Disclosed is synthetic lamellar magnesium phyllosilicate, which has as the main component a composition represented by the following formula:
wherein n is a number of up to 5, and has X-ray diffraction peaks at spacings of 4.5-4.6 Å, 2.5-2.6 A and 1.5-1.6 A, wherein the lamination asymmetry index (Is) defined by the following formula: wherein 6, 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 Å, and θ 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 at least 3.0, the BET specific area is at least 300 m 2 /g and the Methylene Blue decolorizing power (JIS K-1470) is at least 100 ml/g. This synthetic lamellar magnesium phyllosilicate is prepared by a process comprising subjecting active silicic acid or active aluminosilicic acid an an oxide or hydroxide of magnesium or a compound capable of forming said oxide or hydroxide under reaction conditions to a hydrothermal treatment.