Pseudomorphous transformation of goethite needles into hematite in sediments of the Triassic Skagerrak Formation, Denmark

The TEM of the red pigment in the Skagerrak Formation has revealed the following: (1) The main colouring agents in the Skagerrak Formation are goethite needles in the shallow parts, and hematite needles in the deeper parts. (2) Dissolved iron in the pore fluid precipitates as goethite needles at shallow burial depths. (3) As the needle shape is characteristic of goethite, a pseudomorphous transformation of goethite into hematite must have taken place during increased burial (burial temperatures of 56-75°C). The conversion of goethite to hematite, as suggested by Larson & Walker (1975), has been confirmed. (4) Hematite needles in several orientations occur as a rim on the detrital grains, whereas parallel hematite needles may occur in between clay-sized phyllosilicates, the latter being of only minor importance as a colouring agent in the Skagerrak Formation, but possibly similar to Fe-oxide crystallites in association with clays. Even though this hematite phase is of minor importance as a colouring agent in the Skagerrak Formation, it may be more common in other red beds with different burial histories.