Mp. Mazurov et At. Titov, Composition and features of crystallization and transformation of spinels at the contacts of dolerites with dolomites and rock salt, GEOL GEOFIZ, 42(7), 2001, pp. 1100-1109
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.