Combined Raman spectroscopic and Rietveld analyses as a useful and nondestructive approach to studying flint raw materials at prehistoric archaeological sites

摘要

A set of six lithic tools, unearthed along 2011 in two karst sites of the Guadalteba County (Malaga, Spain), has been nondestructively investigated by Raman spectroscopy and laboratory x-ray diffraction. From a chemist's point of view, our goal is to develop a systematic screening protocol for a quick, easy, low cost and nondestructive characterization of archaeological flints based on Raman spectroscopy, x-ray diffraction and Rietveld refinement. In this paper, we firstly made use of Raman spectroscopy to determine, in a semiquantitative way, but with the generic advantage of a faster data acquisition than x-ray diffraction, the surface content of moganite of each lithic tool, from the ratio between the relative intensities of the two Raman-active symmetric stretching vibrations (A(1) modes) of alpha-quartz (465 cm(-1)) and moganite (501 cm(-1)). The precise bulk quartz/moganite weight content was then accurately quantified by means of high-quality x-ray diffraction, followed by Rietveld refinement. We found a general good correlation between Raman and x-ray data. Nonetheless, as recently reported in the scientific literature by other authors, the vibrational spectroscopic quantification of moganite in silica rocks like flint and chert have to be performed very cautiously, to avoid undesired interferences from other Raman features due to the eventual presence of silanol (SiOH) groups, which could finally lead to an overestimation of the surface moganite concentration. As reported in such a recent article, a useful treatment to reduce the interference from silanol-bands is to heat the samples prior to their Raman analysis at a minimum of 600 A(0)C (but better at 700 or 800 A(0)C) for silanol "dehydration". This, in our opinion, may be for sure a satisfactory procedure when studying flint or chert samples of a "geological origin". But not of practical use when studying lithic tools which were manufactured by men thousands and thousands of years ago.