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 synthetic compound is 4Mg0.-5Al2O3. 2Si02. An unidentified crystalline phase previously noted in certain magnesia-aluminasilica 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 Mg0-A1203-Si02. Some revision of the earlier work on this system therefore appears to be necessary. Experiments designed to prove the existence of sapphirine as a synthetic 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, and that 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 Mg0-A1203-Si02 has been made.

Critical thermodynamic evaluation and optimization of the MgO-Al2O3, CaO-MgO-Al2O3, and MgO-Al2O3-SiO2 Systems

“…The formula of sapphirine was assumed to be Mg 4 Al 10 Si 2 O 23 following Osborn and Muan [86] and Foster. [87] Sub-solidus phase equilibria were investigated by several authors [85,[87][88][89] using a sintering technique and x-ray phase determination. Smart and Glasser [89] and Foster [87] reported that the (sapphirine + corundum + mullite) phase assem- blage transformed to (spinel + mullite + corundum) above about 1460°C.…”

“…[87] Sub-solidus phase equilibria were investigated by several authors [85,[87][88][89] using a sintering technique and x-ray phase determination. Smart and Glasser [89] and Foster [87] reported that the (sapphirine + corundum + mullite) phase assem- blage transformed to (spinel + mullite + corundum) above about 1460°C. Smart and Glasser [89] reported that the (sapphirine + cordierite + corundum) phase assemblage transformed to (sapphirine + cordierite + mullite) above about 1386°C.…”

Effect of mineral and phase composition of starting mixtures on solid-phase synthesis of cordierite

“…The total admixture content may be from 5 to 20 vol.%. All well-known conventional methods that activate the synthesis process⎯common fine grinding, increase in annealing temperature and duration, introduction of fluxes and mineralizers as alkaline oxides, B 2 O 3 , MnO 2 , and tungsten and molybdenum compounds⎯decrease the cordierite formation temperature to 1220-1250°C [3] but do not increase its amount. According to [4], the liquid phase formed after introduction of alkali metal oxides into the charge greatly activates crystallization.…”

Melting and transformation temperatures of mineral and allied substances

“…For ternary equilibrium diagram see figure 9; lowest ternary eutectic with MgO• Si02, Tridymite and Cordierite, 1345°C. (Rankin and Merwin, 191a;Bowen and Greig, 1924;Greig, 1927a;Schairer, 1942;Foster, 1950Foster, , 1951aKeith and Schairer, 1952. ) 2Mn0• 2A1203· 5Si02, (2:2:5), Manganese Cordierite, incongruent melting to mullite and liquid about usaoc.…”

“…reprinted by permission of the Journal of Geology; copyright, 1952, by the University of Chicago. Composite diagram based on Rankin and Merwin (1918), Bowen and Greig (1924), Greig (1927a), Schairer (1942), andFoster (1950). Ricker and Osborn (1954); reprinted by permission of the American Ceramic Society.…”