Abstract
A strongly reduced native iron-bearing andesite breccia from Disko contains graphite-rich modified shale xenoliths with magnesian spinels. These spinels are free from or very poor in ferric iron and vary considerably within the MgO-FeO-Cr2O3-Al2O3 compositional space. Through a simple substitution of the type 3 (Mg, Fe)2+ ⇆ 2(Al, Cr)3++□vacancy, the spinels vary from stoichiometric (Mg, Fe)2+ (Al, Cr)3+ 2O4 towards (Al, Cr)2O3. The simple substitution of Cr for Al suggests that Cr was accepted into the spinel structure as Cr3+, despite the reduced nature of the enclosing andesite. The most magnesian spinels are cation deficient spinelss in the synthetic systems MgO-Al2O3 and MgO-Al2O3-Cr2O3. The absence of exolved (Al, Cr)2O3-component is probable due to rapid quenching.
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