Nelsonites are scarce intrusive igneous rocks. They are also known as FTP rocks, in reference to their composition (Fe-rich, Ti-rich, P-rich igneous rocks). Nelsonites are principally composed of dark gray to blackish, metallic-lustered magnetite (Fe3O4) and ilmenite (FeTiO3) (or ilmenomagnetite; titanian magnetite), plus apatite (CaPO4). Minor minerals reported in nelsonites include spinel, olivine, pyrrhotite, and graphite.
These rocks typically occur as Fe-Ti-oxide concentrations in anorthosite complexes. Anorthosite is coarsely-crystalline, intrusive igneous rock overwhelmingly dominated by plagioclase feldspar. Nelsonites appear to form as cumulates in cooling batholiths. The sample shown below is from a dike-like intrusive body, apparently formed by remobilized Fe-Ti-oxide cumulates.
Nelsonite (above & below; above: ~4.5 cm across; below: ~2.0 cm across) from the Precambrian-aged Laramie Anorthosite Complex of Wyoming.
Geologic Unit & Age: Laramie Anorthosite Complex (probably the Sybille Fe-Ti Oxide Deposit; Sybille Oxide Deposit), early Mesoproterozoic, 1.43 b.y.
Locality: Laramie Range (probably from the Sybille Pit), southeastern Wyoming, USA.