Shape of MgAl2O4 Grains in a CaMgSiAlO Glass Matrix

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Abstract
When sintered Al2O3 is annealed with CaMgSiAlO glass at 1600°C, polyhedral MgAl2O4 grains form and glass pockets are entrapped within the grains. After annealing for 13 h at 1600°C, the liquid pockets show a regular octahedron shape which is expected to represent the equilibrium shape. All grain surfaces in contact with the glass matrix show the same shape. The small grains, which must be shrinking, thus have the equilibrium shape, because their shrinkage shape is is identical to the equilibrium and growth shape. However, the octahedral shape also represents the growth shape for the growing large grains. The grains also form grain boundaries with neighboring grains.