Abstract: Wesselsite, SrCu[Si₄O₁₀], is a new mineral species from the Wessels mine, Kalahari Manganese Field, South Africa, and it belongs to the gillespite group. Wesselsite is tetragonal, space group P4/ncc; the unit cell parameters, refined from Gandolfi film data, are a = 7.366(1), c = 15.574(3) Å V = 845.01 ų. The strongest lines are (d_obs/l_obs/hkl) (7.79/35/002), (4.33/20/112), (3.89/20/004), (3.44/40/104), (3.33/100/202), (3.12/55/114), (3.03/50/212), (2.68/25/204), (2.61/30/220) and (2.32/30/116). Wesselsite is associated with hennomartinite, embedded in a matrix of sugilite, xonotlite, quartz and pectolite. Microprobe analyses of 111 samples show that it is the end-member of a solid solution series with effenbergerite, BaCu[Si₄O₁₀], with substitutions of Sr by Ba up to 50 mol.%. Wesselsite forms tiny subhedral plates in sizes not exceeding 50 × 50 × 5 µm, arranged in clusters of up to 200 µm. It shows a perfect cleavage parallel to {001}, has blue colour, white to light blue streak, and is uniaxial negative with ω = 1.630(2), ε = 1.590(5), strongly pleochroic from blue (ω) to pale blue (ε). The calculated density is 3.32 g cm⁻³, the measured density is 3.2(1) g cm⁻³.