Intergrowths of corundum and spinel from the Thurein-Taung deposit (Myanmar)

The article displays data on formation conditions and features of unique corundum-spinel aggregates from the Thurein-Taung deposit in Myanmar (Burma). Chemical composition of corundum and spinel is given, and there is traced the effect of iron admixture on the coloring of spinel and parameters of its crystal lattice on the base of X-ray diffraction analysis. The surface of corundum on contact with spinel was studied in detail with aid of the color cathodoluminescence; it is shown that the process of overgrowing by spinel was preceded by resorption of corundum surface. Mineral and fluid inclusions in these minerals have been studied with use of microprobe analysis and the Raman spectroscopy. The temperatures of homogenization have been determined for liquid carbon dioxide in fluid inclusions inside corundum. On the base of mineral equilibria analysis in the Al—Mg—O—CO₂—H₂S—C system there were determined the optimal activities of magnesium and calcium and the fugacity of carbon dioxide. They are equal to а_Mg = 10^–5; а_Ca = 10^–1 and f_CO2 = 10^–4. It was found that corundum-spinel aggregates formed at temperature about 400 ºС as a result of partial dissolution of corundum protocrystal by high-temperature solutions containing Mg and Ca. Crystallization of the spinel took place with increasing pH (>3.5) and a decrease of redox potential (Eh ≤ –0.3 V).