Abstract: A unique interstratified expandable mica was obtained by cation exchange treatments using an expandable mica synthesized from talc. The 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectrum of the expandable mica used as a starting material showed that it had 2 kinds of Na+: one was exchangeable and the other was not exchangeable. Half of the Na+ per unit cell of the expandable mica was replaced with Mg2+ by cation exchange treatments. The X-ray powder diffraction (XRD) analysis of the Mg2+-exchanged expandable mica, after heating at 73 °C, indicated that Na+ in the interlayer sheets was exchanged with Mg2+ in every second layer and that it had an interstratified structure with a 12.5-Å, layer thickness and a 9.6-Å layer thickness. The structure of the Mg2+-exchanged expandable mica was changed into a unique interstratified structure by the calcination at 600 °C; one component had a stacking sequence of talc and a small amount of OH−, but the other had a different stacking sequence from talc and no structural OH−.