Sursassite occurs as orange-red needles associated with nsutite, hematite, apatite-(CaF), spessartine, manganoan clinochlore, and ardennite in quartz veinlets cross-cutting the red-purple schists of the Ottré Formation (Salm Group, Middle Ordovician) in the core of the Lienne syncline, Stavelot Massif, Belgium. The chemical composition of sursassite shows the presence of about 2 wt.% MgO and up to 4 wt.% CaO, while FeO does not exceed 1 wt.%. The crystal-chemical formula for sursassite, calculated on the basis 16 cations per unit cell, is (Mn3:59Ca0:44)∑=4:00 (Al5:25Mn0:193+Mg0:44Fe0:123+) ∑=6:00 (Si5:97Al0:03) ∑=6:00 (O21:53OH0:47) (OH)6:00· A single-crystal structure refinement has been performed to R1 = 0.0454, in space group P21/m, with a = 8.697(1), b = 5.787(1), c = 9.769(1) Å, and β = 108.91(1)°· The structure is based on chains of edge-sharing aluminium octahedra arranged along the b axis, connected together by sharing corners with the Si(1)O4 tetrahedron and the Si(2)Si(3)O7 group. Two independent manganese cations, Mn1 and Mn2, occur between the aluminium chains and are linked to seven and six oxygen atoms, respectively. The Mn1 site is occupied by Mn and minor amounts of Ca, while the Al1 sites contains aluminium and minor amounts of Mg, Mn3+, and Fe3+. The absence of Mg from the Mn1 and Mn2 positions is in accord with the low-pressure conditions in which sursassite crystallized in the Lienne Valley, conditions that have been estimated at 1–2 kbar.