Abstract

Polarized IR-absorption spectra were measured on inclusion-free spots, 50 mu m 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 (super -1) , the spectra display four weak (alpha lin < or = ca. 7 cm (super -1) ) and sharp (Delta v (sub 1/2) congruent to 20 cm (super -1) ) bands typical of the antisymmetric and symmetric stretching modes of CH (super -3) and -CH (super -) 2 groups of aliphatic hydrocarbons, C n H (sub 2n+2) (V (sub as,CH3) at 2951-2959 cm (super -1) , V (sub as,CH2) at 2920-2923 cm (super -1) , V (sub sym,CH3) at 2871-2874 cm (super -1) , V (sub sym,CH2) at 2850-2851). All bands are polarized in the ac-plane of orthorhombic cordierite. In the temperature range 123< or =T (K)< or =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 Aa wide cavities of the c-parallel channels of the crystal structure. The concentrations of alkanes, C n H (sub 2n+2) from band intensities, are between about 20 and about 100 ppm, corresponding to about 0.7.10 (super -3) and about 2.3.10 (super -3) molecules per formula unit cordierite. Evaluation of the averaged intensities of the antisymmetric as well as symmetric C-H stretching vibrations of either species, CH (super -) 3 and -CH (super -) 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 0.7X10 (super -3) to 2.3X10 (super -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.

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