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High-temperature phase relations and topological constraints in the quaternary system MgO-Al₂O₃-SiO₂-Cr₂O₃: An experimental study

¹ Dipartimento di Scienze della Terra, Università di Milano, Via Botticelli 23, 20133 Milano, Italy
² Petroceramics Srl, Via Botticelli 23, 20133 Milano, Italy

Correspondence: ^* E-mail: stefano.poli{at}unimi.it

Phase relationships in the system MgO-Al₂O₃-SiO₂-Cr₂O₃ (MASCr) were investigated experimentally from 1250 to 1560 °C using a 1-atm vertical quench furnace on 12 oxide mixtures covering a wide range of chemical compositions. Spinel, corundum, mullite, and sapphirine were found to display a wide range of solid solutions toward CR-rich end-members, whereas enstatite, cordierite, forsterite, and glasses were always Cr-poor. Glasses formed in relatively MgO-rich bulk compositions from 1250 °C, in assemblage with cordierite, enstatite, forsterite, and spinel, suggesting a lowering of the eutectic minimum in MASCr compared to available melting relationships in the model system MAS. Although unexpected, the relatively low-temperature melting of Cr-bearing mixtures is supported by topological constraints in the subsystems SACr and MSCr. The extent of Cr solubility in mullite has been found to reach ~14 wt% at 1560 °C, and 33 wt% in sapphirine at 1340 °C, which are the highest Cr values reported so far for these minerals. Liquidus surfaces are constrained by glass compositions that are displaced systematically toward silica compared to corresponding liquids in the ternary system MAS. As a consequence, the primary crystallization fields of Cr-spinel, Cr-sapphirine, and Cr-mullite are found to face the cristobalite/tridymite primary fields. A tentative liquidus diagram for the system MgO-Al₂O₃-SiO₂-Cr₂O₃ is proposed, which could serve as a basis for the interpretation of the high-temperature evolution of planetary materials as well as for the development of new ceramic materials.

Key Words: Mullite • spinel • sapphirine • chromium • glass • liquidus phase diagram