M.A. Sequeira Braga

Weathering of the Oporto granite: geotechnical and physical properties

Abstract This paper characterizes the weathering products of the two-mica Oporto granite in terms of its mineralogical, chemical, geotechnical and physical properties. This information is used to identify a physical property that can be used as an index of the degree of weathering and to estimate other properties. In outcrop, the Oporto granite is always weathered and weathering profiles frequently exceed 20 m. Within these profiles, granitic saprolites are composed of quartz, feldspars and some mica with kaolinite and gibbsite as final weathering products. The average percentage of secondary minerals determined by bulk X-ray diffraction (XRD) is low in both weathered rock (2.4%) and saprolite (8.7%). Geotechnically, the saprolites exhibit no or low plasticity (plasticity index, PI≤12%). The skeleton character of the saprolites is reflected in the significant proportion of fines (29.5%

Weathering of granites in a temperate climate (NW Portugal): granitic saprolites and arenization

Abstract Saprolites or in situ weathering products from granitoid rocks of Northwest Portugal were studied by means of X-ray diffraction (XRD) of their clay fraction and observations of polished thin sections combined with transmission electron microscopy (TEM), scanning electron microscopy with energy dispersive spectrometer (SEM-EDS) microanalyses. Their principal features are: (1) depths of more than 10 m; (2) mean material loss of 40% as calculated by an isovolumetric method; (3) low clay content (mean value of 7%) and a high degree of mineralogical evolution. The clay fraction is characterized by a predominance of kaolinite and gibbsite, with subordinate 2:1 minerals (illite, chlorite, vermiculitic mixed layers, vermiculite). The significance of the secondary minerals in granitic saprolites from Northwest Portugal is compared to published data from Atlantic Europe. This allows the definition of a climatic zonality for weathering products and identifies ‘arenization’ as an important weathering process.

Clay mineral assemblages in weathered basalt profiles from central and southern Portugal: climatic significance

Abstract Mineralogical and chemical changes during weathering of basalts from central and southern Portugal were studied. Both regions have a temperate Mediterranean type climate, the Algarve being more arid. The areas concerned are the Lisbon (72 Ma) and the Runa Volcanic Complexes (100–70 Ma), and Rio Maior (160–130 Ma) in central Portugal, and Budens (170–180 Ma) in the south. The basalts are generally rich in olivine phenocrysts. Clay minerals are in general well crystallised and comprise smectites, associated with 1:1 minerals (principally halloysite), illite and palygorskite. Kaolin minerals prevail in topographic highs, whilst smectites predominate in lower areas. Smectites were analysed from the

Trachyte stones in monuments of the São Miguel and Terceira Islands, Azores (Portugal)

Publisher Summary Among the volcanic rocks from Azores Islands, trachyte stones have been preferentially used through centuries in many monument facades because of their color and softness. After a few centuries, these stones show remarkable decaying. This chapter describes the study conducted to compare the trachyte stones from quarries and monument to point out the similarities and differences in their petrophysical and chemical characteristics and to determine the decay phenomenon. Angra do Heroismo Cathedral in Terceira Island and Misericordia Church in Sao Miguel Island were the monuments selected for the study on degradation causes and mechanisms. The same methodology was applied to quarry and monument samples from both Islands by using petrographic, mineralogical, chemical, and petrophysical techniques. The Well-marked differences were noticed among the trachytes from Sao Miguel and Terceira Islands; but within each island, outcrops or the monument building stones showed several similarities. Some textural aspects observed in the field showed some variability; the stones had a pore-type porosity of genetic nature. The values of porosity accessible to water were very high for monument samples and higher than the values for quarry samples. The samples of Misericordia Church showed a high porosity, dominated by large pores. It was concluded that these highly porous trachyte materials are susceptible to degradation, namely by salt crystallization mechanisms.

Secondary phosphate phases in altered trachyte from S. Miguel Island (Azores, Portugal)—A possible contribution to the stone degradation

Publisher Summary This chapter discusses the geochemical and mineralogical research that were undertaken to determine the origin of degradation observed in stone monuments of the Misericordia church. This facade was constructed using volcanic rocks of trachyte composition. The degradation process in these stones is correlated to the precipitation of secondary phases as saline minerals and rare earth elements (REE) phases. During alteration processes, the behavior of REE depends on several factors, for example Eh, pH, the presence of organic and inorganic ligands, and the mineralogical distribution of REE in the parent material. Among the REE, cerium (Ce) is important because it occur in nature as Ce 3+ like the majority of lanthanides, or as Ce 4+ in oxidizing conditions. The mineralogical transformations (dissolution/precipitation), including the nature of secondary minerals incorporating the REE, is a fingerprint to mark out the degradation processes. Loss of material results from the dissolution reactions of primary minerals inducing an increase in the porosity, whereas the precipitations of secondary phases cause physical constraint increasing the fragility of the stones. This chapter evaluates the REE-bearing minerals in altered trachyte from the church by combining the scanning electron microscopy (SEM) observations made in both monument stones and quarry samples. From the analyses, it is concluded that the occurrence of these secondary phases in the first stages of weathering of trachytes in Azores Islands indicates a high porosity of these rocks allowing an effective water-rock interaction.