Hulsite, , from Brooks Mountain, Seward Peninsula, Alaska, is monoclinic, P2/m, a = 10.695 ± 0.004Å, b = 3.102 ± 0.001Å, c = 5.431 ± 0.001Å, β = 94.21° ± 0.03°. The structure contains two infinite octahedral sheets parallel to (100) in accord with the platy habit of the crystals; the brackets in the above formula enclose the cation compositions of the two sheets. One sheet is formed by edge-sharing of two crystallographically distinct octahedral chains; the other is formed by edge-sharing among three such chains. The sheets are held together by corner-linking to boron-oxygen triangles and by an octahedral corner shared by both sheets. Occupancy and cation multiplicity refinements of the five octahedral sites suggest that the two-chain sheet, which is more readily flexible than the three-chain sheet, contains tin and ferric iron in one cation site, ferrous iron and magnesium in the other. Hulsite belongs in the family of 3Å fiber-axis wallpaper structures and is a less-ordered version of the pinakiolite structure. Its structure is distinct from that reported for members of the chemically similar ludwigite-vonsenite series.