Synthetic Sapphirine and Its Stability Relations in the System MgO-Al₂O₃-SiO₂

Abstract

A compound corresponding in optical properties and in X-ray powder pattern with the natural mineral sapphirine has been synthesized by repeated sintering of the constituent oxides at atmospheric pressure. The most probable formula for this artificial compound is $$4 MgO\cdot5 Al_{2}O_{3} \cdot2 SiO_{2}$$. An unidentified crystalline phase previously noted in certain magnesia-alumina-silica porcelains has been shown to be identical with this synthetic sapphirine. There is reason to believe that sapphirine possesses a small field of primary crystallization on the liquidus surface of the ternary system $$MgO-Al_{2}O_{3}-SiO_{2}$$. Some revision of the earlier work on this system therefore appears to be necessary. Experiments designed to prove the existence of sapphirine as an artificial mineral and to determine its thermal behavior and its stability relations with other crystalline phases in the ternary system have been carried out. The results indicate that synthetic sapphirine melts incongruently at or near 1475° C. to yield spinel and liquid. Immediately below that temperature, sapphirine is compatible with cordierite, spinel, and mullite, and at still lower temperatures with corundum also. On the basis of these results a revision of the equilibrium diagram for the system $$MgO-Al_{2}O_{3}-SiO_{2}$$ has been made. It is hoped that this revision will considerably enhance the usefulness of the diagram to petrologists and ceramic technologists.