a mineral of the montmorillonite-vermiculite group. In terms of its crystallochemical structure, nontronite is a phyllosilicate. Its chemical composition is

Fe₂³⁺ [Al_xSi_4-xO₁₀](OH)₂ · Na_0.33(H₂O)₄

It contains significant quantities of Al₂ OO₃ (up to 14 percent) and MgO (up to 8 percent), as well as small quantities of CaO (up to 2 percent), K₂O, Na₂O, and sometimes NiO and Cr₂O₃. The basis of the structure of nontronite is a three-layer unit of the talc or pyrophyllite type—two layers of SiO tetrahedrons separated by an octahedral layer of FeO₆; between the unit layers are molecules of water with exchange bases of Na. Al replaces the Si in the tetrahedrons.

Nontronite is an isomorphic mixture of nontronite proper and montmorillonite. It crystallizes in the monoclinic system. Aggregates of nontronite are usually compact, claylike, and crypto-crystalline, less frequently in the form of pseudomorphs after dark-colored minerals. Their color ranges from greenish yellow to dark green. Their hardness on Mohs’ scale is about 2, and their density is up to 2,100 kg/m³. Nontronite is a typical super-gene mineral, which forms primarily upon the weathering of iron-bearing silicates of various ultrabasic igneous and meta-morphic rocks. Nontronite masses rich in nickel are economically important.