Reyerite is comprised of a sheet of edge-sharing Ca octahedra with adjacent tetrahedral sheets of Si₈O₂₀ and a double tetrahedral sheet of Si₁₄Al₂O_38, and an overall chemical composition of (Na,K)₂Ca₁₄Si₂₂Al₂O₅₈(OH)₈ ^. 6H₂O (Merlino, 1988a). The tetrahedral sheets are formed from 6-fold rings of tetrahedra with some tetrahedra pointing up and some down. Na, K, and H₂O occupy partially filled sites within the double tetrahedral sheet network. Truscottite, Ca₁₄(Si₂₄O₅₈)(OH)₈ ^. 2H₂O, is similar to reyerite, but with alkali and aluminum generally absent, although a limited amount of K and Al can be present and the composition can approach reyerite. Gyrolite, Ca₁₆Si₂₄O₆₀(OH)₈ ^. (14+x)H₂O, has a Ca octahedral sheet bounded by Si₈O₂₀ tetrahedral sheets as a unit (layer charge may vary from -4 to -5 depending on Al content) separated from other similar units by an interlayer of 2Ca + Na octahedra (Merlino, 1988b). Fedorite, K₂(Ca₅Na₂)Si₁₆O₃₈(OH,F)₂ ^. H₂O, has a sheet of edge-sharing Ca octahedra with double tetrahedral sheets, Si₁₆O₃₈, on adjacent sides. Minehillite, (K,Na)₂Ca₂₈Zn₅Al₄Si₄₀O₁₁₂(OH)₁₆, has a central edge sharing Ca octahedral sheet with a single tetrahedral sheet on one side (similar to reyerite), but a complex slab that differs from reyerite on the other. The structure of cairncrossite, Sr₂Ca_7-xNa_2x(Si₄O₁₀)₄(OH)₂(H₂O)_15-x, where x is between 0 and 1, forms a unit consisting of edge-sharing Ca octahedral sheets with adjacent Si tetrahedral sheets. These units are cross linked by SrO₈ polyhedra. The structure of orlymanite (chemical composition approximately Ca₄Mn₃Si₈O₂₀(OH)₆ ^. 2H₂O), which has not been determined, is believed related to reyerite. Synthetic (“K-phase”, “Z-phase”) phases with structural similarities to reyerite are known. Reyerite group minerals are potentially important in cements used to case geothermal wells where phases form at elevated temperatures and pressures in a steam-rich environment.