a mineral of the oxidized chlorite (leptochlorite) group with a complex, varying composition. The approximate formula is (Fe²⁺, Fe³⁺)₃[Al₂Si₂O₁₀] (OH)₂(Mg, Fe)₃(OH)₆. Thuringite crystallizes in the monoclinic system. It usually forms compact, fine-grained masses but sometimes occurs as minute, readily separable flexible scales having a pearly luster and pronounced pleochroism. Hardness on Mohs’ scale varies from 2 to 2.5, and the density is 3,150–3,190 kg/m³. The mineral has perfect (001) cleavage.

Thuringite can be identified only by using a number of diagnostic methods. In its largest accumulations, it forms a valuable component in the sedimentary deposits of iron ore; it is also formed during the metamorphosis of iron-rich quartzites and the hydrothermal alteration of iron-rich silicates in magmatic rock. The largest known deposits of thuringite are in Thuringia (German Democratic Republic).