Goethite-hematite from the Precambrian of Minnesota, USA.

This rock is from a Minnesota iron mine that targeted banded iron formations, or BIFs, which are unusual, dense, marine sedimentary rocks consisting of alternating layers of iron-rich oxides and iron-rich silicates. Most BIFs are Proterozoic in age, although some are Late Archean. They do not form in today's oceans - they're an example of an “extinct” rock type. Many specific varieties of iron formation are known. The most attractive type is jaspilite, which is a reddish & silvery gray banded rock consisting of hematite, red chert (“jasper”), and specular hematite or magnetite. Most BIFs have been subjected to one or more orogenic (mountain-building) events, resulting in folding and/or metamorphism. BIFs are known from around the world, but some of the most famous & extensive deposits are found in the vicinity of North America’s Lake Superior Basin. Many BIFs have economic concentrations of iron and are mined as iron ores.

This is a cross-section view of a somewhat botryoidal layer of fibrous goethite (= dark colored iron hydroxy-oxide, FeO(OH)) with some reddish hematite (= iron oxide, Fe2O3).

Stratigraphy: float from the Biwabik Iron-Formation, middle Animikie Group, upper Paleoproterozoic, ~1.878 Ga