Two ceremonial polished ‘greenstone’ axe-heads from Mesoamerica were examined with a Raman microprobe in order to identify the mineral phases present, and hence to classify the rock type, as a feasibility test of this non-destructive method, i.e. an alternative to conventional destructive archaeometric methods such as diverse analyses made upon drilled cores, sawn slices, chipped fragments or scraped powders. Axe-head 1 from Mexico, constituted of a green rock with noticeable disseminated red or black minerals, revealed the presence of garnet, clinopyroxene and titanite with probable clinoamphibole; this is an eclogite since the available calibrations of the chemical variation of garnet or clinopyroxene mineral solid solutions with Raman wavenumber shifts permit semi-quantitative chemical analyses of these two critical minerals. Axe-head 2 from Guatemala consists of a uniform bluish green coloured mineral which is shown here to be nearly pure jadeite jade; the rock is thus a jadeitite. Both rock types are dense, hard, aesthetic and valuable. Known geological localities for either of these rock types are rare world-wide, especially in the Americas, as their mineral assemblages with Al predominantly in octahedral coordination require unusually high pressure for their crystallization. Axe-head 1 is the first record for the pre-Columbian utilization of, and commerce in, eclogite in Mesoamerica. This feasibility study demonstrates the utility of the Raman microprobe for the physico-chemical characterization of precious archaeological artefacts in stone. It is suggested that this technique could and should become a routine approach in archaeometry for identifications and provenancing.
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Smith et al. (1997) studied this question.