Electron microprobe analyses, compositions in the magnesioferrite-magnetite series very close to Fe 3 O 4 . All analyzed grains are zoned, with Fe 3 O 4 increasing toward the rim. The cores of magnetites in each of the five intrusions are chemically distinct and distinguishable from each other as indicated by projections from within the Cr-free spinel prism, MgFe 2 O 4 -Mg 2 TiO 4 -MgAl 2 O 4 -Fe 3 O 4 -Fe 2 TiO 4 -FeAl 2 O 4 , and a plot of Mn vs. Mg. The systematic chemical variation and zoning indicate magmatic origin. Magnetite crystals nucleated throughout the crystallization interval of the carbonatites, but most of them show evidence of marginal resorption.--Modified journal abstract.