Origin of the red colour in a red limestone from the Vispi Quarry section (central Italy): A high-resolution transmission electron microscopy analysis

Hematite has been considered as the most common stain to impart a red colour to rocks. During the Cretaceous Period, widely distributed oceanic red beds have been interpreted as evidence for oxic or sub-oxic conditions in the Tethys Ocean. To obtain an accurate understanding of redox conditions during this time, the origin and morphology of hematite must be addressed in detail. In this study, the morphology and distribution of hematite in both red and white limestones from the Vispi Quarry section, central Italy, were carefully investigated using high-resolution transmission electron microscopy and selected-area electron diffraction. The results suggest that nano-grains of hematite endow the limestones with a homogeneous red. This may indicate that the nano-grains of the ferric minerals are authigenic, implying oxic or sub-oxic conditions at the time the red limestones were deposited.