• journal_title：American Mineralogist
• Contributor：Ernesto Mesto ; Fernando Scordari ; Maria Lacalamita ; Emanuela Schingaro
• Publisher：Mineralogical Society of America
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2012.4023
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：97
• issue：8-9
• firstpage：1460
• section：Articles

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 K2p_3/2 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 (K_0.18Na_0.01NH₄⁺ _0.62)_∑=0.81(Al_1.98Fe²⁺_0.02)_∑=2.00 (Si_3.19Al_0.81)_∑=4.00O_10.00OH_2.00 for tobelite and to (K_0.46Na_0.03Ba_0.01NH₄⁺ _0.36)_∑=0.86(Al_1.98Mg_0.01Fe²⁺_0.01V³⁺_0.01)_∑=2.01 (Si_3.13Al_0.87)_∑=4.00O_10.00F_0.08OH_1.92 for NH₄⁺-rich muscovite.