Mineralogy and geochemistry of Fe-Ti oxide and apatite (nelsonite) deposits and evaluation of the liquid immiscibility hypothesis.

Modal mineralogy determined for 32 Fe-Ti oxide and apatite rocks from localities in Virginia, New York, Quebec, Norway, and Sweden largely supports the 2:1 oxide:apatite ratio suggested as a eutectic mixture by Philpotts (1967). A fairly consistent suite of accessory minerals, including biotite, clinoamphibole, spinel, zircon, and sulfides, is present. Some silicate accessory minerals tend to form glomeroporphyritic intergrowths in an equigranular matrix of nelsonite, suggesting that the accessories are xenocrysts that have been trapped in an oxide-apatite liquid. This liquid may represent an extreme case of partitioning of high charge-density cations into a low silica, phosphorus-enriched immiscible melt. Temperature and oxygen fugacity estimates for oxide pairs in nelsonites range from 600 degrees to 1,000 degrees C and 10 (super -20) to 10 (super -11) atm f (sub o ₂ ) and bracket an equilibration path that is close to the FMQ buffer curve. Several stages of exsolution are evident in Fe-Ti oxides of some deposits. Nelsonites contain fluorapatite that is enriched in light lanthanides. The occurrence of nelsonite "dikes" in Roseland anorthosite and associated rocks of the Roseland-Piney River district of Virginia is explained by the infilling of fractures with nelsonite liquid that has settled out of an overlying ferrodiorite pluton.