Synthetic stevensite composed of stevensite-type sodium magnesium phylosilicate of which metallic components consist essentially of magnesium, sodium and silicon. The synthetic stevensite is characterized by having an X-ray diffraction peak at a spacing of 16 to 26 .ANG. when treated with ethylene glycol. It can be produced by a process comprising hydrothermally treating an aqueous composition containing basic magnesium carbonate and a silica-sodium component selected from the group consisting of (i) sodium silicate, (ii) sodium silicate and amorphous silica, and (iii) amorphous silica and sodium silicate.