XRD patterns corresponding to dehydrated (D) and rehydrated (R) Mg-vermiculite samples split into six groups. Group I is represented by a pattern with the position of the first basal diffraction equal to 14.23 Å (with the 4.97 Å water molecules in the interlayer). Group II (100 °C–D), with the 13.75 Å, is formed by unstable partially dehydrated phase (with 4.32 water molecules). Group III (150 °C–D), with the 11.51 Å, is formed by a phase with one sheet of water molecules (2.41). The 11.51 Å phase formed after 12 minutes when the temperature raised from 25 °C to 150 °C and remained unchanged upon prolonged heating at 150 °C. Group IV (300° and 450 °C–D) is represented by the pattern of mixed layer structure with the position of the first basal diffraction equal to 10.4 Å. The next group V (550° and 700 °C–D and R) is formed by a dehydrated stable phase with 9.3 Å spacing. The last group VI (1000 °C) has the XRD pattern of enstatite. The positions of the first basal diffractions of dehydrated Mg-vermiculite samples correlate with the number of water molecules in the interlayer. Mg-vermiculite samples heated to 100°, 150°, 300°, 450°, 550°, and 700 °C exhibit rehydration abilities of 100 %, 100 %, 84 %, 60 %, 25 % and 2 %, respectively.