Fibrous anhydrous magnesium oxysulfate and process for the preparation thereof

A process for the preparation of a fibrous anhydrous magnesium oxysulfate having the formula of MgSO.sub.4.5Mg(OH).sub.2, comprising the steps of:

• preparing a magnesium oxysulfate having a formula of 2MgSO4.Mg(OH)2.3H2O by dispersing magnesium hydroxide or magnesium oxide into an aqueous solution of magnesium sulfate having a concentration of 1.5 to 5.0 mol/l and then subjecting the dispersion to a hydrothermal reaction carried out at a temperature of 100.degree. to 200.degree. C. and under a pressure of 1 to 15 kg/cm.sup.2 for 0.1 to 5 hours;
• cooling and diluting, with water or an aqueous solution of magnesium sulfate, a slurry of said magnesium oxysulfate having a formula of 2MgSO.sub.4.Mg(OH).sub.2.3H.sub.2 O to a concentration of 0.5 to 2.5 mol/l of the aqueous solution of magnesium sulfate and to a temperature of 10.degree. to 60.degree. C. to thereby decompose the 2MgSO.sub.4.Mg(OH).sub.2.3H.sub.2 O and produce fibrous magnesium oxysulfate having a formula of MgSO.sub.4.5Mg(OH).sub.2.8H.sub.2 O;
• and heat treating the fibrous magnesium oxysulfate having a formula of MgSO.sub.4 Mg(OH).sub.2.8H.sub.2 O to obtain the fibrous anhydrous magnesium oxysulfate having the formula of MgSO.sub.4.5Mg(OH).sub.2.