Grootfonteinite, Pb ₃O(CO ₃) 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 H _0.345Na _0.275Ca _0.045Pb _2.645C _2O7 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 …-[PbCO ₃]-[PbO]-[PbCO ₃]-… The composition of the resulting electroneutral 2D block is {[Pb ₂(CO ₃) ₂][(Pb _0.7Na _0.3)(O _0.7(OH) _0.3)]} ⁰. The stereochemically active 6s2 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 [PbCO ₃] 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.