.alpha.-alumina powder and process for producing the same

A process for producing .alpha.-alumina powder comprising the step of calcining under atmospheric pressure at least one of transition alumina or a transition alumina precursor capable of being converted to transition alumina on heating, in a gas atmosphere containing a hydrogen halide selected from the group consisting of hydrogen chloride, hydrogen bromide and hydrogen iodide, in the presence of a seed crystal to obtain .alpha.-alumina powder comprising .alpha.-alumina particles having a substantially octahedral or eicosahedral shape and having a D/H ratio of from 1 to 2 wherein D represents a maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of .alpha.-alumina and H represents a maximum particle diameter perpendicular to the hexagonal lattice plane;

• wherein said seed crystal is a crystal serving as a nucleus for crystal growth around the seed crystal, and a crystal of .alpha.-alumina grows around the seed crystal and wherein said seed crystal is at least one compound selected from the group consisting of .alpha.-alumina, an oxide of titanium, vanadium, chromium, iron and nickel and a nitride of titanium, vanadium, chromium, iron and nickel.