Scanning electron microscope study has revealed that in Proterozoic sandstones, shales, and carbonate rocks of the Visingsö Group in southern Sweden, detrital Fe-Ti oxides have undergone alteration by dissolution and/or replacement mainly by titanium oxides, hematite (reddish rocks), or pyrite (greenish sandstones, shales, and carbonate rocks). The titanium oxides occur as either poorly crystalline masses, cryptocrystalline aggregates, or discrete, euhedral crystals of anatase, brookite, and rutile that attain a variety of crystal habits.

Microprobe analyses have shown that the alteration of ilmenite occurs through several intermediate phases, each successively enriched in titanium and depleted in iron, to an almost pure form of TiO2. Some of the iron and titanium which is released during alteration of the Fe-Ti oxides is incorporated in associated clay minerals.

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