Magadiite is a hydrous sodium silicate mineral (NaSi₇O₁₃(OH)₃·4(H₂O)) which precipitates from alkali brines as an evaporite phase. It forms as soft (Mohs hardness of 2) white powdery monoclinic crystal masses. The mineral is unstable and decomposes during diagenesis leaving a distinctive variety of chert (Magadi-type chert).