Composition and features of crystallization and transformation of spinels at the contacts of dolerites with dolomites and rock salt

Skarn and exsolution spinels from the contacts of intrusions of a trap comp lex with carbonate-salt-bearing deposits of the Siberian Platform cover hav e been studied using scanning and translucent electron microscopy and elect ron microprobe. Spinel at dolerite-dolomite contacts occurs within exoskarn zones of magnesian skarns formed at the magmatic stage and in the endoskar n zone of metasomatic column formed at the postmagmatic stage, The composit ions of skarn spinels deviate from the stoichiometric ones. These spinels a re solid solutions of the system MgAl2O4-FeAl2O4-MgFe2O4-FeFe2O4 with negli gible concentrations of manganese, zinc, and titanium. In calciphyre, spine ls are represented by the series MgAl2O4-MgFe2O4-FeFe2O4; in the spinel-for sterite zone, by MgAl2O4-FeAl2O4-FeFe2O4; and in the spinel-fassaite zone, by MgAl2O4-FeAl2O4. Among exsolution spinels there are four groups differin g in composition, form, and time and mechanism of crystallization. Under ex solution of magnetite solid solution, first nonstoichiometric spinels of th e series MgAl2O4-FeAl2O4-MgFe2O4-ZnFe2O4 formed by the mechanism of heterog eneous nucleation, which disintegrated into pleonaste, magnesioferrite and franklinite. Then lamellae of magnesian-ferruginous Al-spinel and hercynite were successively generated by the mechanism of homogeneous nucleation. Sp inel was a source of alumina for phlogopite, clinochlore, hydrotalcite, and other minerals of postskarn associations.