We solved the crystal structures of the phyllosilicates shlykovite, KCaSi₄O₉(OH)^.3H₂O, and cryptophyllite, K₂CaSi₄O₁₀^.5H₂O, two minerals recently discovered in the Khibiny alkaline complex (Kola Peninsula, Russia). The structures were solved from synchrotron diffraction data collected on the same intergrown crystal. Shlykovite and cryptophyllite are representatives of two new, closely related structure types. The main structural units of both minerals are TOT blocks consisting of tetrahedral Si layers (T) and an octahedral component (O), sandwiched between them. Each T-layer consists of 4- and 8-membered rings of SiO₄-tetrahedra and can be considered as a half of a double layer described in the structures of the members of the rhodesite mero-plesiotype series. A topologically closely related Si layer also forming T-fragments of TOT blocks was recently discovered in mountainite, KNa₂Ca₂[Si₈O₁₉(OH)]^.6H₂O. The O-fragment of the TOT blocks in shlykovite and cryptophyllite is formed by columns of edge-sharing Ca-centered octahedra CaO₅(H₂O). In both minerals K cations are located in the voids of the Si layer. The content of the interlayer space is different in each mineral: only H₂O molecules in shlykovite, K atoms and H₂O molecules in cryptophyllite. Mountainite, shlykovite and cryptophyllite are closely related in configuration of the TOT blocks, their main structure unit, and could be used to define the mountainite structural family.