Abstract: A novel magnesium hydroxide of the formula Mg(OH).sub.2 having (i) a strain in the direction of not more than 3.0 .times. 10.sup.-3, (ii) a crystallite size in the direction of more than 800 A, and (iii) a specific surface area of less than 20 m.sup.2 /g. The magnesium hydroxide, if desired as coated with anionic surfactants, is useful, for example, as fire retardants for thermoplastic synthetic resins and aqueous paints. The novel magnesium hydroxide is prepared by heating in an aqueous medium at an elevated pressure a novel basic magnesium chloride or nitrate of this invention expressed by the formula Mg(OH).sub.2-x A.sub.x .sup.. mH.sub.2 O wherein A is Cl or NO.sub.3, x is a number of more than 0 but less than 0.2, and m is a number of 0 to 6.

Abstract: A novel magnesium hydroxide of the formula Mg(OH).sub.2 having (i) a strain in the (101) direction of not more than 3.0 .times. 10.sup.-3, (ii) a crystallite size in the (101) direction of more than 800 A, and (iii) a specific surface area of less than 20 m.sup.2 /g. The magnesium hydroxide, if desired as coated with anionic surfactants, is useful, for example, as fire retardants for thermoplastic synthetic resins and aqueous paints. The novel magnesium hydroxide is prepared by heating in an aqueous medium at an elevated pressure a novel basic magnesium chloride or nitrate of this invention expressed by the formula Mg(OH).sub.2-x A.sub.x.mH.sub.2 O wherein A is Cl or NO.sub.3, x is a number of more than 0 but less than 0.2, and m is a number of 0 to 6.