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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.

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