The paper contains description of quintinite [Mg ₄ Al ₂ (OH) ₁₂ ](CO ₃ )(H ₂ O) ₃ for the first time found in the Mariinskoe deposit (the Middle Urals, Russia). The mineral occurs there as white tabular crystals in cavities of metasomatically altered gabbro in association with prehnite, calcite and a chlorite group mineral. Its formation seems to be connected with the late hydrothermal alteration of primary mafic and ultramafic rocks hosting the emerald-bearing glimmerites. The Mg:Al ratio was determined as ∼ 2:1 by electron microprobe analysis. IR-spectroscopy revealed in the mineral the presence of H2O molecules, hydroxyl and carbonate groups. Quintinite is monoclinic, space group C2/m, a =5.233(1), b = 9.051(2), ñ = 7.711(2) A, b = 103.09(3)°, V = 355.7(2) A ³ . Based on single-crystal structure refinement the polytype should be denoted as 1M. This is the third find of quintinite-1M in the world (after Kovdor massif and Bazhenovskoe chrisotile-asbestos deposit).