Synthetic magnesium silicates are white, odorless, finely divided powders formed by the precipitation reaction of water-soluble sodium silicate (water glass) and a water-soluble magnesium salt such as magnesium chloride, magnesium nitrate or magnesium sulfate. The composition of the precipitate depends on the ratio of the components in the reaction medium, the addition of the correcting substances, and the way in which they are precipitated.