The new mineral somersetite, has been found at Ton Works ('Merehead quarry') in Somerset, England, United Kingdom. Somersetite is green or white (typically it is similar visually to hydrocerussite-like minerals but with a mint-green tint), forms plates and subhedral grains up to 5 mm across and up to 2 mm thick. In bi-coloured crystals it forms very thin intergrowths with plumbonacrite. The empirical formula of somersetite is Pb8.00C5.00H4.00O20. The simplified formula is Pb8O(OH)(4)(CO3)(5) ,which requires: PbO = 87.46, CO2 = 10.78, H2O = 1.76, total 100.00 wt.%.

The infrared spectrum of somersetite is similar to that of plunbonacrite and, to a lesser degree, bydrocerussite. Somersetite is hexagonal, P6(3)/mmc, a = 5.2427(7), c = 40.624(6) , V = 967.0(3) (3) and Z = 2. The eight strongest reflections of the powder X-ray diffraction (XRD) pattern [d,(I)(hkl)] are: 4.308(33) (103), 4.148(25)(104), 3.581(40)(107), 3.390(100)(108), 3.206(55)(109), 2.625(78)(110), 2.544(98)(0.0.16) and 2.119(27)(1.0.17). The crystal structure was solved from single-crystal XRD data giving R-1 = 0.031. The structure of somersetite is unique and consists of the alternation of the electroneutral plumbonacrite-type [Pb5O (OH)(2)(CO3)(3)](0) and hydrocerussite-type [Pb-3(OH)(2)(CO3)(2)](0) blocks separated by stereochemically active lone electron pairs on Pb-2(+). There are two blocks of each type per unit cell in the structure, which corresponds to the formula [Pb5O(OH)(2)(CO3)(3)][Pb-3(OH)(2)(CO3)(2)] or Pb8O(OH)(4)(CO3)(5) in a simplified representation. The 2D blocks are held together by weak Pb-O bonds and weak interactions between lone pairs.