Clay mineral assemblages as tracers of fireplaces in Pleistocene archaeological sites (Cueva del Ángel, Spain)

The mineralogical characterization (bulk and clay fraction) of the endokarstic deposit from Cueva del Ángel site, Lucena (Córdoba, Spain) has been carried out. The samples of the surroundings (within an approximate radius of 25 km) and one sample (A-1) from non-archaeological zone in the cave was also studied in order to perform a mineralogical correspondence with the samples of the sedimentary record. Samples of three different profiles and Stratigraphic Units (J/K: Units I, II, III; J/7: Units I, II; 7/8: Unit I) were considered. The mineralogical composition of the samples from the three profiles is similar. Calcite, phyllosilicates and phosphates (mostly apatite group), with minor quartz and dolomite, constitute the mineralogical association of the sediment. The surroundings samples have a similar bulk mineralogical composition than profiles of the sedimentary record, except for the absence of phosphates (associated with the deposit). On the contrary, significant differences were observed in the clay minerals assemblages of the three studied profiles, and also between the Stratigraphic Units. Stratigraphic Unit III of J/K profile and 7/8 profile are dominated by a clay mineral assemblage consisting of smectite, illite and kaolinite. In these samples, the phyllosilicates show a higher degree of ordering, and they are similar to the surroundings samples. The same clay minerals are detected in samples of Stratigraphic Units II of J/7 and J/K profiles (in the last one kaolinite only appears in traces), but the degree of ordering of phyllosilicates is lower. The phyllosilicates present in Stratigraphic Units I from J/K and J/7 profiles exhibited a high degree of degradation, being impossible their identification. This fact could be related to a thermal effect in this zone of the sedimentary record. In fact, X-ray patterns of sample A-1 (located in a non-archaeological zone in the cave) heated to 600 °C and 1000 °C are similar to the XRD of degraded samples from Stratigraphic Units I from J/K and J/7 profiles. SEM study confirms the heating action on phyllosilicates present in some zones of the sedimentary record, where evidences of cracking due to thermal events can be observed on aluminium silicates belonging to Stratigraphic Unit I from J/K and J/7 profiles. Considering the clay mineralogical study, the areas of the sedimentary record affected by fire have been delimited. Therefore, the clay mineralogy study could be used as fireplace tracers in sedimentary records.