Mineralogical and geochemical characterization of Mo-rich black shales in the Grand Duchy of Luxembourg

Abstract

“Schistes carton”, Jurassic black shale rocks occurring in the south-western part of the Grand Duchy of Luxembourg, were investigated, based on a combined mineralogical and geochemical approach. The major aim of this study was to understand the lateral and vertical variation in mineralogy, and the distribution of major and trace elements within the different black shale lithologies, as well as in the overlying soils. The mobility of trace metal(loids) in the black shale and in the soil developed upon the black shale was also addressed. The “Schistes carton” bedrock consists mainly of illite and kaolinite, quartz, calcite and pyrite. Framboidal pyrite is found in the clay matrix, while stratiform pyrite lenses are found in calcitic beds or in lag deposits. At a depth of 4.25 m, pyrite is altered to goethite and gypsum. The acidity generated by pyrite oxidation is neutralized by calcite dissolution, resulting in neutral to slightly alkaline pH conditions. The “Shistes Carton” bedrock is characterized by variable concentrations of Ca, S, Cd, Mo and Zn. Compared to the surrounding matrix, the (stratiform) pyrites found in the “Schistes carton” express an enrichment with As, Cd, Pb and Zn, while the lag deposit contained high concentrations of As, Cd, Mn, Zn, and P. Mo concentrations in the bedrock and the soils up to 74 mg/kg were found, which is comparable with Mo concentrations in soils developed on black shales at other locations in the World. Most trace elements are not leached from the shales under the prevailing pH conditions (7–7.5), except Mo, which leaches in concentrations up to 85 μg/l. This study highlights the importance of small and large-scale variations in bedrock composition and its influence on soil geochemistry, which should be taken into account during the development of national and regional guidelines and regulations on soil quality.