Colours of Gemmy Phosphates from the Gavà Neolithic Mines (Catalonia, Spain): Origin and Archaeological Significance

In the Neolithic Gavà mines, variscite and turquoise were exploited for ornaments manufacturing, although some prospective pits and tunnels were dug on other similar greenish minerals such as smectite or kandite. A 3D study of the distribution of mineral phases allows us to determine the parameters involved in variscite colours. Methods are comprised of quantitative colourimetry, thin section petrography, SEM-BSE-EDS, EMPA, XRD, Raman spectroscopy, and57Fe Mössbauer spectrometry. Mapping of the mines indicates that colour is not directly dependent on depth. Although variscite from Gavà is poor in Cr3+ and V+3 compared with gemmy variscite from other localities, the deep green samples content has the highest values of Cr3+. In the case of cryptocrystalline mixtures with jarosite, phosphosiderite, or goethite, variscite tends to acquire a greenish brown to olivaceous hue. If white minerals such as quartz, kandite, crandallite, or alunite are involved in the mixtures, variscite and turquoise colours become paler.
This paper has been granted by the projects 2014 SGR 1661, 2014/100820, 2017 SGR 995, and CLT009/1800044 of the Generalitat de Catalunya; by the project RYC2016-21108 of MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future” (EU).