MEREHEADITE, PB2O(OH)CL - A NEW LITHARGE-RELATED OXYCHLORIDE FROM MEREHEAD QUARRY, CRANMORE, SOMERSET

Mereheadite, ideally Pb2O(OH)Cl, is a new mineral related to litharge and which is structurally similar to synthetic bismuth-oxyhalides. Wit h other lead-and lead-copper oxychlorides, it occupies lenses and cavi ties in veins of manganese and iron oxide minerals which cut through a sequence of dolomitic limestones at Merehead quarry, Cranmore, Somers et (51 degrees 12'N, 2 degrees 26'W). Mereheadite is pale yellow to re ddish-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 refle ctance data on randomly orientated grains from 400 to 700 nm are provi ded, 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/cm(3), D-c alc = 7.31 g/cm(3). 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 mendipit e, blixite, cerussite, hydrocerussite and calcite in lenses and pods i n the veins. Other minerals which occupy cavities in these veins inclu de chloroxiphite, paralaurionite, parkinsonite and the borosilicate da tolite. Mereheadite is monoclinic, space group C2/c, and its cell para meters, refined from powder X-ray diffraction are: a = 5.680(2), b = 5 .565(3), c = 13.143(9) Angstrom, beta = 90.64(4)degrees, V = 415.4(8) Angstrom(3), Z = 4. The ten strongest reflections in the X-ray powder diffraction pattern are [d in Angstrom, (I, hkl)]: 2.930(10,113), 3.78 5(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 .76)Cl-4.09. Although it is very similar chemically to blixite, it has notably different cell parameters. There is some uncertainty about th e essential nature of boron and carbon in natural mereheadite. This st ems from the impossibility of ensuring the purity of samples for wet-c hemical analysis, and from the predominance of lead in the structure o f the mineral which has meant that the location of boron and carbon wi thin the mereheadite structure is unresolved. B-11 MAS NMR does show, however, that boron is present as BO3 groups. The structure consists o f alternating PbO sheets and layers of chlorine atoms. Each lead atom is coordinated to four chlorines and four O/OH in a square antiprism c onfiguration. As such, it is structurally-related to nadorite, thoriko site and schwartzembergite. Comparisons with structurally analogous ph ases such as bismuth oxychlorides and bismutite (Bi2O2CO3) suggest tha t the BO3 and CO3 groups are likely to replace chlorine in the layer b etween PbO sheets. The composition of natural mereheadite is defined b y three end-members: the mereheadite end-member Pb2O(OH)Cl, and two fi ctive end-members Pb-2(OH)(2)CO3 and Pb4O(OH)(3)BO3.