Stv/Ker mixed layers occurs in the same association, and formed finer unlaminated aggregates associated with the more intense dolomitization and silicification processes. Saponite occurs associated with detrital minerals forming clayey levels intercalated with microcrystalline carbonates. Fluids with a high Mg/Si and pH < 9 favor the precipitation of kerolite. The increase in pH during diagenesis may be responsible for the formation of Stv/Ker mixed layers.

The characterization of Mg-clays in rock samples (well P1) from the Barra Velha Formation (Early Cretaceous) allowed the establishment of mineral assemblages on the basis of their kerolite and Mg-smectite (stevensite and saponite) content. Kerolite-rich assemblages (A and B) rarely contain saponite. Assemblage B is composed of kerolite-stevensite mixed layers, while assemblage A consists of more than 95% kerolite. Mg-smectite-rich assemblages (C and CB) are made up of both Mg-smectites. The predominance of stevensite in the lower interval of the stratigraphic succession suggests evaporative conditions, higher salinity and pH, which would favor its authigenesis by neoformation. In the upper portion, the occurrence of thick kerolite-rich intervals suggests regular water inputs, contributing with a decreasing in salinity and pH, favoring the neoformation of kerolite and later kerolite-stevensite mixed layering. The saponite would be the result of the transformation from Al-smectite into Mg-smectite in a Mg²⁺ rich medium. The results indicate that lake hydrochemical processes would have allowed the establishment of a basic depositional sequence, from base to top, as follows: (i) initial lake expansion stage marked by the occurrence of saponite, (ii) later kerolite neoformation, (iii) formation of kerolite-stevensite mixed layer with increasing salinity, and (iv) neoformation of stevensite, marking a final stage of maximum salinity (evaporation) and alkalinity of the lake.

This paper presents research on the influence of quartz deformation in aggregates for the development of the alkali-silica reaction in concrete and its relationship with silica dissolution. The study also compares these characteristics with the field behavior of such rocks in concrete. The paper proposes parameters to classify the different degrees of deformation of quartz. Transmission electron microscopy showed the presence of walls even in slightly deformed quartz, which indicate the presence of the internal paths available to react with the alkaline concrete pore solutions and point to the potential development of an alkali-silica reaction. The presence of the deformation bands in the quartz grains leads to the alkali aggregate reaction occurring more rapidly. The visible spectrophotometer test was performed to evaluate the dissolution potential of the different samples of deformed quartz, which confirmed that the reactivity of the quartz increases as the deformation of the crystalline structure increases. The parameters established in the present study could be verified by analyzing the behavior of reactive and innocuous aggregates from the buildings.

Defects in the crystalline structure of quartz facilitate the connection with the alkali hydroxides, since under a high alkalinity condition (e.g., in concrete), the Si-O bonds of quartz are easily broken. This study set out to investigate the influence of the deformation structures of quartz on its susceptibility to the alkali–silica reaction. A granite, a protomylonite, and a mylonite were selected for this study. Using optical microscopy, the quartz grains contained in these rocks were quantified and their texture characterized. The quartz samples extracted from the rocks were analyzed by magnetic nuclear resonance, to evaluate their potential for dissolving silica as well as changes in their atomic scale before and after the reaction with alkali hydroxides. These analyses were compared with the results of the accelerated mortar bar test. The study showed that the quartz with intense undulatory extinction and deformation bands denotes the most favorable condition to the development of the alkali–silica reaction. However, on an atomic scale, the slightly deformed grains were highly prone to react. Thus, in a high alkalinity condition, over a long period of time, any quartz tends to develop the alkali–silica reaction, regardless of the deformation degree of the grain.

A Mina do Seival é constituída por rochas vulcânicas e diques de composição andesítica e traqui-andesítica, dispostas em duas sequências. A sequência I inclui rochas piroclásticas e efusivas, e a sequência II é representada pelos diques de composição andesítica. Ambas são incluídas na Associação Shoshonítica de Lavras do Sul. Este magmatismo é relacionado ao estágio pós-colisional do ciclo Brasiliano/Pan-Africano, situando-se estratigraficamente no Alogrupo Bom Jardim, pertencendo à Formação Hilário na Bacia do Camaquã (Neoproterozoico). A área possui intensa alteração hidrotermal e mineralizações de Cu. A mineralização e o magmatismo da Sequência II são controlados por estruturas tectônicas orientadas segundo N/NE e NO, que são relacionadas à distensão regional no período pós-colisional da Orogênese Brasiliano/Pan-Africana. Processos hidrotermais em diferentes temperaturas atuaram sobre estas rochas originando produtos de alteração pervasiva, principalmente clorita, corrensita e esmectita, com veios preenchidos por quartzo, carbonato, barita e minerais de cobre. Em ambas as sequências encaixantes da Mina do Seival é possível identificar a afinidade shoshonítica das rochas. Os elevados teores de Cu, Zn e Ni nos diques em relação às rochas piroclásticas e às efusivas, mesmo nas mais hidrotermalizadas, sugerem que as principais ocorrências de mineralização de Cu têm origem magmática. Os dados químicos de rocha total indicam que o enriquecimento dos elementos componentes da mineralização, Ag, Au, Cu e Zn, está relacionado aos diques da Sequência II. Os teores de Au e Cu são mais elevados nas amostras com menores concentrações de carbonatos, sugerindo que a carbonatação não tem relação com a deposição dos minérios.