Patterns of transparent spinel formation at Kukhilal deposit, south-western Pamir

Two groups of spinel crystals, holohedral and anhedral, were recognized at Kukhilal deposit. The latter are transparent raw crystals. It was found that anhedral transparent spinels formed due to reaction minerals (hydrotalcite and Mg chlorite) formation as envelopes between forsterite and spinel. These envelopes generated magals. They are viewed as bimetasomatic mineral associations produced in contacts of spinel and forsterite crystals. It was found that neogenic minerals (hydrotalcite, Mg chlorite, serpentine, brucite, etc.) preserved fragile spinel relics from Alpine orogenesis tectonic deformations due to their ductility. Transparent spinel was preserved in hydrotalcite-chlorite magal part. Magals are proposed to be regarded as jewelry spinel orebodies. Microprobe and X-ray structural analyses of spinel, forsterite, hydrotalcite, chlorite and serpentine are presented.

Oxide scale behavior of Al–7.5 mass%Mg alloy containing trace Ca during short-term oxidation

In this study, using transmission electron microscopy and phase diagrams from thermodynamic calculations, we investigated the oxide film formation of Al–7.5 mass%Mg alloy containing Ca traces during short-term oxidation in terms of the thermodynamic stability and multi-element oxides by inter-diffusion based on the results of analysis for the oxide film. For the oxidation test at 515 ˚C, for 1 h, its results showed that there is no significant difference between the Ca-added and Ca-free Al–7.5 mass%Mg alloys was observed, while further exposure caused the Ca-free alloy to gain significant weight. Based on the standard Gibbs free energy for oxide production calculated in this study, CaO was the most preferential product among the single metal oxides examined. As per calculations for MgAl2O4-spinel formation reactions, the spinel formation from MgO was thermodynamically the most favorable. According to the phase diagrams calculated in this study, various multi-element oxides including Ca could possibly form in the oxide layer of Ca-added alloy. The analysis results of transmission electron microscopy confirmed that MgO is the primary oxide in the Al–Mg binary alloys. In oxidation tests conducted for less than 1 h, the spinel was rarely found. The outmost areas of oxide layers correspond to MgO and CaO in Ca-free and Ca-added alloys, respectively. However, in the Ca-added alloy, the inner layer contains certain amounts of Ca, Al, and Mg.

MAGNESIOCHROMITE (MgCr2O4) SYNTHESIS: EFFECT OF MECHANICAL AND MICROWAVE PRETREATMENT

Magnesiochromite spinel synthesis based on oxides, hydroxides and nitrates of magnesium and chromium was investigated. The precursors reactivity was compared by means of effective rate constants calculated by Ginstling-Brounshtein equation. The possibility of use of this equation was confirmed by the dependences linearity with high linear approximation coefficients. The reactivity of MgO various forms (soft-burned, or caustic magnesite, and dead-burned, or periclase) in the spinel formation was compared. Oxide precursors (especially with the periclase participation) reacted with the substantially less rate in comparison with hydroxides and salts. The influence of a preliminary mechanical activation by impact-and-attrition (planetary mill) and attrition (ball-ring mill) action as well as a microwave treatment (2.45 GHz) was analyzed. The most positive effect of a mechanical treatment in a planetary mill that was associated with an activation of Mg and Cr compounds became apparent in the field of relatively low temperatures (700-1100 °С). For example, MgCr2O4 yield at 1000 °С and the joint impact treatment of oxides was twice as much than under the simple mixing. The subsequent temperature rising lead to some decrease of a pretreatment effect so long as diffusion coefficients in these conditions grew, so the reaction run rapidly even without a preliminary mechanical treatment. It was noticed that an appreciable reduction of MgO reactivity in the spinel formation after a certain attrition in a ball-ring mill linked to the plane sliding in cubic crystals and resulted in the removing of the most disordered and defective layer from grains and the plane surface uncovering. The combined method consisting of a mechanical treatment of magnesium and chromium nitrates mixture in a planetary mill and the subsequent burning in a thermal kiln (1000 °С) was considered as the most effective as it resulted in practically single product. The microwave treatment took up an intermediate position by the effectiveness.