Abstract: A boron nitride complex comprising hBN in which lithium or an alkaline earth metal is diffused and supported in the form of its boron nitride. The boron nitride complex is prepared by heating hBN powder or a sintered product thereof and lithium, an alkaline earth metal, a lithium nitride or boride, or an alkaline earth metal nitride or boride in a non-oxidizing atmosphere to diffuse in and deposit on hBN powder or sintered product thereof, the lithium or alkaline earth metal in the form of its boron nitride. Also disclosed is a process for preparing a light-transmitting dense body of cubic system boron nitride, which comprises diffusing in and depositing on a sintered product of hexagonal system boron nitride, from 0.15 to 3.0 molar % of Me.sub.3 B.sub.2 N.sub.4 where Me is an alkaline earth metal, and sintering the Me.sub.3 B.sub.2 N.sub.4 -containing product thereby obtained, at a temperature of at least 1350.degree. C. under a thermodynamically stable pressure for cubic system boron nitride.

Abstract: A process for producing a sintered body of cubic system boron nitride comprises steps of:(a) mixing raw material boron nitride selected from the group consisting of hexagonal system boron nitride, cubic system boron nitride, and a mixture thereof with Me.sub.3 B.sub.2 N.sub.4 (where: Me represents an alkaline earth metal) in an amount of from 0.15 to 3.0 mol %; and(b) treating said mixed material at a temperature of 1350.degree. C. and above under a thermodynamically stabilized pressure condition of the cubic system boron nitride.

Abstract: A process for preparing cubic boron nitride comprises heating hexagonal boron nitride and magnesium boron nitride at a temperature of at least 1350.degree. C. under a pressure at which the cubic boron nitride is thermodynamically stable, whereby the cubic boron having high strength and high purity can readily be obtainable. Also disclosed is a process for the preparation of magnesium boron useful as a starting material for the above process. This process comprises mixing hexagonal boron nitride and magnesium nitride or metal magnesium in a molar ratio of hexagonal boron nitride/magnesium being at least 0.6, and heating the mixture thus obtained, at a temperature of from 950.degree. to 1250.degree. C. under atmospheric pressure in a non-oxidizing atmosphere, e.g. a nitrogen atmosphere.

Abstract: A clay composition comprising silica intercalated between the interlayers of the clay.

Abstract: A process for producing cubic system boron nitride comprises contacting calcium or strontium boron nitride with hexagonal system boron nitride and heating at higher than 1,450.degree. C. under the thermodynamically stable pressure for cubic system boron nitride.