The palygorskite-sepiolite group consists of palygorskite, sepiolite, falcondoite, kalifersite, loughlinite, raite, tuperssuatsiaite, yofortierite, windhoekite, and an un-named species, ~NaCa(Fe2+, Al, Mn)5[Si8O19(OH)](OH)7.5H2O. Palygorskite and yofortierite are dioctahedral and all others are trioctahedral. The palygorskite-sepiolite group has infinitely extending tetrahedral sheets involving 6-fold rings of tetrahedra. Tetrahedral sheets have a continuous basal oxygen-atom plane, but the palygorskite-sepiolite group has apical oxygen atoms pointing in opposing directions within a continuous sheet. Each section of like-pointing tetrahedra form a strip or ribbon pattern, and each ribbon consists of a tetrahedral ring (or two pyroxene-like chains) in palygorskite and 1.5 rings (or three pyroxene-like chains) in sepiolite. In palygorskite and sepiolite, the octahedra, which are linked via edge sharing, form strips that are not continuous sheets. In sepiolite, the octahedral strips are eight octahedra wide, whereas strips that are five octahedra in width occur in palygorskite. The terminal anion at the edges of the octahedral strip involves four OH2 groups per formula unit and are required for charge balance. Because these groups are well bonded to the octahedral metal cation and not isolated, they are not referred to as H2O. Vacant regions, zeolitic H2O, and exchangeable cations may reside in the channels formed at the edges of the octahedral strips in palygorskite and sepiolite. Exchange reactions with organic molecules are possible if the size of the organic cations is appropriate, because steric constraints control what can enter this channel. Larger molecules also may be adsorbed by the structure, but this is probably because of defects. Environments of formation range from low-temperature aqueous solutions to high-temperature hydrothermal (< 350 oC) conditions, and natural solutions tend to be alkali-rich with (Na + K)/Al > 1. See individual species for chemical compositions.