Stone Beads from Late Bronze Age and Early Iron Age Settlements from South-Western Portugal: Analyses by X-Ray Diffraction A.P. Gonc¸alves, A.M. Monge Soares, A.C. Silva, and L. Berrocal-Rangel
1 Introduction The use of non-destructive techniques is very important in the field of archaeology, as frequently the specimens are unique and often small, and damaging them may be very problematic. X-ray diffraction can be used in a non-destructive way, and provides important information not only on the crystal structure but also on the composition of the specimens. It can be used in the characterisation of small and big specimens, and it is also possible to scan their surfaces in order to detect and identify inhomogeneities. In the present study, a set of 19 stone beads (Fig. 1) discovered during archaeological excavations or surveys at three proto-historic settlements from south-west ´ lamo and Salsa 3, were analysed using X-ray Portugal, namely Castro dos Ratinhos, A diffraction in order to identify the rocks or minerals used for their manufacture. Castro dos Ratinhos is a fortified settlement with Late Bronze Age and Early Iron Age occupations (Silva and Berrocal-Rangel 2005; Berrocal-Rangel and Silva 2007). Fourteen stone beads ascribed to these two occupations and a small black glass bead, this one pointing out to an Oriental connexion, were retrieved during archaeological excavations that took place during the last 4 years. Salsa 3 was the object of archaeological investigations during 2006 and 2007; Late Bronze Age and Early Iron Age occupations were also identified at this archaeological site. Two ´ lamo is a fortified settlement, apparently with beads were selected from this site. A only one occupation, of Late Bronze Age chronology. The three beads analysed were recovered during an archaeological survey. All these proto-historic settlements are
A.P. Gonc¸alves (*) and A.M.M. Soares Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953 Sacave´m, Portugal e-mail:
[email protected] A.C. Silva Direcc¸a˜o Regional de Cultura do Alentejo, R. de Burgos, 5, 7000-863 E´vora, Portugal L. Berrocal-Rangel Universidad Auto´noma de Madrid, Ciudad Universita´ria de Cantoblanco, 28049 Madrid, Spain I. Turbanti-Memmi (ed.), Proceedings of the 37th International Symposium on Archaeometry, 227 DOI 10.1007/978-3-642-14678-7_32, # Springer-Verlag Berlin Heidelberg 2011
228
A.P. Gonc¸alves et al.
Fig. 1 Stone beads and location of the archaeological sites where the beads were discovered
located in the Guadiana Valley, close to the Portuguese–Spanish border, with distances between them smaller than 40 km (Fig. 1). To the best of our knowledge, this work represents the first study of a set of protohistoric stone beads from Portuguese archaeological sites. Green stone beads from the Neolithic and Chalcolithic periods have been previously subject to analyses using powder X-Ray diffraction, which, due to the destructive nature of the method, were only carried out on broken beads (Caneˆlhas 1973; Gonc¸alves 1979). More recently, a bead from a burial necklace with 17 stone beads found in a cist of the south-western Middle Bronze Age necropolis of Carapinhais (see Fig. 1) was analysed by SEM, revealing a mineral belonging to the chlorite group (Gonc¸alves 2007). Using the same technique that we use in our study, supplemented by an Energy Dispersive X-ray Fluorescence (EDXRF) analysis, a bead of talc from a south-western Bronze Age burial was also previously analysed (Gonc¸alves et al. 2005).
2 Experimental X-ray diffraction was used to identify the rocks and/or minerals that constitute the stone beads. A particular sample holder was developed in order to hold and place the entire collection of stone beads in the proper positions required to perform the
Stone Beads from Late Bronze Age and Early Iron Age Settlements
229
X-ray diffraction experiments. The X-ray diffraction system consisted of an X’Pert Panalytical diffractometer with a Bragg-Brentano assembly. The scans were performed at room temperature, in a reflection mode, using monochromatic CuKa ˚ ) and under set conditions of 45 kV and 40 mA. The data were radiation (1.54056 A measured with a 2y step size of 0.02 in a 2y-range of 5 –65 and a counting time of 3 s per step, in order to obtain good statistical data. Phase identification was derived from the X-ray diffraction data using the PowderCell programme (Nolze and Krauss 2000).
3 Results and Discussion A first observation of the X-ray diffraction data of all samples immediately shows that most of the stone beads are composed of the same rock or mineral (Fig. 2). Only one bead, sample A, collected at Castro dos Ratinhos, shows a significantly different X-ray diffraction spectrum, pointing to a dissimilar composition. This is in agreement with the macroscopic observation of the stone beads, where only sample A appeared different and was made of a soft green stone, in contrast with the other beads, which were made of hard brown-red stones. The spectra obtained for the majority of the samples have the most intense peak at 2y ~26.65 , followed by intense peaks at ~20.87 , ~39.44 , ~50.14 , and ~59.97 . This clearly indicates that the material used for all of the stone beads, with the exception of sample A, is quartz. The quartz used in the manufacture of these beads is of the carnelian variety. This aspect is also rendered in some identical elemental and X-ray diffraction patterns, pointing out to a similar mineralogical composition and suggesting the same lithological origin for the carnelian beads.
