A platy, hydrous alkali silicate of composition approximating Na2O . 14SiO2 . 9H2O with exchange properties, and which forms by precipitation from alkaline lakes rich in carbonate/bicarbonate brines, such as that found at Lake Magadi, Kenya. Suggested formulae include NaSi7O13(OH)3 . 3H2O and NaSi6O12(OH). The latter formula, which differs somewhat from the chemical ratio (due to supposed impurities), is derived from a proposed structure model (Garcés et al., 1988) based on the zeolites in the mordenite group: the model consists of continuous sheets of six-fold SiO4 rings of tetrahedra and adjacent five-fold rings pointing away from the sheet surfaces. Other structure models are possible. Kenyaite, with an approximate formula of NaSi11O20.5(OH)4 . H2O, forms under a similar environment as magadiite and is probably a layer structure also.