Portable Raman verification and quantification of jade in Olmec ceremonial axes from El Manatí, Veracruz, Mexico

The El Manatí archaeological site in Veracruz State, Mexico, is famous for its deposits of ceremonial stone axes and beads from the Olmec culture. Some of these have been considered to be in jadeite‐jade, but this rock type in these artefacts has never before been physico‐chemically verified. A portable Raman spectrometer was employed in situ inside the Centro‐INAH Veracruz reserve for this remote analytical operation of non‐destructive direct hand‐held analysis to identify some of the mineral species present. The key points of interest were as follows: which objects can correctly be called ‘jade’ because they contain the high‐pressure minerals jadeite or omphacite and, if so, with what mol. % Jd (Jd%) in the solid‐solution jadeite–diopside–hedenbergite. The Jd% value was then quantified from the wavenumber shift of the symmetric Si–O–Si Raman vibration band. Although some artefacts did not yield any usable Raman spectra, exploitable spectra were obtained from 41 stone axes or beads. Jadeite was found in 13 artefacts and omphacite in another 11. Jd% varied from 95 to 38% such that many of these jades are actually omphacite‐jade rather than jadeite‐jade. The complicated terminology of jades is thus re‐examined. A partial correlation is shown to exist between the Jd% value and the density of the artefacts. These results provide important, hitherto unknown, mineralogical data for archaeologists studying artefacts at different horizons at different sites of Mesoamerican cultures. These results also allow these artefacts to be now labelled correctly in their Museum reserve or exhibition display. Copyright © 2017 John Wiley & Sons, Ltd.