Aliphatic hydrocarbons in structural channels of cordierite; a first evidence from polarized single-crystal IR-absorption spectroscopy

Abstract

Polarized IR-absorption spectra were measured on inclusion-free spots, 50 μm in diameter of (100)-, (010)-, and (001)-oriented single-crystal plates of orthorhombic cordierites extracted from anatectic granitoids and their pegmatite from the western part of the Ukrainian shield. In the range 3100-2700 cm -1 , the spectra display four weak (α lin ≤ ca. 7 cm -1 ) and sharp (∆ν 1/2 ≅ 20 cm -1 ) bands typical of the antisymmetric and symmetric stretching modes of CH 3 - and -CHH 2 - groups of aliphatic hydrocarbons, C n H 2n+2 (ν as,CH₃ at 2951-2959 cm -1 , ν as,CH₂ at 2920-2923 cm -1 , ν sym,CH₃ at 2871-2874 cm -1 , ν sym,CH₂ at 2850-2851). All bands are polarized in the ac-plane of orthorhombic cordierite. In the temperature range 123 ≤T (K) ≤573, the degree of polarization decreases as temperature increases. The band polarizations and their temperature dependence ensure that the hydrocarbons are incorporated in the cordierite matrix, i.e., in the ca. 5.8 Å wide cavities of the c-parallel channels of the crystal structure. The concentrations of alkanes, C n H 2n+2 from band intensities, are between about 20 and about 100 ppm, corresponding to about 0.7·10 -3 and about 2.3·10 -3 molecules per formula unit cordieriite. Evaluation of the averaged intensities of the antisymmetric as well as symmetric C-H stretching vibrations of either species, CH 3 - and -CH 2 - , yields a ratio of 1:1 between them consistent with n = 4 only, realized in butane C 4 H 10 or in a butane-rich mixture with n = 4 on average and concentrations between 0.7 × 10 -3 to 2.3 × 10 -3 molecule pfu. Polarizations as well as molecular and cordierite-cavity sizes are consistent with an allocation of butane molecules in the channel cavities of the cordierite structure, with the molecular axes of butane predominantly parallel to b.