The development of crystalline structure in aluminium hydroxide polymorphs on ageing

SUMMARY. Infrared absorption spectra of the precipitated solids separated from aluminium-hydroxide solutions below the pH of minimum solubility of AI(OH)3 demonstrate he slow ageing of polymeric A1 complexes to gibbsite. On comparison with the spectra of natural gibbsites, hifts in absorption frequency during crystal growth are interpreted in terms of changes in hydrogen bonding as a consequence of deprotonation and stacking of aluminium hydroxide layers to form gibbsite. In alkaline solutions the bayerite polymorph precipitates and IR frequencies are compatible with protonation as aluminate ions associate with the bayerite crystal having a lower crystal energy than its low pH polymorph. The infrared spectrum of nordstrandite, which is probably the stable polymorph in alkaline solution, shows evidence of strongly polarized hydroxyl ions similar to gibbsite. O F the many studies of the speciation of aluminium-hydroxide solute in water, recent work at the U.S. Geological Survey (Hem and Roberson, I967; Hem, 1968; Schoen and Roberson, I97O) has provided much useful information on the forms of and controls on aluminium in solution, and the nature of the aluminous olids that precipi-tate from such solutions. For example, it has been demonstrated that solution pH plays a vital role in controlling the structure of the A1 hydroxide precipitates; th