A white coloured 6-layer serpentine mineral was found in the serpentine rock at Ogose, Iruma-gun, Saitama Prefecture. It occurs filling cracks of the serpentine rock. Its X-ray pattern clearly differs from those of published data for 6-layer ortho-serpentine. The reflections are indexed on the basis of a monoclinic cell with a=5.420Å, b=9.200Å, c=43.92Å and β=93.62°. Chemical composition: SiO₂ 41.37%; Al₂O₃ 1.23%; Fe₂O₃ 0.60%; MgO 42.86%; CaO 0.33%; TiO₂ 0.09%; H₂O-0.23%; H₂O+13.29%; Total 100.00%. The structural formula is (A1_0.01, Fe_0.04, Mg_5.07, Ti_0.01, Ca_0.03)(Si_3.87, Al_0·13) O₁₀ (OH) ₈.