AbstractThe usual method of polishing the surface of a solid sample for examination using the x‐ray microprobe technique cannot be adopted for clay minerals. A modified method is suggested and its validity is tested using vermiculite mineral. It is observed that precision of quantitative elemental analysis increases significantly according to the present pretreatment procedure; variation from values obtained by wet chemical method is<1% for SiO2, MgO and Fe2O3and 2–3% for the rest. However, values obtained with the x‐ray microprobe technique vary enormously from point to point and this non‐uniformity is explained in terms of the structural characteristics of the clay mineral. The variation increases with the rise in temperature of treatment due to structural modifications occurring during heat treatment. The x‐ray microprobe technique has been found to be quite effective in determination of trace elements such as