Mineralogical characteristics of cherts during and after heat treatment

Abstract Eleven chert samples from various geographical and geological locations and ages were heat treated, with two of the samples also undergoing in situ X-ray diffraction (XRD) during heat treatment. Characterisation was performed using X-ray fluorescence (XRF), XRD and Raman spectroscopy. The results indicated that the presence of moganite in cherts is independent of whether they are quartzitic or cristobalitic/tridymitic. After heating, cherts cannot be differentiated into these two types using XRD, as all the samples might contain quartz, cristobalite and tridymite. Heat treatment induced transformations in both siliceous and non-siliceous phases, forming new crystalline phases. The observed transformations at 1000°C agree with the literature data. The combination of XRD and Raman spectroscopy is crucial for comprehensive chert analysis, with Raman methods detecting lower concentrations and textural variations than XRD. A correlation exists between chert type (quartzitic or cristobalitic/tridymitic), i.e. mineralogy and age, but not between elemental composition and age. In situ XRD during heating shows distinct crystalline transformations. These transformations can affect significantly the properties of the cherts and has implications for conservation and provenance studies of archaeological tools. The analytical tools used to characterise the mineralogy of cherts are a fast and efficient alternative to conventional petrographic microscopy, especially in the case of heat-treated cherts.