English: Nelsonite in the Precambrian of New York State, USA.

Nelsonites are scarce intrusive igneous rocks. They are also known as FTP rocks, in reference to their composition: iron-rich (Fe), titanium-rich (Ti), and phosphorus-rich (P). 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. They appear to form as cumulates in cooling batholiths.

The weathered rock surface shown here is part of a nelsonite body, interpreted as an igneous dike, in Port Leyden, New York. This occurrence was first identified in the 1990s but the rocks were mined as iron ores in the late 1800s. Early mineral descriptions of the iron ore were inaccurate. Modern analysis by Darling & Florence (1995) has shown that Port Leyden nelsonites have 32-43% magnetite, 30-45% apatite, 8-15% ilmenite, and 5-11% iron sulfide (= brassy gold colored areas in the rock). Minor and trace minerals include chlorite, garnet, molybdenite, monazite, siderite, sphalerite, and zircon.

The nelsonite rocks at Port Leyden are black-colored, heavy, and extremely hard. Crack samples are very difficult to obtain - whacking with geology hammers usually results in small chips spalling away. Small pyrite patches and veinlets occur in the nelsonite.

Locality: waste rocks around old, flooded iron mine shaft, just south of North Street on the eastern side of the town of Port Leyden, just west of the Black River, western Adirondacks, southern Lewis County, north-central New York State, USA (~43° 35’ 05.47” North latitude, ~75° 20’ 27.28” West longitude)

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Reference cited:

Darling & Florence (1995) - Apatite light rare earth element chemistry of the Port Leyden nelsonite, Adirondack Highlands, New York: implications for the origin of nelsonite in anorthosite suite rocks. Economic Geology 90: 964-968.