Far-infrared spectroscopy and X-ray powder diffraction Rietveld structure refinement for the hydrothermal synthetic muscovite series, (a) KAl3(Si3–yGey)O10(OH,OD)2, (b) KGa3(Si3–yGey)O10(OH,OD)2, and (c) K(Al3–xGax)Ge3O10(OH,OD)2, where x = 0.0–3.0, y = 0.0–3.0, and the muscovite–tobelite series, (d) [K1–A(NH4,ND4)A]Al3Si3O10(OH,OD)2, where A = 0.0–1.0, indicate that there is a complete solid-solution in each series. In the 200–50 cm−1 far-infrared region, four kinds of bands are observed: two bands due to octahedral deformation coupled with OH deformational vibrations between 240 and 130 cm−1, an in-plane tetrahedral torsional band between 175 and 130 cm−1, an interlayer I–Oinner stretching band, and an I–Oouter stretching band. The weak and broad 140 cm−1 band is assigned to K–Oinner stretching and the strong broad 110 cm−1 band to K–Oouter stretching in muscovite. The broad 175 cm−1 band is (NH4/ND4)–Oinner stretching and the 140 cm−1 broad strong band is (NH4/ND4)–Oouter stretching in tobelite. With increasing Ga→Al substitution, the I–Oinner and I–Oouter stretching bands shift to lower frequency, while with increasing Ge→Si substitution, the I–Oinner stretching band shifts to higher frequency; the I–Oouter stretching bands do not change as a function of composition.