Diagenesis and geochemistry of the Visingo group (Upper Proterozoic), southern Sweden: a clue to the origin of color differentiation.

Detailed petrographic examination, XRD, microprobe analysis and iron-content analysis suggest a diagenetic origin for the colours of the sandstones and shales of the Visingo group, Lake Vattern region, Sweden. Iron, derived from the alteration of biotite, ilmenite and possibly other minerals is the principal colouring agent. In the drab-coloured (grey to greenish) rocks the iron is chiefly ferrous and occurs as authigenic chlorite and pyrite. In the red rocks it occurs as fine-grained hematite, clearly derived from altered biotite. In the yellow to orange rocks the iron is also present as hematite, but in much smaller amounts. Hydromuscovite, quartz, K-feldspar, kaolinite, illite and mixed-layer clay minerals are additional diagenetic products and occur as grain coatings and pore-filling cements. -H.R.B.