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Russian Geology and Geophysics

2001 year, number 7

COMPOSITION AND FEATURES OF CRYSTALLIZATION AND TRANSFORMATION OF SPINELS AT THE CONTACTS OF DOLERITES WITH DOLOMITES AND ROCK SALT

M. P. Mazurov and A. T. Titov
Keywords: Spinel, magnesian skarn, metasomatism, dolerite, dolomite, rock salt
Pages: 1100-1109

Abstract

Skarn and exsolution spinels from the contacts of intrusions of a trap complex with carbonate--salt-bearing deposits of the Siberian Platform cover have been studied using scanning and translucent electron microscopy and electron microprobe. Spinel at dolerite-dolomite contacts occurs within exoskarn zones of magnesian skarns formed at the magmatic stage and in the endoskarn zone of metasomatic column formed at the postmagmatic stage. The compositions of skarn spinels deviate from the stoichiometric ones. These spinels are solid solutions of the system MgAl2O4-FeAl2O4-MgFe2O4-FeFe2O4 with negligible concentrations of manganese, zinc, and titanium. In calciphyre, spinels are represented by the series MgAl2O4-MgFe2O4-FeFe2O4; in the spinel-forsterite zone, by MgAl2O4-FeAl2O4-FeFe2O4; and in the spinel-fassaite zone, by MgAl2O4-FeAl2O4. Among exsolution spinels there are four groups differing in composition, form, and time and mechanism of crystallization. Under exsolution of magnetite solid solution, first nonstoichiometric spinels of the series MgAl2O4-FeAl2O4-MgFe2O4-ZnFe2O4 formed by the mechanism of heterogeneous 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. Spinel was a source of alumina for phlogopite, clinochlore, hydrotalcite, and other minerals of postskarn associations.