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FigureS1.tifTIFF image7703KThe black slag studied: (a) M1, (b) M1 T, (c) M2, (d) M2 T, (e) M3 and (f) M4.
FigureS2.tifTIFF image227KMicro-Raman spectrum of sample M1 T: lepidocrocite (LE), high magnesium calcite plus aragonite (HMCA), magnesioferrite (MF), magnetite (M), tefroite (T) and silicates (SI).
FigureS3.tifTIFF image4023KM1 T sample: a) SEM microphotography (100 µm) of a zone of crushed M1 T sample (Figure central part) together with the elements: Cr, Mn, Ca, Fe, Al, Si, S and Mg; b) micro-Raman spectrum of hematite (H), walstromite (W), magnetite (M) and high magnesium calcite (HMC).

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