Colour and chemical variations in adamite-olivenite solid solution minerals from the Lavrion mines, Attica, Greece

The Lavrion mining district is as world-class site of geological and historical value. In terms of geology, an impressive succession of geodynamic processes i.e., metamorphism, tectonic and magmatic activity, ore-formation, and supergene oxidation, created over time a unique diversity of primary and secondary minerals. Until today, more than 600 minerals have been remarked at the broad Lavreotiki area, corresponding to nearly 12% of all known species (Voudouris et al., 2021, and references therein). Historically, Lavrion comprises one of the most important mining districts on Earth, exhibiting mining works that date back to at least the 3rd millennium B.C. However, it is mostly known for its silver mines that flourished during the classical period (6th-4th centuries B.C.). Among other mineral species, adamite-olivenite solid solution mineral specimens (hereafter “adamite”) from Lavrion, are of exceptional quality since they exhibit significant morphological and colour variations. Collectors use several names (e.g., cuproadamite, aluminium-adamite, zincian olivenite, etc.) to describe “adamite” specimens based on colouration and/or crystal habit. However, the terminology used often does not reflect either the chemistry of the specimen, or the official nomenclature. Thus, a proper characterization of “adamite” specimens from Lavrion remains partly inaccurate. Here we present preliminary mineral-chemical data for the Lavrion “adamites” collected by SEM-EDS (HSGME, Athens, Greece) and EPMA (WWU-Institute of Mineralogy, Munster, Germany). Among other arsenates, minerals of the adamite group and especially adamite [Zn2(AsO4)(OH)] and olivenite [Cu2(AsO4)(OH)] are scarcely present in oxidation zones of polymetallic deposits (Southwood et al., 2020). Zinc can be completely substituted by Cu, thus forming a solid solution series between the two end-member species. Other impurities commonly comprise Fe, Co and Mn substituting for the cation sites and P replacing As. Recently, zincolivenite was identified as new mineral species with intermediate composition and an ideal Zn:Cu ratio of 1:1, corresponding to a compositional range of [Cu0.5Zn1.5(AsO4)(OH)] to Cu1.5Zn0.5(AsO4)(OH)], (Chukanov et al. 2007). At Lavrion, “adamite” exhibits a significant variety of both crystal habits and colours. It is found in many localities throughout the mining district (e.g., Kamariza, Plaka, Sounion) forming fine, radiating crusts or fan-shaped rosettes, pseudo-octahedral, bow-tie shaped or prismatic crystals, commonly set on a limonitic matrix (Fig. 1). It is commonly associated with other minerals like calcite, aragonite, smithsonite, hemimorphite, zincaluminite, etc., forming very aesthetic specimens. Colours range from various shades of green to blue, colourless, or rarely, black or with a yellowish tint. Traditionally, green-coloured varieties are described as ‘cuproadamite” or “cuprian adamite”, a term partly applicable to blue-colored varieties as well. Vivid blue crystals are often referred to as “aluminian adamite”. Pinkish/purple varieties are extremely rare at Lavrion and are described as “cobaltoan or manganoan adamite”. The latter two varieties are not considered in the present study. Blue crystals correspond to adamite composition (Fig. 2 and 3a-c) with a limited substitution of Zn by Cu (up to 2.73 wt %). No other impurities were found (e.g., Fe, Co, Al, Mn, P are mostly below the detection limit). An increasing Cu content results in greenish hues: green “adamite” specimens yielded a compositional range covering both fields of zincolivenite and olivenite (Cu content from 26 wt % to 58.16 wt %). None of the green-coloured crystals yielded Cu content lower that 0.5 apfu (Fig 2), at least in the studied specimens. Many analyses cluster around the Zn:Cu ratio of 1:1, corresponding to typical, green-coloured zincolivenite. The rare “black adamite” is built of a μm-sized film of colourless to light bluish adamite that grows on top of green zincolivenite (Figs. 2 and 3d-f), thus giving the overall impression of a black tint to the crystals. This adamite carries significantly more Cu (aver. 8.34 wt %) compared to the pure, blue-colored crystals, and evolves into zincolivenite as the Cu content exceeds 0.5 apfu towards the inner part of the crystals. Both yellowish and transparent crystals carry no significant impurities: the analyses plot in the adamite s.s. field and exhibit very minor Cu content (up to 0.30 wt% Cu). Based on EPM analyses, we conclude that adamite s.s. from Lavrion exhibits significant colour variations, from transparent, to yellow, to blue. No green-coloured adamite s.s. was identified in the studied samples. Crystals with green hues cover a significant compositional range from zincolivenite to olivenite. Further study will include other colourful varieties of “adamite” and will also investigate a larger number of trace elements using LA-ICP-MS, to fully define the chromophore elements responsible for the large variation in the colour of the desirable “adamite” specimens from Lavrion and RAMAN spectroscopy for identifying their spectral signature.