Mississippi Valley type galena deposits emplaced into Carboniferous limestones throughout the Mendip Hills during the late Permian or Triassic period were locally exposed to the action of seawater during the Jurassic period following regional uplift and erosion of the intervening strata. Oxidation of galena initiated the deposition of manganate minerals from the seawater, and these adsorbed heavy metals from both the seawater and local environment. A subsequent hydrothermal event heated the lead-manganate deposits causing decomposition of the galena and creating the conditions which led to the formation of the suite of unusual secondary minerals – including a number of rare oxychlorides – now found at Merehead. Heating of the manganate phases converted them to Mn oxides and released the entrained heavy metals which were then incorporated into unusual mineral phases. The impervious Mn oxide coating which enclosed the cooling Pb-rich areas isolated them chemically, leading to closed-system behaviour. The high-T phases at Merehead are similar to those found in the Pb-bearing silicic skarns at Långban, whilst the suite of secondary minerals which evolved in the closed-system environments bears striking similarities to the ‘anomalous sequence’ of minerals found at the Mammoth-St. Antony Mine. The complexity of these formation processes probably explains the rarity of Mendip-type Pb-Mn deposits. The collective importance of the disconformity, the hydrothermal event, and subsequent sealing of the deposits are recognized for the first time, and the temperature of the hydrothermal event is shown to have been much greater than has heretofore been realized. Silurian volcanic strata underlying the Carboniferous limestones which have previously been assumed to be the source of heavy metals are shown to have been uninvolved in the process.

The crystal structure of mereheadite (monoclinic, Cm, a = 17.372(1), b = 27.9419(19), c = 10.6661(6) Å, β = 93.152(5)°, V = 5169.6(5) Å3) has been solved by direct methods and refined to R1 = 0.058 for 6279 unique observed reflections. The structure consists of alternating Pb–O/OH blocks and Pb–Cl sheets oriented parallel toth e (201) plane and belongs toth e 1:1 type of lead oxide halides with PbO blocks. It contains 30 symmetrically independent Pb positions, 28 of which belong to the PbO blocks, whilst two positions (Pb12 and Pb16) are located within the tetragonal sheets of the Cl– anions. Mereheadite is thus the first naturally occurring lead oxychloride mineral with inter-layer Pb ions. The coordination configurations of the Pb atoms of the PbO blocks are distorted versions of the square antiprism. In one half of the coordination hemisphere, they are coordinated by hard O2– and OH– anions whose number varies from three to four, whereas the other coordination hemisphere invariably consists of four soft Cl– anions located at the vertices of a distorted square. The Pb12 and Pb16 atoms in between the PbO blocks have an almost planar square coordination of four Cl– anions. These PbCl4 squares are complemented by triangular TO3 groups (T = B, C) so that a sevenfold coordination is achieved. The Pb–O/OH block in mereheadite can be obtained from the ideal PbO block by the following list of procedures: (1) removal of some PbO4 groups that results in the formation of square-shaped vacancies; (2) insertion of TO3 groups into these vacancies; (3) removal of some Pb atoms (that correspond to the Pb1A and Pb2A sites), thus transforming coordination of associated O sites from tetrahedral OPb4 tot riangular OHPb3; and (4) replacement of two O2– anions by one OH– anion with twofold coordination; this results in formation of the 1×2 elongated rectangular vacancy. The structural formula that can be derived on the basis of the results of single-crystal structure determination is Pb47O24(OH)13Cl25(BO3)2(CO3). Welch et al. (1998) proposed the formula Pb2O(OH)Cl for mereheadite, which assumes that neither borate nor carbonate is an essential constituent of mereheadite and their presence in the mineral is due to disordered replacements of Cl– anions. However, our study demonstrates that this is not the case, as BO3 and CO3 groups have well-defined structural positions confined in the vacancies of the Pb–O/OH blocks and are therefore essential constituents. Our results also show that mereheadite is not a polymorph of blixite, but is in fact related to symesite. Symesite thus becomes the baseline member of a group of structurallyrelated minerals.

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, Dcalc = 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) Å3, 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 Pb8O4.19(BO3)0.51 (CO3)0.62(OH)0.76Cl4.09. 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.