Ammonium quantification in muscovite by infrared spectroscopy

Single muscovite grains of different chemical composition (in particular Si-content from 3.05 to 3.69 per formulae unit (pfu)) and origin (metamorphic and granitic rocks) were analysed by infrared (IR) spectroscopy. Their ammonium concentrations were subsequently measured by capacitance manometry after extraction as N2 using a sealed-tube combustion technique. The values range from 0 to 2203 ppm NH4+. Correlation between ammonium concentration and ammonium IR absorbance corresponding to NH4+ bending permits an assessment of the ammonium molecular absorptivity at 1430 cm−1 (εN–H1430=462±32 l mol−1 cm−1). In order to avoid uncertainties on thickness estimates using an optical technique, a relationship between sample thickness and IR absorbance was determined. Ammonium concentration of muscovite can be directly derived from its IR spectrum using the relationship [NH4+] (ppm)=1142.5×[(AN–H1430−A2514)/(A1282−A2514)]−606 where A1282, AN–H1430 and A2514 are absorbances corresponding to wavenumbers 1282 (Si–O vibration peak), 1430 (NH4+ bending) and 2514 cm−1 (silicate network vibration), respectively. The precision of ammonium concentration estimates using this method is better than 20% (2σ) for muscovites thicker than 30 μm. This is independent of mineral composition, thus implying that the ammonium concentration vs. IR absorbance relationship is independent of the muscovite–celadonite solid solution.