Crystal chemistry and petrology of coexisting galaxite and jacobsite and other spinel solutions and solvi

Galaxite (Gx:MnAl₂O₄) and jacobsite (Jb:MnFe₂O₄)-rich spinels occur together in the metamorphosed manganese deposit near Bald Knob, North Carolina. They show limited mutual solubility with 12-19% Jb in Gx and 19-22% Gx in Jb. The implied solvus is consistent with syntheses of Ishida et al. (1977) along the Gx-Jb pseudobinary. Refinement of the structure of one galaxite (Gx₈₅Jb₁₀Sp₄Gh₁) reveals that Mn is located on tetrahedral sites and Al on octahedral sites and that galaxite is therefore a normal spinel. All available data suggest that jacobsite is also a normal spinel at low temperatures, and the solvus between galaxite and jacobsite does not seem to be caused by differences in normality/inverseness. Chemical and experimental data available for (Mg,Zn,Mg,Fe²⁺)(Fe³⁺,Al)₂O₄ spinels suggest that the ferrites (jacobsite, franklinite, magnesioferrite and magnetite) show extensive solid solution and are separated from alumínate solid solutions (galaxite, gahnite, spinel (s.s.) and hercynite) by wide solvi at metamorphic temperatures.