Original paper
Grootfonteinite, Pb3O(CO3)2, a new mineral species from the Kombat Mine, Namibia, merotypically related to hydrocerussite
Siidra, Oleg I.; Jonsson, Erik; Chukanov, Nikita V.; Nekrasova, Diana O.; Pekov, Igor V.; Depmeier, Wulf; Polekhovsky, Yury S.; Yapaskurt, Vasiliy O.
European Journal of Mineralogy Volume 30 Number 2 (2018), p. 383 - 391
29 references
published: Aug 20, 2018
published online: Jan 10, 2018
manuscript accepted: Jun 27, 2017
manuscript revision received: Jun 17, 2017
manuscript received: Apr 1, 2017
Abstract
Grootfonteinite, Pb3O(CO3)2, is a new Pb oxycarbonate found in a mineralogically complex, banded assemblage from the Mn(-Fe) oxide ore unit of the Kombat mine. Grootfonteinite is named after the locality in the Grootfontein district. The mineral forms platy grains up to 1 mm across and up to 0.2 mm thick included in and intergrown with massive cerussite. Grootfonteinite is colourless, with white streak and adamantine lustre. It is brittle with perfect cleavage on (0 0 1). The density calculated using the empirical formula H0.345Na0.275Ca0.045Pb2.645C2O7 is 6.856 g · cm−3.The strongest five reflections in the X-ray powder-diffraction pattern [(d in Å)-(Intensity)-(hkl)] are: 4.586-25-0 1 0, 3.244-100-0 1 3, 2.652-30-1 1 0, 2.294-21-0 2 0, 2.053-39-0 2 3. Grootfonteinite crystallizes in space group P63/mmc (No. 194), a = 5.303(1), c = 13.770(3) Å, V = 335.3(1) Å3, Z = 2. The crystal structure of grootfonteinite is formed by layered blocks which consist of sheets with composition [PbCO3] and (ideally) [PbO], the stacking of which can be described as …-[PbCO3]-[PbO]-[PbCO3]-… The composition of the resulting electroneutral 2D block is {[Pb2(CO3)2][(Pb0.7Na0.3)(O0.7(OH)0.3)]}0. The stereochemically active 6s 2 lone electron pairs of the two Pb atoms are located in between the blocks, resembling the classical case of the structure of litharge. Grootfonteinite is structurally related to hydrocerussite, abellaite, and plumbonacrite. A characteristic structural feature of all these minerals is the presence of [PbCO3]0 sheets in the upper and lower parts of invariably electroneutral 2D blocks, the middle part being variable. The topology of 2D blocks in the crystal structure of grootfonteinite can be considered as intermediate between those of abellaite and hydrocerussite. These three minerals can be considered to form a merotype family. Other members of this family can be hypothesized which differ in the nature of the interleaved sheets.
Keywords
grootfonteinite • carbonate • lead • hydrocerussite • abellaite • layered structure • new mineral • Kombat mine