Fe³⁺-bearing and dioctahedral member of the smectite group of minerals. A representative formula is: R0.33Fe3+2(Si3.67Al0.33)O10(OH)2^.nH₂O, where R refers to the exchangeable cation, commonly Na, Ca and Mg, and n is a rational number, not necessarily an integer. The layer charge originates by substitution primarily in the tetrahedral sheet. One of the dominant minerals along mid-ocean ridges.
Cf., smectite