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Mereheadite, ideally Pb 2 O(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 (51812'N, 2826'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 10À30 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, VHN 100 = 171, D (meas) = 7.12(10) g/cm 3 , D calc = 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 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 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.11 B MAS NMR does show, however, that boron is present as BO 3 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 (Bi 2 O 2 CO 3 ) suggest that the BO 3 and CO 3 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 Pb 2 O(OH)Cl, and two fictive end-members Pb 2 (OH) 2 CO 3 and Pb 4 O(OH) 3 BO 3 .

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S U M M A R V. Petrographic descriptions, bulk chemistry, and partial analyses of several mineral phases in each stone are presented. Medanitos, previously classed as a eucrite, is now shown to be a howardite: Putinga is an equilibrated olivine-hypersthene chondrite. THESE stony meteorites are described together because specimens of both were acquired by the British Museum from the same donor, the Vatican Collection of Meteorites, through the auspices of Fr. E. W. Salpeter, S.J. MEDANITOS The specimen described is a 6-g slice, B.M. I968, 3, in excellent condition, from the larger of the two stones found (Hey, I966), and represents some 2o % of the material collected. Beneath a dark fusion crust, the central portion is leucocratic, being composed of clear or milky-white feldspar, pale brown pyroxene, and occasional spots of sulphide. Rounded intergrowths of coarser-grained crystals, up to I mm long, are set in a finer-grained groundmass. The intergrowths range from 2 to 4 mm in diameter. Although the groundmass is generally fine-grained (o.2 mm), some large, subophitic patches of pyroxene have lengths up to 2 mm. Groundmass textures are due to granu-lation; the subophitic pyroxenes do not exhibit complete optical continuity and are probably relicts of even larger grains; some thin exsolution lamellae, probably parallel to (00I), are occasionally present. Feldspars are unzoned. Sulphide is unevenly distributed, but is mostly in grains less than o.2 mm in length; it was absent from the fragment taken for analysis. A translucent red-brown phase occurs as inclusions in feldspar and pyroxene, and a microprobe study has shown it to be chromite. Calcite has been identified by an X-ray diffraction technique, and a subsequent test for CO2 proved positive. The chemical analysis is presented in table I, together with the Wahl (weight) norm and partial analyses of the feldspar and pyroxene. Compositions of feldspar derived from the bulk chemical and the microprobe analyses agree closely at about An96. The normative pyroxene is richer in Ca than that measured directly; this is partly due to the concentration of this element in exsolution lamellae and partly to the presence of calcite. The presence of calcite also helps to explain the appearance of normative olivine when none was found in the section; calcium actually present as carbonate appears as wollastonite in the norm and reduces the amount of SiQ available for normative hypersthene. 9

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