Sub-micrometer clay particles are of interest in clay-polymer applications, especially when transparency is important. The scattering of light can be reduced by the adjustment of the refractive index (RI) of the clays to that of the matrix. In this study, the RI of sub-micrometer illite particles was changed by treatment with 5 M HCl for treatment times ranging between 2 and 24 h. The dissolution of Fe leads to a decrease in the RI of illite from 1.587 for the unaltered material to 1.502 after 24 h. The layer structure of the illite particles was preserved during the treatment. The RI of the sub-micrometer illite particles was determined by means of a photospectrometer measuring the light intensity passing through suspensions containing the clay particles, with varying refractive indices.

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