Abstract: Magnesium hydroxide of the formula Mg(OH).sub.2 having a crystallite size in the direction <004> greater than 150 .ANG. and smaller than 500 .ANG., an aspect ratio in the range from 2 to 5, a strain in the direction <004> not greater than 4.2.times.10.sup.-3 and a strain in the direction <110> not greater than 3.0.times.10.sup.-3. 50% of secondary particles with diameters smaller than 1.4 .mu.m and 100% of secondary particles with diameters not greater than 4.0 .mu.m, and a specific surface, determined by the BET method, smaller than 25 m.sup.2 /g. It is prepared by a two-step procedure; in the first step, magnesium nitrate is treated by an alkaline substance, preferably by ammonia, in an aqueous medium, with 1.5 to 6.0-multiple excess of ammonia, at a temperature of not more than 85.degree. C., at the atmospheric pressure, causing that basic magnesium nitrate is formed which, in the second step, decomposes at a temperature of 110.degree. to 150.degree. C.