This sample of ardennite is displayed in the Smithsonian Museum of Natural History. Ardennite is a silicate of magnesium and manganese. It also contains arsenic with the composition Mg_1+xMn₄Al_5-x(AsO₄)(SiO₄)₂(Si₃O₁₀)(OH)₆. The sample at left is about 8 cm across and is from Ottre, Ardennes, Belgium.