Chromium isotope composition of organic-rich marine sediments and their mineral phases and implications for using black shales as a paleoredox archive

•Sequential leaching identified carbonates/phosphates, silicates (clays) and pyrite as Cr hosts in black shales.•Clays are the most important Cr host phase in the investigated black shale samples, suggesting a mainly detrital Cr pool.•The clays are characterised by a positively fractionated Cr isotope composition compared to the igneous baseline.•All Cr hosting phases have similar Cr isotope compositions suggesting diagenetic homogenisation.