Synthesis of smectite clay minerals: A critical review

Smectites are one of the most important groups of phyllosilicates found in soils and sediments, and certainly one of the most difficult to study. New information about the formation mechanisms, impact of structural features o n surface properties, and long-term stability of smectites can best be gain ed from the systematic study of single-phase specimens. In most instances, these specimens can only be obtained through synthesis under controlled con ditions. Syntheses of smectites have been attempted (1) at ambient pressure and low-temperature (<100 degrees C), (2) under moderate hydrothermal cond itions (100-1000 degrees C, pressures to several kbars), (3) under extreme hydrothermal conditions (>1000 degrees C or pressures >10 kbars), and (4) i n the presence of fluoride. Of these approaches, syntheses performed under moderate hydrothermal conditions are the most numerous and the most success ful in terms of smectite yield and phase-purity. Using hydrothermal techniq ues, high phase-purity can be obtained for beidellites and several transiti on-metal smectites. However, synthesis of montmorillonite in high purity re mains difficult. Starting materials for hydrothermal syntheses include gels , glasses, and other aluminosilicate minerals. The presence of Mg2+ seems t o be essential for the formation of smectites, even for phases such as mont morillonite which contain low amounts of Mg. Highly crystalline smectites c an be obtained when extreme temperatures or pressures are used, but other c rystalline impurities are always present. Although the correlation between synthesis stability fields and thermodynamic stability fields is good in ma ny instances, metastable phases are often formed. Few studies, however, inc lude the additional experiments (approach from under-and over-saturation, r eversal experiments) needed to ascertain the conditions for formation of th ermodynamically stable phases. Thorough characterization of synthetic produ cts by modern instrumental and molecular-scale techniques is also needed to better understand the processes leading to smectite formation.