nontronite (non'-tron-ite). A member of the dioctahedral montmorillonite group of minerals in which the layer charge is the result of substitution primarily in the tetrahedral sheet. An ideal formula is: (Ca_0.5,Na)_0.3Fe³⁺₂(Si,Al)₄O₁₀(OH)₂• n H₂O . One of the dominant minerals along mid-ocean ridges. Cf: smectite .