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

• Published in: The American mineralogist Vol. 84; no. 7-8; pp. 1181 - 1185
• Main Authors: Khomenko, Vladimir M, Langer, Klaus
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.08.1999; Walter de Gruyter GmbH
• Subjects: Absorption spectroscopy; Aliphatic hydrocarbons; alkanes; butane; Chemical elements; cordierite; crystal chemistry; Crystallography; fluid inclusions; host rocks; Hydrocarbons; inclusions; infrared spectra; Mineralogy; Minerals; organic compounds; polarization; ring silicates; silicates; Slabs; spectra; Spectrum analysis
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-1999-7-822

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 (αlin ≤ ca. 7 cm-1) and sharp (Δv1/2≅20 cm-1) bands typical of the antisymmetric and symmetric stretching modes of CH-3 and -CH-2 groups of aliphatic hydrocarbons, CnH2n+2 (Vas,CH3 at 2951-2959 cm-1, Vas,CH2 at 2920-2923 cm-1, Vsym,CH3 at 2871-2874 cm-1, Vsym,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, CnH2n+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 cordierite. 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 C4H10 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.