Summary: Comparison of the infrared spectra (4000-50 cm−1) of natural and synthetic samples of margarite, beryllian margaritc, ephesite, and clintonite indicates that the tetrahedral layers of margarite and ephcsite, containing Si and Al in equal atomic proportions, are ordered as the spectra are sharp and show no Al-O-Al vibrations. Clintonite of this tctrahedral composition is disordered. The hydroxyl-stretching frequencies are affected by ionic substitutions in both the octahedral and tetrahedral layers. The spectra permit the distinction of the different species of brittle mica. They also indicate the presence of beryllium in margarite, and allow an estimate of the tetrahedral composition of clintonites.