Abstract: A method of determining a quantity of amorphous silica-containing impurities in a montmorillonite clay, comprising the steps of: (a) analyzing the clay sample to determine the weight percent of silica contained in the clay; (b) analyzing the clay sample for non-montmorillonite silica-containing crystalline components; (c) calculating the weight percent of silica from step (a) that is derived from the montmorillonite and amorphous silica-containing impurities by lowering the weight percent of silica in step (a) based on the non-montmorillonite silica-containing crystalline impurities found in step (b); and (d) determining the amorphous silica-containing impurity portion of the silica calculated in step (c) by further lowering the amount of silica from step (c) until the amount of montmorillonite-derived silica is consistent with one or more properties of the clay sample.