An abrasive grain is disclosed and may include a body. The body may define a length (l), a height (h), and a width (w). In a particular aspect, the length is greater than or equal to the height and the height is greater than or equal to the width. Further, in a particular aspect, the body may include a primary aspect ratio defined by the ratio of length:height of at least about 2:1. The body may also include an upright orientation probability of at least about 50%.

A plate-like alumina powder production method of the present invention comprises placing a transition alumina and a fluoride in a container such that the transition alumina and the fluoride do not come into contact with each other and then performing heat treatment to obtain a plate-like -alumina powder. The transition alumina is preferably at least one selected from the group consisting of gibbsite, boehmite, and -alumina. It is preferable that the amount of the fluoride used is set such that the percentage ration of F in the fluoride to the transition alumina is 0.017% by mass or more. The container preferably has a volume such that a value obtained by dividing the mass of F in the fluoride by the volume of the container is 6.510.sup.5 g/cm.sup.3 or more. The heat treatment is preferably performed at the temperature of 750 to 1,650 C.

An alumina sintered body according to the present invention has a degree of c-plane orientation of 90% or more as determined by Lotgering's method from an X-ray diffraction profile obtained by irradiating a plate surface with X-rays in a range of 2=20 to 70. The alumina sintered body has no pores when a cross-sectional surface formed in a direction perpendicular to the plate surface is polished using an Ar.sup.+ ion beam and a mask and is examined under a scanning electron microscope at a magnification of 5,000 times. The alumina sintered body has a total mass fraction of impurity elements other than Mg and C of 100 ppm or less. This alumina sintered body has a high degree of orientation, high density, and high purity and thus has a higher optical translucency than those known in the art.

A method for producing a transparent alumina sintered body includes (a) the step of preparing an alumina raw material powder containing a plate-like alumina powder having an aspect ratio of 3 or more so that the mass ratio R1 of F to Al in the alumina raw material powder is 5 ppm or more, and forming a compaction raw material containing the alumina raw material powder into a compact, and (b) the step of pressure-sintering the compact at a temperature at which F evaporate to yield a transparent alumina sintered body.

96 parts by mass of a -alumina powder, 4 parts by mass of a an AlF.sub.3 powder, and 0.17 parts by mass of an -alumina powder as a seed crystal were mixed by a pot mill. The purities of each raw material were evaluated, and it was found that the mass ratio of each impurity element other than Al, O, F, H, C, and S was 10 ppm or less. In a high-purity alumina-made sagger having a purity of 99.9 percent by mass, 300 g of the obtained mixed powder was received, and after a high-purity alumina-made lid having a purity of 99.9 percent by mass was placed on the sagger, a heat treatment was perforated at 900 C. for 3 hours in an electric furnace in an air flow atmosphere, so that an alumina powder was obtained. The value of AlF.sub.3 mass/container volume was 0.016 g/cm.sup.3.

An abrasive grain is disclosed and may include a body. The body may define a length (l), a height (h), and a width (w). In a particular aspect, the length is greater than or equal to the height and the height is greater than or equal to the width. Further, in a particular aspect, the body may include a primary aspect ratio defined by the ratio of length:height of at least about 2:1. The body may also include an upright orientation probability of at least about 50%.

There are provided processes for converting alumina into -Al.sub.2O.sub.3 or transition alumina that comprise heating the alumina at a temperature of about 900 C. to about 1200 C. in the presence of steam and optionally at least one gas under conditions suitable to obtain the -Al.sub.2O.sub.3 or transition alumina. For example, the alumina can comprise a transition alumina (such as -Al.sub.2O.sub.3), an amorphous alumina or a mixture thereof.

The present invention aims to provide a simple and efficient method for manufacturing -alumina particles, main component particles of which each have a crystal face other than the face [001] as a main crystal face and a polyhedral shape other than a hexagonal bipyramidal shape. According to the method for manufacturing -alumina particles of the present invention, when an aluminum compound is calcined in the presence of a specific content of a metal compound, -alumina particles each having a particle diameter of 50 m or less, a degree of crystallization of 90% or more, and a polyhedral shape can be obtained.

An abrasive grain is disclosed and may include a body. The body may define a length (l), a height (h), and a width (w). In a particular aspect, the length is greater than or equal to the height and the height is greater than or equal to the width. Further, in a particular aspect, the body may include a primary aspect ratio defined by the ratio of length:height of at least about 2:1. The body may also include an upright orientation probability of at least about 50%.

A plate-like alumina particle, in which a ratio I(006)/I(ll3) of a peak intensity I(006) at 20=41.60.3 degrees which corresponds to a (006) face to a peak intensity I(113) at 20=43.30.3 degrees which corresponds to a (113) face of diffraction peaks obtained by X-ray diffraction measurement using a Cu-Ka ray, is 0.2 or more. A method for manufacturing the plate-like alumina particle including mixing an aluminum compound including an aluminum element, a molybdenum compound including a molybdenum element, and a shape-controlling agent to produce a mixture and firing the mixture.