Rumseyite, ideally [Pb2OF]Cl, is a new mineral species which is associated with calcite, cerussite, diaboleite, hydrocerussite and undifferentiated Mn oxides in a small cavity in ‘hydrocerussite’ from a manganese pod at Merehead quarry, Somerset, England. Rumseyite is tetragonal, I4/mmm, a = 4.065(1), c = 12.631(7) Å, V = 208.7(1) Å3, Z = 2. The mineral is translucent pale orange-brown with a white streak and vitreous lustre. It is brittle with perfect {100} cleavage; Dcalc = 7.71 g cm−3 (for the ideal formula, [Pb2OF]Cl). The mean refractive index in air at 589 nm is 2.15. The six strongest reflections in the X-ray powder-diffraction pattern [dmeas in Å, (Irel), (hkl)] are as follows: 2.923(100)(013), 2.875(68)(110), 3.848(41)(011), 6.306(17)(002), 1.680(14)(123), 2.110(12)(006). The crystal structure of rumseyite is based on alternating [OFPb2] and Cl layers. Rumseyite is related to other layered Pb oxyhalides. Fluorine and oxygen are statistically disordered over one crystallographic site. Rumseyite is named in honour of Michael Scott (Mike) Rumsey (1980–), Curator and Collections Manager at the NHM (London), who discovered the mineral. The mineral and name have been approved by the IMA Commission on New Mineral Names and Classification (IMA 2011-091). The holotype specimen is in the collections of the Natural History Museum, London (specimen number BM1970,110).

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