Knowledge Bank

The absorption near 3$\mu$ in muscovite and biotite has been investigated using polarized radiation and various orientations of the crystals. Muscovite exhibits a single maximum while biotite gives two maxima. The polarization properties of these bands are very different in the two cases indicating the hydrogens are in different positions in these two micas. According to x-ray studies, the crystallographic unit cell in muscovite contains eight OH groups, the space group being $C2h6$, and biotite has generally been assumed to be the same. The Bravais cell in muscovite contains only four OH groups, and the selection rules predict a maximum of two OH fundamentals active in absorption. If the interaction across the cleavage plane is so small that these are coincident, then the directions of the OH groups can be uniquely determined. The presence of two distinct OH frequencies in biotite indicates that the Bravais cell in this case contains eight OH groups and that the space group should be $C2h4$. The directions of these eight OH groups can be determined and are found to be very different from those of the corresponding OH groups in muscovite.