Vermiculites and smectites are best diagnosed by performing a number of different cation saturations, solvations and heatings. See page 12 of handout. Mg, Ca vs. Na vs. K saturations. A monovalent cation has a low hydration energies than an equivalent sized divalen cate. Therefore, monovalent cations do not hold the layers apart as well as divalent cations.
Divalent cations have higher hydration energies.
The presence of a strongly held hydration sphere better holds the layers apart. The response of the interlayer distance is then going to be a result of the balance of forces necessary to dehydrate the cation and the forces of attraction between the layers and the cation. Talc has no interlayer cation therefore, no water to be drawn into the structure.