Morphogenesis in Inorganic Precipitates in Gels

[摘要] This thesis is concerned with the precipitation of inorganic materials and in particular Induced Morphology Crystal Aggregates (IMCA's). This strange phenomenon occurs when an akaline earth metal carbonate and an alkaline earth metal silicate precipitate simultaneously in basic silica gel. The metal silicate precipitates as a sheath which acts to control the crystal-to-crystal relations of the metal carbonate aggregate and therefore the overall morphology of the precipitate. Very strange morphologies result such as sheet-like and spiral structures in the case of BaCO3. BaCO3 was studied extensively in this project. A glass cassette reaction vessel was developed in collaboration with J. M. Garcia Ruiz which enabled in-situ light microscopical analysis of morphology. Precipitates were then removed from their gels and subjected to chemical analysis and further morphological observation by scanning electron microscopy (SEM). The surface analysis techniques of energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were utilised to determine the chemical composition of the sheath or membrane which controls morphology. Some key factors which affect BaCO3 IMCA formation were established. A very clear morphogenetical transition from dendrites (normal crystal growth) through to fully induced morphologies was observed. This transition depends on the amount of soluble silicate in the gel and its ability to react with the metal and form a membrane. The main factors which affect this are pH and the presence of NaCl in the gel. High pH values promote IMCA growth. This is attributed to increased silicate ion concentrations at higher pH values. The presence of NaCl also promotes the formation of IMCA's. This may be due to charge effects stabilising the precipitating membrane. Growth mechanisms are proposed which have similarities to cement chemistry and the silica garden effect. Experiments carried out on the nature of the membrane have shown it to be a barium silicate carbonate in the region of one micron thick. Other insoluble barium precipitates such as BaSO4 (studied extensively), BaCrO4 and BaHPO4 have been found to form IMCA's suggesting that the effect may be quite widespread and experiments on iron carbonate and iron sulphide suggested that silica may influence morphology. In the case of iron sulphide, precipitates were obtained which were remarkably similar to naturally occurring iron sulphide framboids, unusual spherical aggregates recently implicated in origin of life studies.

[发布机构] University:University of Glasgow

[关键词] Inorganic chemistry