M. Isabel Dias

RAW MATERIAL SOURCES FOR THE ROMAN BRACARENSE CERAMICS (NW IBERIAN PENINSULA)

The Bracarense ceramics are characterized by a fine, pale yellow paste covered with a brownish yellow slip. The name is derived from Bracara Augusta, the Roman town located in the north of Portugal, where this type of ceramic paste was first found and identified. Various forms with the same type of paste occur, such as imitations of sigillata, terra sigillata and thin walls from the Augustus-Tiberius period, and common ware. Later, similar ceramics were also found in other archeological sites, e.g. Aquis Querquennis (Galiza, Spain), which question the location of the production center of this type of ceramic paste.Mineralogical and chemical analyses showed that the majority of the Bracarense shards studied differ from the common ware of the Braga region. Despite minor differences, the Bracarense shards collected in Aquis Querquennis have the same geochemical pattern as those found in Bracara Augusta, i.e. they appear to have been manufactured with the same clay type. The firing products found indicate a kaolin character of the source clay, and point to firing temperatures near 900°C. The Aquis Querquennis shards have greater Br contents, which can be explained by use-wear and/or post-depositional processes, as this site is located in a thermal-water region.

UPPER CRETACEOUS CLAYEY LEVELS FROM WESTERN PORTUGAL (AVEIRO AND TAVEIRO REGIONS): CLAY MINERAL AND TRACE-ELEMENT DISTRIBUTION

Clay-rich deposits of Upper Cretaceous levels in the Taveiro (Reveles and S. Pedro) and Aveiro (Bustos) regions of west-central Portugal are economically and environmentally important, but detailed chemical and mineralogical characterization is lacking. The purpose of this study was to partially fill that gap by correlating the trace-element geochemistry (particularly the rare earth elements, REE) with the mineralogy of both the whole rock and of the <2 μm fraction along selected stratigraphic levels of the formations. The results will help the ceramics industry in the region and will be important in paleoreconstruction environmental studies.Mineralogical and chemical characterizations were carried out using X-ray diffraction (XRD), X-ray fluorescence (XRF), and instrumental neutron activation analysis (INAA). The following clay-mineral associations were identified: (1) at Reveles — smectite, illite, and kaolin minerals; (2) at S. Pedro — kaolin minerals and illite; and (3) at Bustos — illite, kaolin minerals, and mixed-layer illite-smectite. The distribution of trace elements in the <2 μm fraction depended on the clay mineralogy, suggesting that the trace elements were incorporated in, adsorbed to, or even replaced major elements in the clays, as follows: (1) first-row transition elements, particularly Zn and Ga, were enriched when smectite predominated; (2) As, Rb, and Cs were enriched in this fraction of the S. Pedro deposit, the only one with Fe (oxyhydr)oxides and a high proportion of illite; and (3) REE were more concentrated when kaolin minerals predominated. Eu was enriched in the <2 μm fraction, which was due to preferential incorporation in the Fe (oxyhydr)oxides and/or carbonates.