A process for coating inorganic particles with dense amorphous silica is provided in which a suspension of inorganic particles is coated by adding an acid to a solution of a silicate, preferably an alkali metal silicate. During at least a part of the addition the suspension is subjected to the effects of ultrasonic vibrations. Coatings of dense amorphous silica which are substantially continuous and impervious can be produced using the new method at temperatures of about 60 DEG C and in an acceptable time whereas previously known economical methods for producing a coating of dense amorphous silica generally employed temperatures of approximately 90 DEG C. The process is particularly useful for producing titanium dioxide pigments having very high durability.