Trade name for fused crystalline alumina or artificial corundum used as an quartz and beta quartz. b. Adj. Of or relating to one of two or more closely related minerals and specifying a particular physical structure (esp. a polymorphous modification); specif. said of a mineral that is stable at a temperature lower than those of its beta and gamma polymorphs (e.g., "alpha cristobalite" or "alpha -cristobalite," the low-temperature tetragonal phase of cristobalite). Some mineralogists reverse this convention, using alpha for the high-temperature phase (e.g., "alpha carnegieite," the isometric phase of carnegieite stable above 690 degrees C). c. In crystallography the angle between the b and c axes.