Mereheadite, ideally Pb₂O(OH)Cl, is a new mineral related to litharge and which is structurally similar to synthetic bismuth-oxyhalides. With other lead- and lead-copper oxychlorides, it occupies lenses and cavities in veins of manganese and iron oxide minerals which cut through a sequence of dolomitic limestones at Merehead quarry, Cranmore, Somerset (51°12′N, 2°26′W). Mereheadite is pale yellow to reddish-orange, transparent to translucent and has a white streak and a vitreous or resinous lustre. It is not fluorescent. Individual grains, up to a few mm across, cluster together in compact masses of 10–30 mm in size, but discrete crystals have not been observed. Specular reflectance data on randomly orientated grains from 400 to 700 nm are provided, and refractive indices calculated from these at 590 nm range from 2.19 to 2.28. H = 3.5, VHN₁₀₀ = 171, D_(meas) = 7.12(10) g/cm³, D_calc = 7.31 g/cm³. The mineral is brittle with an uneven, conchoidal to hackly fracture and has a perfect (001) cleavage which is parallel to the sheets of PbO and Cl. It is intimately associated with mendipite, blixite, cerussite, hydrocerussite and calcite in lenses and pods in the veins. Other minerals which occupy cavities in these veins include chloroxiphite, paralaurionite, parkinsonite and the borosilicate datolite. Mereheadite is monoclinic, space group C2/c, and its cell parameters, refined from powder X-ray diffraction are: a = 5.680(2), b = 5.565(3), c = 13.143(9) Å, β = 90.64(4)°, V = 415.4 (8) ų, Z = 4. The ten strongest reflections in the X-ray powder diffraction pattern are [d in Å, (I, hkl)]: 2.930(10,113), 3.785(5,111,-111), 2.825(4,200), 6.581(4,002), 2.182(4,115), 2.780(4,020), 3.267(4,004), 1.980(3,-220), 1.695(3,224,132,117), 1.716(3,026). Its empirical formula is Pb₈O_4.19(BO₃)_0.51 (CO₃)_0.62(OH)_0.76Cl_4.09. Although it is very similar chemically to blixite, it has notably different cell parameters. There is some uncertainty about the essential nature of boron and carbon in natural mereheadite. This stems from the impossibility of ensuring the purity of samples for wet-chemical analysis, and from the predominance of lead in the structure of the mineral which has meant that the location of boron and carbon within the mereheadite structure is unresolved. ¹¹B MAS NMR does show, however, that boron is present as BO₃ groups. The structure consists of alternating PbO sheets and layers of chlorine atoms. Each lead atom is coordinated to four chlorines and four O/OH in a square antiprism configuration. As such, it is structurally-related to nadorite, thorikosite and schwartzembergite. Comparisons with structurally analogous phases such as bismuth oxychlorides and bismutite (Bi₂O₂CO₃) suggest that the BO₃ and CO₃ groups are likely to replace chlorine in the layer between PbO sheets. The composition of natural mereheadite is defined by three end-members: the mereheadite end-member Pb₂O(OH)Cl, and two fictive end-members Pb₂(OH)₂CO₃ and Pb₄O(OH)₃BO₃.