(from the village of Chamoson in Switzerland). (1) A mineral of the subclass of phyllosilicates; an iron chlorite. Chamosite, whose chemical composition is (Fe, Al, Mg)₃[(Si, Al)₄O₁₀](OH)₂(Fe, Mg)₃(O, OH)₆, contains more than 30 percent iron oxides. The distance between layers in its structure is 14 angstroms.

(2) A mineral of the family of septechlorites; also known as berthierine or septechamosite. Its chemical composition is (Fe²⁺, Fe³⁺, Al, Mg)₆[(Si, Al)₄O₁₀](OH)₈. The mineral contains 40–45 percent iron oxides. The composition varies. The structure of chamosite resembles that of kaolinite; the distance between layers is 7 angstroms.

The two minerals may be found together and are difficult to distinguish. They crystallize in the monoclinic system (septechamosite also exhibits a hexagonal modification). They form green, yellow, or brown oolitic or fine-grained (to earthy) aggregates. The hardness on Mohs’ scale is 3, and the density varies from 3,000 to 3,300 kg/m³. Both are hypergenic in origin.

Chamosites are found in clay beds and sedimentary (oolitic) iron ores. Deposits in the USSR occur in the Urals (Alapaevsk, Aiat’, and Khalilovo). Other deposits are found in the Lorraine Basin in France, Northhamptonshire in Great Britain, Thuringia in East Germany, and Nucice in Czechoslovakia.