The crystal structures of tobelite and NH4+-rich muscovite from the sedimentary rocks of the Armorican sandstones (Brittany, France) have been solved for the first time by single-crystal X-ray diffraction. The structural study was integrated by electron probe microanalyses, and X-ray photoelectron and micro-Fourier transform infrared spectroscopy. The crystals belong to the 2M2 polytype with the following unit-cell parameters: a = 9.024(1), b = 5.2055(6), c = 20.825(3) Å, and β = 99.995(8)° for tobelite and a = 9.027(1), b = 5.1999(5), c = 20.616(3) Å, and β = 100.113(8)° for NH4+-rich muscovite. Structure refinements in the space group C2/c converged at R1 = 8.01%, wR2 = 8.84% and R1 = 5.59%, wR2 = 5.63% for tobelite and NH4+-rich muscovite, respectively.

X-ray photoelectron spectroscopy revealed nitrogen environments associated either with inorganic (B.E. 401.31 eV) or organic (B.E. 398.67 eV) compounds. Infrared spectra showed, in the OH-stretching region (3700–3575 cm−1), two prominent bands, centered at ~3629 and ~3646 cm−1, and two shoulders at ~3664 and ~3615 cm−1 that were assigned to Al3+Al3+□-OH arrangements having OH groups affected by different local configurations. In addition, a series of overlapping bands from about 3500 to 2700 cm−1 characteristic of the NH4+-stretching vibrations, a main band at ~1430 and a shoulder at ~1460 cm−1 that were associated to the NH4+-bending vibration (ν4) were also present.

The ammonium concentration was semi-quantitatively estimated in both crystals from the absorbance of the OH-stretching and NH4+-bending vibrations in the infrared spectra. An additional estimate was obtained for the NH4+-rich muscovite by considering the normalized peak area between K2p3/2 and N1s in the X-ray photoelectron spectrum. The obtained values are in agreement with those derived from the interlayer spacing in the simulated X-ray powder diffraction spectra.

The results of this integrated approach converged to (K0.18Na0.01NH4+0.62)∑=0.81(Al1.98Fe2+0.02)∑=2.00 (Si3.19Al0.81)∑=4.00O10.00OH2.00 for tobelite and to (K0.46Na0.03Ba0.01NH4+0.36)∑=0.86(Al1.98Mg0.01Fe2+0.01V3+0.01)∑=2.01 (Si3.13Al0.87)∑=4.00O10.00F0.08OH1.92 for NH4+-rich muscovite.

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