NELSONITE
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.
