EQUILIBRIUM RELATIONSHIPS ON THE LIQUIDUS SURFACE IN PART OF THE MnO‐Al2O3‐SiO2 SYSTEM*

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Abstract
The minerals found to have a stability range on the liquidus surface in the system MnO‐Al2O3‐SiO2 are cristobalite and tridymite, SiO2; mullite, 3Al2O3. 2SiO2; corundum, Al2O3; rhodonite, MnO.SiO2; tephroite, 2MnO.SiO2; galaxite, MnO.Al2O3; spessartite, 3MnO.Al2O3.3SiO2; and a new compound, 2MnO.2Al2O3.5SiO2. The quintuple points and boundary lines found in that part of the MnO.Al2O3.SiO2 system bounded by the systems 3MnO.Al2O3.3SiO2‐SiO2, 3MnO.Al2O3.3SiO2‐Al2O3, and Al2O3‐SiO2have been determined. Quintuple points and boundary lines involving the area inclosed by the systems 2MnO . SiO2‐3MnO . Al2O3 . 3SiO2, 3MnO . Al2O3 . 3SiO2‐SiO2, and 2MnO.‐SiO2‐SiO2 are indicated. Isotherms and isofracts (lines of equal refractive index) are given for the portion of the system investigated.X‐ray data show that crystals of the compound 3MnO.Al2O3.3SiO2 have the same structure as the garnet mineral spessartite (manganese garnet). The compound 2MnO. 2Al2O3 . 5SiO2, which is shown by X‐ray data to have a basic structure similar to cordierite, 2MgO.2Al2O3.5SiO2, seldom crystallizes from the glass, but a third substance which has a high extinction angle crystallizes readily from the reheated glass. These high extinction angle crystals are replaced by 2MnO .2Al2O3.5SiO2 when a proper heating cycle is employed, and they are not found again until the product is melted and crystallization is repeated.

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