## 3 Results
### Mineralogical Characterization  
The observation of the polished sections under an optical microscope enabled the rapid identification of the sulfurrous minerals contained in the solid unreacted samples. Using optical microscope observations, the vase was determined to be characterized by the presence of large amounts of pyrite (FeS\({}_{2}\)), chalcopyrite (CuFeS\({}_{2}\)), iron oxides and traces of covellite (Figure 3). Chalcopyrite was present in circular form, which would indicate that the mineral has undergone a mineralurgical treatment. Pyrite was in the form of an angular fragment, but mainly in the form of framboidal pyrite (Figure 3), composed of micro-cubes of pyrite, which is the most common form of pyrite in anoxic environments and also the most reactive one [36]. The sediments contained traces of sulfides such as pyrite and iron oxides.  
Quantitatively, in the sediments, the main occurring minerals were quartz (14-28 wt. %), epidote (3-7 wt. %), albite (19-30 wt. %), augite (2-5 wt. %), hornblende (4-8 wt. %), chlorite (9-14 wt. %) and traces of hematite (Figure 3 and Figure S1). Carbonate minerals, which are responsible for acidity buffering, were below the detection limit of XRD. Additionally, the mineralogical composition of the vases was similar to that of the sediments. The vases were mainly composed of silicates (quartz: 9-26 wt. %; epidote: 7 wt. %), feldspars (albite: 5-21 wt. %) and pyroxene (augite, enstatite), micas (chlorite) and oxides, such as hematite and traces of sulfides (arsenopyrite). Scanning electron microscope (SEM) observations of the rounded grains of sulfide (Figures 4 and 5) allowed their identifications as isocalcopyrite, a mineral close to isocuprite, formed at more than 200 \({}^{\circ}\)C. Three phases can be distinguished (Figure 5): a rounded phase surrounded by a halo of iron oxide, which is also surrounded by an exsolution phase of the same nature as the grain. The iron oxides and the exsolution texture highlighted by the X-mapping may result from the first  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{6}{c}{**Criteria for Assessing the Quality of Freshwater Sediments**} \\ \hline  & & & & **Concentrations (mg/kg)** & & \\ \hline
**Groups** & **Substances** & **REC** & **TSE** & **COE** & **CPE** & **CFE** \\ \hline Metals and & Arsenic & 4.1 & 5.9 & 7.6 & 17 & 23 \\ metalloids & Cadmium & 0.33 & 0.6 & 1.7 & 3.5 & 12 \\  & Chromium & 25 & 37 & 57 & 90 & 120 \\  & Copper & 22 & 36 & 63 & 200 & 700 \\  & Mercury & 0.094 & 0.17 & 0.25 & 0.49 & 0.87 \\  & Nickel & ND & ND & 47 & ND & ND \\  & Lead & 25 & 35 & 52 & 91 & 150 \\  & Zinc & 80 & 120 & 170 & 310 & 770 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Quality criteria for freshwater sediment (REC: rare effect concentration; TCE: threshold concentration producing an effect; COE: concentration of occasional effects; CPE: concentration producing a probable effect; CFE: concentration of frequent effects).  
alteration of the mineral. Iron oxides are also visible in some places of the grain, mainly on the inner periphery, a sign of oxidation.  
The observation of a larger area on the polished section allowed us to highlight that copper and zinc are not only contained in sulfurous minerals, but they are also associated with the vases (on organic matter). Zinc is not always concentrated in the places where copper is. Their large dispersion suggests that they are attached to the small grain size, and thus readily available for oxidation.  
Figure 4: Image observed at the SEM, energy-dispersive X-ray spectroscopy (EDS) spectrum and quantification of an isochalcopyrrite from vase sample number 81,850.  
Figure 3: Images of minerals observed by optical microscope in the polished section of sample vase number 81,850, (**a**) pyrite, (**b**,**c**,**e**) chalcopyrite with exsolution texture, (**d**) silicates, and (**f**) framboidale pyrite and silicate.