Relationships between soil colour and nnineralogical composition: application for the study of waste-rock dumps in abandoned mines

摘要

Soil colour is mainly influenced by the presence of colour-bearing minerals. In sulphides mine dumps (similar to natural soils but without significant organic and biological components), the main colouring agents are represented by several Fe-oxides, -oxyhydroxides and -oxyhydroxysulfates that form as a consequence of the AMD (Acid Mine Drainage) processes. In this work, after a review on the state of art, we present a case study about the relationships between colour and mineralogical composition from a sulphide-bearing waste-rock dump of the Libiola mine (Eastern Liguria, Italy). The dump is highly heterogeneous in granulometry, lithology, mineralogy and chemistry, either laterally or in vertical sections. Mineralogical analyses (optical microscopy, SEM-EDS, XRPD), associated with colour data (Munsell and CIE-L*a*b* colour parameters) confirm the correlation of the yellow hue/+b* (yellow) with the diffuse presence of goethite and the correlation of the red hue/+a* (red) with the presence of hematite. The results suggest that the characterization of the colour of waste-rock samples is a valuable and quick method to evaluate the presence and the distribution of the secondary products of AMD within highly heterogeneous mine waste disposals. Moreover, the results can be used as a quick indicator to plan field works and to design a sampling campaign.