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Mg/Al-hydrotalcites containing NO3-, Cl-, SO42- or ClO4- were synthesised under N2 to prevent incorporation of CO32-. The presence of the anions in the hydrotalcite structure was confirmed by infrared and Raman spectroscopy. The CO3- and the NO3-hydrotalcites contained both NO3- and CO32-, while the Cl-hydrotalcite also contained some CO32-. It is known that during thermal treatment of hydrotalcites dehydroxylation and decarbonisation strongly overlap. Mass spectrometry following TGA enables one to identify both reactions. For CO3-hydrotalcite CO2 is released simultaneously with water (dehydroxylation) around 335 degrees Celsius followed by NO around 365 and 500 degrees Celsius. The stability of the NO3-hydrotalcite is different showing a major loss of CO2 and H2O (dehydroxylation) around 410 degrees Celsius with losses of NO around 345 and 450 degrees Celsius. The Cl-hydrotalcite shows a similar behaviour for the H2O loss (dehydroxylation), but Cl is lost over a range from 400 to 900 degrees Celsius and CO2 comes off in steps around 360 and 500 degrees Celsius. Completely different is the thermal behaviour of SO4- and ClO4-hydrotalcites. SO4-hydrotalcite shows a gradual weight-loss due to dehydroxylation with two minor water peaks around 260 and 375 degrees Celsius, while the sulphate remains in the structure. The sulphate is not lost until heated to 900 degrees Celsius. The ClO4-hydrotalcite shows a complex thermal behaviour with 2 steps of water loss around 375 and 440 degrees Celsius, where the second step is accompanied by the loss of O2. A possible explanation is a redox reaction between perchlorate and the cations giving metal-chlorides and O2.

This paper describes an X-ray diffraction and spectroscopic study, including infrared, near-infrared and Raman spectroscopy of some selected zinnwaldites. In general, zinnwaldite forms a member of the trioctahedral true micas with characteristically Li in the octahedral positions and low iron contents. Although the infrared spectrum of zinnwaldite has been described before, near infrared and Raman spectroscopy have not been used so far to study this mineral. X-ray diffraction showed that all the samples reported in this study have the 1M structure. The Raman spectra are characterised by a strong band at 700-705 cm-1 plus a broad band associated with the SiO modes around 1100 cm-1. Less intense bands are observed around 560, 475, 403 and 305 cm-1. The corresponding IR spectra show strong overlapping SiO modes around 1020 cm-1 plus less intense bands around 790, 745, 530, 470-475 and 440 cm-1. Two overlapping OH-stretching modes can be observed around 3550-3650 cm-1, in agreement with a broad band in the IR around 3450 cm-1 and a complex band around 3630 cm-1. The near-IR spectra basically reflect combination and overtone bands associated with protons in the zinnwaldite structure. A very broad band observed around 5230 cm-1 is characteristic for adsorbed water while bands around 4530, 4435 and 4260 cm-1 can be ascribed to metal-hydroxyl groups.