the magnesia-alumina (spinel) system
/ spin-ELL /
The magnesia-alumina system is the classic textbook binary with a well-behaved intermediate compound. Its two components are periclase (MgO) on one end and corundum (Al2O3) on the other, and squarely in the middle sits the compound spinel, MgAl2O4 — the mineral that gives the whole spinel crystal-structure family its name, and the same structure that hosts the magnetic ferrites. Unlike mullite, spinel is beautifully cooperative: it melts congruently, at a high temperature around 2105 degrees C, so you can melt and crystallize it as a single pure phase.
Because spinel melts congruently, the diagram splits cleanly into two simple eutectic halves. On the magnesia side there is a eutectic between periclase (MgO) and spinel; on the alumina side there is a eutectic between spinel and corundum (Al2O3). Each eutectic is an invariant point (three phases, F = 0) sitting well below spinel's melting point. A useful subtlety: spinel is not a knife-edge line but opens into a solid-solution field at high temperature, dissolving extra alumina (and to a lesser extent extra magnesia), so its stoichiometry drifts — a real example of the intermediate compound also being a solid solution.
This system underpins some of the toughest refractories made. Spinel and magnesia-spinel bricks line steelmaking ladles and cement rotary kilns because spinel resists corrosive slags, has a high melting point, and takes thermal shock well. The two eutectics warn the refractory maker where the first liquid appears on either side of the ideal composition. And transparent spinel armor — clear MgAl2O4 that stops bullets — is the same compound grown pore-free, a striking reminder that the modest midpoint of a simple binary diagram can be a high-performance engineering ceramic.
Line a steel ladle with magnesia-spinel brick. Spinel (MgAl2O4) sits at the diagram's midpoint, melting congruently near 2105 degrees C, with eutectics on each side well below it. That high, clean melting plus slag resistance is exactly why this humble compound in the middle of a simple binary is a premier high-temperature refractory.
MgO-Al2O3: congruently melting spinel (MgAl2O4) in the middle, flanked by a periclase eutectic and a corundum eutectic.
Spinel is a congruently melting compound (contrast mullite), which is why it splits its binary into two tidy eutectic sub-systems. But do not read it as fixed-stoichiometry: at high temperature it dissolves excess alumina and becomes a wide solid solution.