| Formula | FeO.TiO2.nFe2O3Ca3(F,Cl)(PO4)3 | |
|---|---|---|
| Hardness | 5 - 6 | |
| Specific gravity | 3.7 - 4.1 bulk; ilmenite: 4.5-5.0 | |
| Streak | ilmenite: black to brownish-redapatite: white | |
| Color | black metallic and light-colored | |
| Habit | even-granular texture | |
| Luster | metallic to sub-metallic | |
| Fracture | conchoidal | |
| Tenacity | brittle | |
| Texture | holocrystalline |
Nelsonite is an igneous rock primarily constituted of ilmenite and apatite, with anatase, chlorite, phosphosiderite, talc and/or wavellite appearing as minor components. Rocks are equigranular with a grain size around 2 – 3 mm.[1] The black ilmenite is slightly magnetic while the whitish apatite is not.
It was named for Nelson County, Virginia, and is also found in that state's Amherst and Roanoke counties. In 2016, the Virginia legislature designated it as the official State Rock of Virginia.[2]
At one time, it was mined for the primary extraction of titanium dioxide. Titanium dioxide from ilmenite is used as a white paint pigments[3] and in the early 1900's as a colorant of artificial teeth.[4] The calcium phosphate from apatite is used as agricultural fertilizer. While no active mining of Nelsonite occurs in Virginia, active mining occurs in parts of China for rare-earth elements.[5]
Anorthosite-related nelsonite occurs mainly as veins and lensoidal intrusions in anorthosite complex or wall rocks. The Fe–Ti–P-rich magma liquids formed immiscible and eutectic mixtures crystallizing around 850-1,000 °C followed by intense fractionation.[6] Disagreement exists on the role liquid immiscibility plays in generating nelsonite and Fe–Ti oxides ore. It is not common to have such relatively large density differentials of mineral constituents as seen in Nelsonite; ilmenite (4.7–5.2 g/cm3) and apatite (~ 3.5 g/cm3). Nelsonites experience differential weathering. Exposed apatite within the Nelsonite is readily removed through solution by meteoric waters that can result in a cellular or sponge-like ilmenite mass.
Nelsonites are generally associated with anorthosite intrusions and scarce worldwide.[7] Nelsonite occurs at various localities, not limited to: Nelson, Amherst, & Roanoke Counties, Va.; Carthage & Cheney Pond, N.Y.; Laramie, W.Va.; Washington State; Quebec, Canada; and China.[8]
The nelsonite occurs in alkalic Roseland anorthosite, and in gneiss, granulite, and charnockitic rocks surrounding the anorthosite. Multiple varieties of equigranular nelsonite are present in the Roseland-Piney River district.
The prefixes ilmenite, rutile, magnetite, biotite, and hornblende denote special richness of the rocks in these minerals.