Thermal analyses of aluminium hydroxide and hydroxyaluminosilicates

Hydroxyalummosilicates (HAS(A) and HAS(B)) are important secondary mineral phases in the biogeochemical cycle of aluminium. HAS(A) 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 HAS(A) 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 AI(OH)(3(s)) the thermal signature of which was a sharp exothermic peak (ca. 282 degrees C) which though present in HAS(A)-like structures (260-275 degrees C) was absent from HAS(B)-like structures. Solid-state NMR of heated HAS and AI(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 HAS(A) to HAS(B). The identification of a novel form of purely amorphous AI(OH)(3(s)) may be of interest to chemists and materials scientists alike. (C) 2005 Elsevier Ltd. All rights reserved.