Surface modification of calcium carbonate using aqueous fluosilicic acid (H2SiF6) was carried out in terms of the mole ratio of fluosilicic acid to calcium carbonate as well contact time. The resulting surface compounds of amorphous silica and calcium fluoride after surface modification were visualized on the surface of calcium carbonate. Amorphous silica was generated by the hydrolysis of silicon hexafluoride (SiF62-) dissociated from fluosilicic acid. Calcium fluoride was generated by the reaction of dissociated fluoride ion and calcium ion. The compounds were widely distributed throughout the surface of calcium carbonate like thin coating layer. An effect of fluosilicic acid concentration on the formation of surface compounds was more significant than contact time. The surface compounds contributed to enhance an acid resistance of calcium carbonate compared to unmodified calcium carbonate. (C) 2002 Published by Elsevier Science B.V.