Preferred orientation of precipitates in alumina-rich manganese aluminate spinels

\u3cp\u3eSingle crystals of manganese aluminates having compositions round about MnAl\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e have been grown by means of a floating zone technique. Precipitation of γ-(Mn, Al)\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e and α-Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e has been studied using optical microscopy and micro-Laue back-reflection technique. The γ phase precipitates as lamellae on {001} planes of the Mn-Al spinels, and α-Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e precipitates as platelets on {111} planes with orientation relations {0001}\u3csub\u3eα\u3c/sub\u3e/{111}\u3csub\u3espine1\u3c/sub\u3e and 〈1̄0〉\u3csub\u3eα\u3c/sub\u3e/〈1̄〉\u3csub\u3espine1\u3c/sub\u3e in majority and occasionally {0001}\u3csub\u3eα\u3c/sub\u3e/ {001}\u3csub\u3espine1\u3c/sub\u3e. Nucleation of corundum in the vicinity of the intermediate γ phase is discussed, the formation of the latter being shown to produce the contraction of the oxygen framework necessary for the precipitation of corundum.\u3c/p\u3