Thermal analyses of aluminium hydroxide and hydroxyaluminosilicates

Hydroxyaluminosilicates (HASA and HASB) are important secondary mineral phases in the biogeochemical cycle of aluminium. HASA is formed by the reaction of silicic acid (Si(OH)4) with an aluminium hydroxide (Al(OH)3(s)) template with further substitution of Si(OH)4 into HASA resulting in HASB. Recently, fluoride and phosphate-substituted forms of HAS have been synthesised and characterised. Thermogravimetric analysis incorporating differential scanning calorimetry (TGA–DSC) is an effective method for studying the structure of mineral phases and was used herein as a possible tool to discriminate between different forms of HAS as well as to elucidate further upon their mechanism of formation. All of the HAS studied exhibited distinctive thermal behaviour dependent upon their Si:Al ratio and the inclusion of fluoride or phosphate in their structure. The observed thermal characteristics were sufficient to allow different HAS to be identified though they did not offer any structural information in addition to that which was obtained previously by solid-state NMR and microprobe analysis. We have identified an hitherto unrecognised form of a purely amorphous Al(OH)3(s) the thermal signature of which was a sharp exothermic peak (ca. 282 °C) which though present in HASA-like structures (260–275 °C) was absent from HASB-like structures. Solid-state NMR of heated HAS and Al(OH)3(s) could not identify any significant changes in the coordination of aluminium which might be associated with this characteristic exotherm. However, NMR was successful in identifying Al(V) as a potential intermediate in the transformation of HASA to HASB.The identification of a novel form of purely amorphous Al(OH)3(s) may be of interest to chemists and materials scientists alike.

When the thermal behaviour of Al(OH)3(s) was investigated, we made the entirely novel observation of a sharp exothermic peak at ca. 282 °C. Exotherms at similar temperatures were also observed for HASA and F-substituted HASA. Exotherms were only observed for solid phases collected from solutions in which the pH was tightly controlled using a biological buffer. Such conditions favoured the formation of an hitherto unrecognised purely amorphous form of Al(OH)3(s).Figure optionsDownload as PowerPoint slide