A
20
Rat
inho
40
Fig. 2 X-ray diffraction patterns of the stone beads
2θ (°
)
Sals
60
a3
Ála
mo
s
230
A.P. Gonc¸alves et al.
The diffractogram obtained for sample A has a much larger number of peaks, the most intense ones being at 2y ~12.47 , ~18.76 , ~19.70 , ~20.98 , ~23.08 , ~25.10 , ~35.43 , ~36.95 , ~45.37 , ~60.18 , and ~67.19 . This indicates that: (1) either the constituent stone is composed of several different minerals or (2) it is composed of a mineral with low structural symmetry. The analysis of the position and intensity of the peaks allowed us to conclude that stone bead A is mainly composed of only one mineral, belonging to the chlorite group, most probably clinochlore. The clinochlore constitution of sample A has parallels with the aforementioned necklace with 17 similar beads found in the SW Bronze Age necropolis of Carapinhais, not far (approximately 30 km) from Castro dos Ratinhos (Fig. 1). The uniformity of the material used for the manufacture of the beads found in ´ lamo and Salsa 3 contrasts with the general variety of stones Castro dos Ratinhos, A used in earlier times (in Neolithic or Chalcolithic times, for instance) for producing this type of artefacts. Beads in carnelian seem to be common in SW Iberian protohistoric settlements (Gibson et al. 1998). However, due to the lack of geological studies concerning this type of rock, it is not possible to identify the lithological origin of the carnelian beads found in the south-west of the Iberian Peninsula.
4 Conclusion In conclusion, the analysis of the samples by X-ray diffraction, a non-destructive technique, allowed us to identify the raw material of all the beads as quartz, with the exception of one (Sample A) made of a soft dark green stone, belonging to the chlorite group, most probably clinochlore, with parallels in a necklace with stone beads found in the SW Bronze Age necropolis of Carapinhais, not far from Castro dos Ratinhos. Moreover, the quartz used in the manufacture of most of the beads is of the carnelian variety. Identical elemental and X-ray diffraction patterns point toward a similar mineralogical composition, suggesting the same lithological origin for the carnelian beads, an origin which, however, was impossible to determine. This uniformity of the material used for the manufacture of the beads contrasts with the variety of stones used in earlier times (Neolithic or Chalcolithic) in the making of this type of artefacts.
References Berrocal-Rangel L, Silva ACS (2007) O Castro dos Ratinhos (Moura-Alqueva, Portugal): Um complexo defensivo no Bronze Final do Sudoeste peninsular. In: Moret P, Berrocal-Rangel L (eds) Paisajes Fortificados de la Edad del Hierro. Las Murallas Prohistoricas de la Meseta y de la Vertiente Atla´ntica en su contexto europeo. Real Academia de la Historia/Casa de Velazquez, Madrid, pp 169–190 Caneˆlhas MGS (1973) Estudo radiogra´fico de “calaı´tes” portuguesas. Revista de Guimara˜es 83(1/4):125–145
Stone Beads from Late Bronze Age and Early Iron Age Settlements
231
Gibson C, Correia VH, Burgess CB, Boardmann S (1998) Alto do Castelinho da Serra (Montemor-oNovo, E´vora, Portugal). A Preliminary Report on the excavations at the Late Bronze Age to Medieval Site, 1990–1993. J Iberian Archaeol 0:189–244 Gonc¸alves AAHB (1979) Elementos de adorno de cor verde provenientes de estac¸o˜es arqueolo´gicas portuguesas. Importaˆncia do seu estudo mineralo´gico. In: Actas da 1a Mesa-Redonda sobre o Neolı´tico e o Calcolı´tico em Portugal (Porto, Abril de 1978), pp 209–225 Gonc¸alves AAHB (2007) Identificac¸a˜o mineralo´gica de uma conta da Necro´pole dos Carapinhais (Sobral da Adic¸a, Moura). Vipasca. 2a Se´rie, vol 2. pp 191–193 Gonc¸alves AP, Vale´rio P, Soares AMM, Arau´jo MF (2005) A stone bead from a SW Bronze Age burial: analysis by EDXRF and X-ray diffraction, O Arqueo´logo Portugueˆs, Se´rie IV, vol 23. pp 257–264 Nolze G, Krauss W (2000) PowderCell 2.3 Program. BAM, Berlin Silva AC, Berrocal-Rangel L (2005) O Castro dos Ratinhos (Moura), povoado do Bronze Final do Guadiana: 1a campanha de escavac¸o˜es (2004). Revista Portuguesa de Arqueologia 8(2): 129–176
