Rocks consisting essentially of iron-titanium oxides and apatite occur as small intrusive-like bodies associated with anorthosites and some alkaline igneous complexes. They have a consistent composition of two-thirds by volume oxides and one-third apatite, and invariably have dikes rich in ferromagnesian minerals and apatite associated with them.Reconnaissance experiments in the system magnetite-fluorapatite indicate a eutectic at a composition of approximately two-thirds by volume of magnetite and one-third apatite, which provides an explanation for the common occurrence of rocks with this composition. Experiments indicate that eutectic mixtures of magnetite and apatite form immiscible liquids with silicate melts having the composition of the dioritic dike rocks commonly associated with oxide-apatite deposits. Mixtures of magnetite, diorite and apatite, containing apatite in excess of thirty percent form three immiscible liquids on melting: an apatite-rich one, a magnetite-apatite melt and a silicate melt.Analysis of coexisting magnetites and ilmenites from naturally occurring oxide-apatite rocks indicate temperatures of formation in the range of 850 to 1,000 degrees C.Oxide-apatite rocks are concluded to have formed as immiscible liquids, which separated from magmas that underwent strong differentiation. It is postulated that high sodium contents in the silicate magmas play an important role in forming these immiscible liquids.