Aluminum from AlO.sub.4 -tetrahedra of as-synthesized zeolites is extracted and substituted with silicon to form zeolite compositions having higher SiO.sub.2 /Al.sub.2 O.sub.3 molar ratios and exhibiting distinctive chemical and physical properties. The preparative procedure involves contact of the starting zeolite with an aqueous solution of a fluorosilicate salt using controlled proportions and temperature and pH conditions which avoid aluminum extraction without silicon substitution.