Abstract: A metal hydroxide solid solution and a metal oxide solid solution, wherein the crystal form is an octahedron comprising upper and lower parallel basal planes and six peripheral pyramidal planes, the pyramidal planes consisting of upward-inclined planes and downward-inclined planes which are alternatively located. The ratio of the major axis diameter of the basal plane to the thickness between the upper and lower basal planes (major axis diameter/thickness) if 1 to 9. This improves fluidity, processability and the like when the solid solutions are kneaded into resins and the like.

Abstract: A composite metal hydroxide is produced by reacting magnesium-containing aqueous solution (X) including water soluble zinc compound wherein magnesium ion concentration is 0.01 to 1 mol/liter with alkaline material (Y) at a specific reaction equivalent ratio. Further, a composite metal hydroxide represented by the following general formula (1) is produced by hydrothermally treating thus obtained composite metal hydroxide within a temperature range of 100.degree. to 200.degree. C. for aging in chlorine-containing aqueous solution having a specific chlorine ion concentration:Mg.sub.1-x Zn.sub.x (OH).sub.2 (1)wherein x indicates a positive number within a range of 0.003.ltoreq.x.ltoreq.0.1. A uniform metallic solid solution can be obtained as a composite metal hydroxide by the above series of processes. Further, its crystal shape can be controlled thereby so as to restrain occurrence of secondary aggregation.