Tsumebite is a rare phosphate mineral named in 1912 after the locality where it was first found, the Tsumeb mine in Namibia, well known to mineral collectors for the wide range of minerals found there. Tsumebite is a compound phosphate and sulfate of lead and copper, with hydroxyl, formula Pb₂Cu(PO₄)(SO₄)(OH). There is a similar mineral called arsentsumebite, where the phosphate group PO₄ is replaced by the arsenate group AsO₄, giving the formula Pb₂Cu(AsO₄)(SO₄)(OH). Both minerals are members of the brackebuschite group.

The minerals in the brackebuschite group have the general formula A₂B(H₂O,OH)(TO₄)₂ and crystallise in the monoclinic system, space group P2₁/m. The group includes:^[5]

• brackebuschite Pb₂Mn³⁺(VO₄)₂(OH)
• arsenbrackebuschite Pb₂(Fe³⁺,Zn)(AsO₄)₂(OH,H₂O)
• gamagarite Ba₂Fe³⁺(VO₄)₂(OH)
• goedkenite Sr₂Al(PO₄)₂(OH)
• bearthite Ca₂Al(PO₄)₂OH
• tsumebite Pb₂Cu(PO₄)(SO₄)(OH)
• arsentsumebite Pb₂Cu(AsO₄)(SO₄)(OH)
• vauquelinite Cu²⁺Pb₂(CrO₄)(PO₄)(OH)
• fornacite CuPb₂(CrO₄)(AsO₄)(OH)
• molybdofornacite CuPb₂MoO₄AsO₄(OH)

The structure of the brackebuschite group minerals is composed of B-(O,OH)₆ octahedra, two non-equivalent TO₄ tetrahedra, TO₄(1) and TO₄(2), and two different irregular polyhedra of large cations. B and T represent different elements in different members of the group. Chains formed from the B octahedra link through the oxygens of TO₄(2) tetrahedra, while the large cation polyhedra form double chains parallel to the b crystal axis through edge sharing with TO₄(1) tetrahedra. The result is a tight three-dimensional structure.^[5] In tsumebite copper ions occupy the B sites, and phosphorus and sulfur occupy the T sites. Lead is the large cation.

Tsumebite belongs to the monoclinic crystal class 2/m, meaning that it has one twofold axis of symmetry perpendicular to a mirror plane. The space group is P 2₁/m, meaning that the crystal lattice is a primitive lattice, with structural elements only at the vertices of the unit cell. These structural elements are made up of two formula units (Z = 2).
Unit cell parameters are a = 8.69 Å, b = 5.78 Å, c = 7.86 Å, β = 111.87°^[4][5][6] or a = 8.70 Å, b = 5.80 Å, c = 7.85 Å, β = 111.5°.^[2][3]

Tsumebite is an emerald-green color, transparent and green in transmitted light, with a green streak and an adamantine (diamond-like) to vitreous (glassy) luster. It is biaxial (+) with refractive indices N_x = 1.885 to 1.900, N_y = 1.920 and N_z = 1.942 to 1.956. It is faintly pleochroic with X = Y = very pale blue to colorless and Z = robin's-egg-blue.^[2][3]

The mineral typically occurs as crusts of intergrown crystals on matrix. Cleavage is absent, but twinning is almost universal, and twins may be multiple, with serrated re-entrants.^[2] It is brittle, with an uneven fracture, hardness 3+1⁄2 and specific gravity 6.13. It is readily soluble in hydrochloric acid HCl and slowly soluble in nitric acid HNO₃.^[4] It is not radioactive.^[6]

Tsumebite is a rare secondary mineral in the oxidised zone of some arsenic-bearing lead-copper deposits, with other lead-bearing phosphates and sulfates.^[2] Associated minerals include azurite, smithsonite, malachite, cerussite, mimetite, wulfenite and olivenite.^[2] The type locality is the Tsumeb mine, Tsumeb, Otjikoto Region, Namibia.^[4] The Handbook of Mineralogy^[2] states that the type material was destroyed by bombing, but does not indicate when or where.

Tsumebite occurs at Morenci, Arizona, predominantly as twinned crystals associated with wulfenite, olivenite and the hyalite variety of opal.^[3] At Broken Hill, New South Wales, Australia, tsumebite has been found as lustrous pale blue to bluish green crystals. It usually occurs with yellow crusts of corkite-hinsdalite, colorless to white pyromorphite needles and sprays of pale greyish green zincian libethenite. Less commonly found with scholzite and torbernite.^[7]