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 alumina sintered body according to the present invention includes a surface having a degree of c-plane orientation of 5% or more, the degree of c-plane orientation being determined by a Lotgering method using an X-ray diffraction profile obtained through X-ray irradiation at 2=20 to 70. The alumina sintered body contains Mg and F, a Mg/F mass ratio is 0.05 to 3500, and a Mg content is 30 to 3500 ppm by mass. The alumina sintered body has a crystal grain size of 15 to 200 m. When a field of view of 370.0 m long372.0 m wide is photographed with a 1000-fold magnification and the photograph is visually observed, a number of pores having a diameter of 0.2 to 0.6 m is 250 or less.

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.

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 0.17% 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.

In an embodiment, an abrasive particle comprises a body including alumina, the alumina including a plurality of crystallites having an average crystallite size of not greater than 0.18 microns. In other embodiments, the body further comprises magnesium and zirconia. The abrasive particle has at least one of an average strength of not greater than 1000 MPa or a relative friability of at least 105%.

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.

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%.

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%.

An alumina sintered body according to the present invention includes a surface having a degree of c-plane orientation of 5% or more, the degree of c-plane orientation being determined by a Lotgering method using an X-ray diffraction profile obtained through X-ray irradiation at 2=20 to 70. The alumina sintered body contains Mg and F, a Mg/F mass ratio is 0.05 to 3500, and a Mg content is 30 to 3500 ppm by mass. The alumina sintered body has a crystal grain size of 15 to 200 m. When a field of view of 370.0 m long372.0 m wide is photographed with a 1000-fold magnification and the photograph is visually observed, a number of pores having a diameter of 0.2 to 0.6 m is 250 or less.

A high-purity microparticle alumina powder which has excellent slurry properties and sintering properties, excellent fluidity and formability, and excellent dielectric properties in the high-frequency region. In this high-purity microparticle alumina powder, the 50% particle diameter (D.sub.50) in the volume particle size distribution and the BET specific surface area (S.sub.BET) satisfy the relations represented by the formula D.sub.500.20 m and the formula D.sub.50S.sub.BET2.010.sup.6 m.sup.3/g, and the content of sodium (Na), silicon (Si), iron (Fe) and calcium (Ca) is each less than or equal to 10 ppm.