K‐Ar dating of synkinematic illite is increasingly recognized as a central method to constrain the timing of shallow crustal faulting. Methods of efficient sample preparation and quantitative identification of illite polytypes are critical to acquiring K‐Ar isotope data for authigenic clays. In this respect, we compared the commonly used clay size separation method through centrifugation with vacuum filtration technology, showing that the former is prone to extract fractions with finer particle sizes under similar conditions, thus improving the error in the authigenic end‐member age. Additionally, we demonstrated that the side‐packed mounting method for X‐ray diffraction analysis can significantly enhance the randomness in powder samples, thus improving the quantification accuracy compared with the front‐packed and back‐packed methods. The validity of our quantification method was confirmed by comparing Profex® modeling patterns with a suite of synthetic mixtures of known compositions, yielding an average analytical error of 3%. Dating results of these artificial mixtures and the reference materials indicated that a large range in percentages of detrital illite and a sufficient amount of age data will produce reliable results for ages of both extrapolated end‐members. However, if the range is limited, the extrapolated age close to those of datasets is still reliable.