The structural properties of synthetic silicates, primarily sheet silicates, crystallized within potash-bearing sesquioxide-silica-water systems, were studied by means of X-ray powder diffraction data. The prototype system of penological importance is the one with alumina as sesquioxide component. As regards this system the main attention was directed at the join pyrophyllite-muscovite. The fairly continuous increase of thebdimension with increasing potassium content recorded for phases along this join is indicative of the formation of solid solutions, the scheme of which is tentatively outlined. The polymorphism of muscovite is also touched upon.