Based on x- and y-coordinates determined by Peng, Wu, and Chang( 1963), acomplete structure has been established for szaibelyite (ascharite) and refined to R = 5.9 percent, using a crystal from Königshall-Hindenburg, Germany, twinned on (100): P21/a, a 12.577(2), b 10.393(2), c 3.139(l)Å, β 95.88(2)° Z = 4[Mg2(OH)B2O4(OH)], calculated density 2.738 g/cm3. Two non-equivalent chains of octahedra formed by oxygen atoms about magnesium are joined by sharing corners to form sheets parallel to (100). The sheets are held together by pyroborate groups, B2O4(OH). Each octahedral chain has a strong twofold screw pseudosymmetry parallel to the c axis. The pseudosymmetry is such that the chain is nearly perfectly transformed by the screw operations into itself with average atomic displacement of only 0.056(4)Å. This situation theoretically permits generation of polymorphic variations of the structure. The cell dimensions and space groups two of simple variations are: (1)a12.511, b10.393, c3.139Å, P212121; (2)a 25.022, b 10.393, c 3.139Å, γ 90.00°, P21/a.