Synthetic lamellar magnesium phyllosilicate and process for preparation thereof

Synthetic lamellar magnesium phyllosilicate, which has as the main component a composition represented by the following formula:

• Mg.sub.3 Si.sub.4 O.sub.10 (OH).sub.2.nH.sub.2 O
• wherein n is a number of up to 5, and has X-ray diffraction peaks at spacings of 4.5-4.6 .ANG., 2.5-2.6 .ANG. and 1.5-1.6 .ANG., wherein the lamination asymmetry index (Is) defined by the following formula:
• Is=tan .theta..sub.2 /tan .theta..sub.1
• wherein
• .theta..sub.1 stands for the angle formed by the peak perpendicular and the peak tangent on the small diffraction angle side in the X-ray diffraction peak at a spacing of 4.5-4.6 .ANG., and
• .theta..sub.2 stands for the angle formed by the peak perpendicular and the peak tangent on the large diffraction angle side in said X-ray diffraction peak,
• is in the range of from 3.5 to 6.0, the BET specific surface area is at least 300 m.sup.2 /g and the Methylene Blue decolorizing power (JIS K-1470) is at least 100 ml/g, said lamellar magnesium phyllosilicate being further characterized by its ability to stably emulsify water and oil, said emulsifying ability being determined by adding 0.4 gram of the phyllosilicate to a mixture of 20 grams of pure water and 20 grams of liquid paraffin, dispersing the resulting mixture for 5 minutes and allowing the mixture to sand still at room temperature for 24 hours, said lamellar magnesium phyllosilicate being obtained by subjecting an acid clay mineral belonging to the montmorillonite group to an acid treatment to such an extent that the X-ray diffraction peak of the plane index (001) of the clay mineral substantially disappears and the Al.sub.2 O.sub.3 /SiO.sub.2 molar ratio of the acid-treated product is within the range of from 1/11 to 1/99 and subjecting the acid-treated product of the clay mineral and an oxide or hydroxide of magnesium or a compound capable of forming said oxide or hydroxide under reaction conditions to a hydrothermal treatment at an MgO/SiO.sub.2 molar ratio of from 2/4 to 6/4 at a temperature of 110.degree. to 200.degree. C.