Single crystal Raman microscopic study of the asbestos mineral chrysotile

Abstract

Polarisation Raman microscopy is used to study tubular chrysotile. The OH-stretching region is characterised by the inner surface OH bands at 3695, 3686 and 3678 cm^-1 and the inner OH band at 3643 cm^-1. The outer OH pointing away from the Mg-layer gives rise to two overlapping bands at 3695 and 3686 cm^-1 due to a positional disorder caused by the folding of the layers. These bands are the in-phase vibrations whereas the bands at 3678 and 3643 cm^-1 represent the out-of-phase vibrations. The 1102 cm^-1 band is an antisymmetric stretching mode of Si–O perpendicular to the sheet. From the 692 and 705 cm^-1 bands the first one is assigned to the symmetric Si–O–Si stretch and the second to an outer symmetric translation mode of the Mg–OH oriented sub-parallel to the a-axis. The 709 cm^-1 band is assigned to the second outer symmetric translation mode of the Mg–OH oriented at a small angle to the b-axis and c-axis. The 629 and 622 cm^-1 bands represent antisymmetric OH–Mg–OH translation modes. The band around 607 cm^-1 is described as the symmetric libration mode of the inner Mg–OH group. The 458 cm^-1 band is assigned as the v₃(a₁) mode of SiO₄. The 466 cm^-1 band is probably an OH translational vibration. A strong band at 388 cm^-1 is ascribed to the antisymmetric v₅(e) mode of the SiO₄ tetrahedron. The band at 432 cm^-1 is assigned as an antisymmetric Mg–OH translation mode. In the region between 450 and 200 cm^-1 five bands can be observed at 374, 345, 318, 304 and 231 cm^-1. The 374, 318 and 304 cm^-1 bands are antisymmetric modes, whereas the 345 and 231 cm^-1 bands are symmetric modes. The band at 374 cm^-1 is associated with a symmetric Mg–OH vibration. At present a more detailed assignment of the other bands is not possible. The band at 199 cm^-1 is assigned to the A_1g mode of a Mg(O,OH)₆ octahedron distorted in the direction normal to the octahedral sheet.

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