The thermal behaviour of silica varieties used for tool making in the Stone Age

Abstract

Before 100,000 years ago, during the Middle Stone Age (MSA) of South Africa, silica varieties of minerals and rocks were sometimes heated during tool making in order to improve their knapping properties. If the heating and cooling process is not controlled, failure results and the nodules fracture. Recently, we postulated that the reversible α- to β-phase transition may play a role in causing silcrete, a type of rock often used to make stone tools in the Western Cape, to fracture. In this new study, we analyse the thermal behaviour (520–620 °C) of silcrete and compare it to that of two chalcedony samples from different origins, together with samples of chert, agate and flint. These minerals and rocks were commonly used to make stone tools. Differential scanning calorimetry (DSC) measurements show that the α- to β-phase transformation is prominent in silcrete, agate and one of the chalcedony samples, weaker in chert and the second chalcedony sample, but non-existent in flint. X-ray fluorescence (XRF), thermogravimetric analysis (TG) and carbon and sulphur analyses show differences in elemental composition between the rocks and minerals. X-ray powder diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy highlight differences in microstructure. These small differences in chemical composition and structure contribute to a variety of chemical reactions and phase transformations that can take place in rocks and minerals, which in combination determine their stability upon heating and show that care should be taken when generalising thermal behaviour.