Crystal chemistry and petrology of coexisting galaxite and jacobsite and other spinel solutions and solvi '
Galaxite (Gx:MnAlzO+) and jacobsite ("ID:MnFezO+)-rich spinels occur together in the metamorphosed manganese deposit near Bald Knob, North Carolina. They show limited mutual solubility with lz-lgVa Jb in Gx and 19-22% Gx in Jb. The implied solvus is consistent with syntheses oflshida et al. (1977) along the Gx-"ID pseudobinary. Refinement of the structure of one galaxite (Gxg5JbrcSpaGhr) 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*XFe3*,Al)zOa spinels suggest hat the ferrites (acobsite, franklinite, magnesioferrite and magnetite) show extensive solid solution and are separated from aluminate solid solutions (galaxite, gahnite, spinel (s.s.) and hercynite) by wide solvi at metamorphic temperatures.
MINERALOGICAL ASPECTS OF THE SYSTEM
The phase relations in the systern FeaOr-Mn:Or-ZnMn:Or-ZnFezOr have been determined at temperatures up to 13000 C. by t}re study of synthetic preparations and natural minerals. The equilibrium diagram shows complete mutual miscibility of the components at temperatures above 1000'and a considerable field of incomplete miscibility at ordinary temperatures. Mix-crystals rich in iron have a cubic lattice, those rich in manganese a tetragonal lattice. Diagrams are given showing the variation with composition in dimensions and volume of the unit cell, and in the axial ratio for tetragonal phases. Analyses of magnetite, jacobsite, franklinite, vredenburgite, hetaerolite and hausmannite are plotted on a composition diagram, and composition limits for these mineral species are proposed. The use of vredenburgite as a geologic thermometer is suggested.
dc Resistivity in the Nickel and Nickel Zinc Ferrite System
The dependence of the resistivities of nickel and nickel zinc ferrites prepared from high purity reagents upon composition and firing temperature has been studied.It has been determined that iron deficient nickel zinc ferrites are of much higher resistivity than iron deficient nickel or zinc ferrites.The iron deficient ferrites prepared, which were fired in oxygen below 1300°C and have Ni:Zn ratios greater than 3:7, show a positive thermoelectric voltage. Other compositions or materials fired above 1300°C have a negative thermoelectric voltage.The effect of excess iron upon resistivity is dependent upon the presence or absence of zinc at temperatures below 1300°C.