Potassium (K(+)) phlogopite was transformed to a vermiculite-like mineral through a topotactic reaction under acidic conditions (pH 2) followed by hydrothermal treatment with Na(+), Mg(2+), Ca(2+), and Al(3+) cations. The resulting Na(+)-, Mg(2+)-, Ca(2+)-, and Al(3+)-altered phlogopites (Phl) denoted as Na-Phl, Mg-Phl, Ca-Phl, and Al-Phl, respectively. Na-Phl, Mg-Phl, and Ca-Phl all exhibited the same high adsorption capacity as natural vermiculite and the absorption of Cs(+) and Sr(2+) ions on these materials followed the Langmuir model. High-angle annular dark-field scanning transmission electron microscopy showed that Cs(+) ions in the Mg-Phl layers were intercalated deep within the crystal structure, along specific interlayer regions. These adsorbed anhydrous Cs(+) ions were firmly fixed at the centers of hexagonal rings positioned simultaneously in the upper and lower tetrahedral silicate sheets, whereas Sr(2+) ions adsorb into the interlayer in the hydrous state. Al-Phl formed a hydroxyl-interlayered vermiculite and demonstrated significant selectivity for Cs(+) at very low concentrations of the isotope. Consequently, the artificially altered phlogopites prepared in this study showed controllable and versatile adsorption capabilities making them significantly more suitable than natural vermiculite for Cs and Sr decontamination.