Studies on the Synthesis of Layer Silicates

Magnesium hydroxide reacts with silica gel to form a hectorite-like phyllosilicate. The reaction is believed to involve initial interlayer adsorption of silicate ion into the brucite crystals. Subsequent argillification is thought to occur by dissolution of the adsorbate and recrystallization of the complex magnesium silicate ions thus formed. If this reaction scheme is valid, silicates should be most easily formed from hydroxides whose crystal structure can be easily penetrated by silicate ion. The reactions with silica gel of the divalent hydroxides of magnesium, calcium, cadmium, copper, zinc and beryllium were investigated. In each case, a 10% aqueous slurry of a mixture of the solid hydroxide with silica gel was refluxed at pH 10. The hydroxide-silica ratio was 0.75, to satisfy the formula of an ideal 2:1 trioctahedral layer silicate. The reactions were fallowed by infrared spectroscopy and electron microscopy, and evidence was sought for the existence of an initial intrastructural adsorbate. In some cases, differential thermal analysis was used for further characterisation of the samples. The reaction products were, where possible, identified by powder x-ray diffraction and electron diffraction. In the case of polymorphous hydroxides, samples of consisting of single polymorphs were used as starting materials. Amorphous silica gel has a strong Si-O stretch band at 1070 cm-1 in the infrared. Intrastructural adsorption of silicate ion produces a new Si-O stretch hand at around 1020 cm-1. This hand increases in intensity as the adsorbate is formed. Recrystallization of the adsorbate leads to further changes in the Si-O stretching pattern. Adsorption leads to the breaking up of the hydroxide crystals, yielding a material of very low particle size having a weak microcrystalline electron diffraction pattern. Recrystallization of this material is not always observed under the reaction conditions used. With all the hydroxides except epsilon-Zn(OH)2 and beta-Be(OH)2, the above changes were observed. Recrystallization of the Mg(OH)2 and gamma-Zn(OH)2 adsorbates led to hectorite and hemimorphite, respectively. Ca(OH)2 and Cu(OH)2 gave adsorbates which recrystallized partially. The Cd(OH)2 adsorbate did not recrystallize. epsilon-Zn(OH)2 and beta-Be(OH)2 did not form adsorbates. The adsorbate half-time is defined from the infrared spectra for each hydroxide, and correlated with the hydroxide structural parameters. Hydroxides with layer or chain structures have short adsorbate half- times, while those with framework structures have long or infinite adsorbate half-times. The reaction of magnesium hydroxide with Pyrex glass at 11