THE HORNBLENDE MINERAL IN NEOLITHIC POTS. TECHNOLOGICAL SIGNIFICANCE.

Several pots belonging to Neolithic cultures from the Romanian Plain contain crystals of brown hornblende. The brown hornblende is a calcic mineral belonging to the amphibole family, a highly complex series of inosilicates. Interestingly, the clay used as raw material for these Neolithic pots contains green, not brown, hornblende. No neighbouring sands (which could be added as a temper) contain such brown crystals. Therefore, the existence of the brown hornblende can be explained by in situ formation at the expense of the green hornblende during the firing process. We have designed and performed heating experiments to test this hypothesis. Crystals of green hornblende were added to clays similar to those used in the ceramics of the Sultana Culture Gumelnița. Crystal with sizes ranging 0.1-8 mm were selected to reproduce the crystal size distribution found in the Neolithic ceramics of interest. All crystals have an angle of extinction (c/Ng) varying between 20°-22°. Heating was performed in 5 steps, with a temperature range of 500°C-900°C.The main results are: - Up to about 600°C, the green hornblende does not change colour, regardless of its shape or size; - Between 600°C-700°C, at a high rate of heating, the green hornblende remains unchanged. At a slower rate of heating the small crystals become brown entirely while the larger ones change colour only along the border. In the large crystals colour and extinction angle change gradually from the brown border to the green core; - At 800°C regardless of the rate of heating, all crystals are tainted brown. Larger crystals, when heated rapidly develop a slight colour zoning and a gradual change in the extinction angle; - Between 800°C and 900°C, irrespective of size and rate of heating all green hornblende crystals become brown with a c/Ng=0. We interpret our observations as the result of oxy-substitution, i.e. hydrogen removal from the group hydroxyl (OH) group in the crystal structure of hornblende, coupled with an oxidation of the iron present in the crystal (Fe²⁺ to Fe³⁺). Our experimental results can be used, along other criteria, to estimate the temperature and burning rate of hornblende-bearing Neolithic pots. [ABSTRACT FROM AUTHOR]

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