Somersetite, Pb₈O(OH)₄(CO₃)₅, a new complex hydrocerussite-related mineral from the Mendip Hills, England

Abstract

The new mineral somersetite, has been found at Torr 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 plumbonacrite and, to a lesser degree, hydrocerussite. Somersetite is hexagonal, P63/mmc, a = 5.2427(7), c = 40.624(6) Å, V = 967.0(3) Å3 and Z = 2. The eight strongest reflections of the powder X-ray diffraction (XRD) pattern [d,Å(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 R1 = 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 [Pb3(OH)2(CO3)2]0 blocks separated by stereochemically active lone electron pairs on Pb2+. There are two blocks of each type per unit cell in the structure, which corresponds to the formula [Pb5O(OH)2(CO3)3][Pb3(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.

Hydrocerussite-related minerals and materials: structural principles, chemical variations and infrared spectroscopy

“…Strong positional disorder of the Pb2 atoms over six neighbouring positions and a corresponding occupancy of 1/6 in the [Pb(OH) 2 ] 0 sheet has been discussed above for the structure of hydrocerussite. A similar splitting of corresponding Pb2-type sites, but now over three positions, is observed in the topologically similar [PbO] 0 sheets of grootfonteinite Pb 3 O(CO 3 ) 2 (Siidra et al, 2018a) and somersetite Pb 8 O(OH) 4 (CO 3 ) 5 (Siidra et al, 2018c). In grootfonteinite, a minor amount of Na + substitutes for Pb 2+ in the [PbO] 0 -sheet with correspondingly OH À in the oxygen site playing the role of a charge-compensating agent.…”

“…Even additional C-type sheets of [PbCO 3 ] 0 composition can be incorporated in the interior of the blocks, as found for the structures of plumbonacrite Pb 5 O(OH) 2 (CO 3 ) 3 (Krivovichev & Burns, 2000a;Rumsey et al, 2012) and somersetite Pb 8 O(OH) 4 (CO 3 ) 5 (Siidra et al, 2018c). In these interleaved cerussite-type sheets, each (in somersetite) or half (in plumbonacrite) of the Pb sites are disordered over two neighbouring positions in agreement with the 1/2 occupancy of these sites; accordingly, these disordered sheets are denoted C d .…”

“…(b) By inversion, as observed for the unnamed slag phase from Lavrion (Siidra et al, 2018b), somersetite (Siidra et al, 2018c) and susannite (Steele et al, 1999).…”

“…Identification of these minerals using the currently prevailing methodology of scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS) is problematic owing to their nearly identical EDS spectra. Available analytical data obtained using wavelength-dispersive spectrometry (WDS) (Cameca SX-100, 20 kV, 20 nA and 1 mm beam diameter) for hydrocerussite and Pb oxo-hydroxide carbonates (Rumsey et al, 2012;Siidra et al, 2018aSiidra et al, , 2018c, suggest that the compositions of these minerals are close to their ideal formulae and lead is the only detectable element. The concentration of other elements included in the analytical protocols (e.g.…”

“…Mineral source references: hydrocerussite (this work) / synthetic analogue 1 (Martinetto et al, 2002) / synthetic analogue 2 (this work); plumbonacrite (Rumsey et al, 2012) / synthetic analogue (Krivovichev & Burns, 2000a); synthetic Na (Krivovichev et al, 2000b;Belokoneva et al, 2002) / K, Pb-hydroxycarbonates (Brooker et al, 1983;Bette et al, 2017); grootfonteinite (Siidra et al, 2018a); abellaite (Ibá ñ ez-Insa et al, 2017); unnamed slag phase from Lavrion (Siidra et al, 2018b); somersetite (Siidra et al, 2018c); susannite (Steele et al, 1999); macphersonite (Steele et al, 1998); leadhillite (Giuseppetti et al, 1990 R3m P 3c1 (a) P6 3 /mmc (b) P6 3 /mmc P6 3 /mmc (c) a (Å ) 5.2475 (1) / 5.2465 (6) / 5.257 (3) 9.0921 (5) / 9.0891 (7) 5.268 1 4. 470, 33 / 4.462, 27 / 4.471, 22 4.26, 80 4.568, 29 / 4.631, 25 (d) 4.…”