Birnessite Post J E, Veblen D R American Mineralogist 75 (1990) 477-489 Crystal structure determinations of synthetic sodium, magnesium and potassium birnessite using TEM and the Rietveld method sample is Na-birn _database_code_amcsd 0001300 5.174 2.850 7.336 90 103.18 90 C2/m atom x y z occ Biso Mn 0 0 0 0.5 O1 .376 0 .133 1.0 Na .595 0 .500 .145 1.5 Wat2 .595 0 .500 .30 1.5 Wat3 0 0 .5 .15 1.5