Abstract: To assign far infrared (FIR) absorption bands of K+ in muscovite, dichroic experiments were performed. For a muscovite crystal rotated about a crystallographic axis, c*, a, or b, two bands corresponding to vibration modes of K+ appear, respectively, at 107 and 110 cm−1 (rotation about c*), 107 and 143 cm−1 (rotation about a), and 110 and 143 cm−1 (rotation about b). Two in-plane modes at 107 and 110 cm−1 and one out-of-plane mode at 143 cm−1 are identified for the vibrations of K+ in muscovite. Each of these transition moments are near the crystallographic axes b, a, and c, respectively. These observations match well predictions based on the approximate C3i symmetric environment of K+, although the site symmetry in the space group of muscovite is only C2.