Four kinds of regularly interstratified 25Å minerals, paragonite-beidellite, Ca, Sr and Ba brittle mica-beidellite, were purely synthesized and examined mineralogically. These specimens show similar characteristics to rectorite (regularly interstratified mica-smectite) on X-ray diffractometry, thermal analyses and electron microscopy. Basal spacings of them depend on ionic radii of interlayer cations of non-expansible layers. Chemical analyses and infrared absorption spectra show that Ca-brittle mica-beidellite has a large Al-for-Si substitution in the tetrahedral sheets of 2: 1 silicate layers.