Paulisite, Ca2Zn(CO3)3<middle dot>2H2O, a new mineral with a novel crystal structure

Paulisite, Ca2Zn(CO3)(3)<middle dot>2H(2)O, is a new mineral species discovered in the underground workings at the abandoned mine adit of the first level of the Staroceske Lode, near the historical shaft safary, Kank near Kutna Hora, central Bohemia, Czech Republic. Paulisite is associated with hydrozincite and aragonite (holotype sample) or calcite, aragonite, hydrozincite, and monohydrocalcite (other samples). The new mineral occurs as crusts, up to 1 cm thick, formed by parallel or radial aggregates of acicular crystals, elongated on [100], up to 5 mm long. Paulisite is colorless to white, with a white streak. It is transparent and has a vitreous luster. Mohs hardness is ca. 4; the calculated density is 2.756 g/cm(3). Paulisite is optically biaxial positive, with alpha = 1.554(1), beta = 1.569(2), gamma = 1.605(1) (589 nm), and 2V((meas)) = 68(2)degrees. The empirical formula, based on electron-microprobe analyses (n = 11), is Ca-2.00(Zn0.97Mg0.02 Cu0.01Al0.01)Sigma(1.01)(CO3)(3)<middle dot>2H(2)O based on three cations (excluding C) per formula unit. The ideal formula is Ca2Zn(CO3)(3)<middle dot>2H(2)O, which requires (in wt%) CaO 31.02, ZnO 22.50, CO2 36.51, H2O 9.97, total 100.00. The strongest reflections of the powder X-ray diffraction pattern [d (& Aring;)/(I-rel)/hkl] are: 8.226/(100)/011, 6.492/(100)/002, 4.112/(18)/022, 3.246/(35)/004, 3.085/(19)/130, and 2.458/(21)/042. According to single-crystal X-ray diffraction data, paulisite is monoclinic, space group Ia, Z = 4 with a = 6.3007(6), b = 10.6236(11), c = 12.9837(12) & Aring;, beta = 90.840(5)degrees, V = 868.99(15) & Aring;(3). The crystal structure was refined to R-1 = 0.0229 for 2330 unique reflections with F-o > 4 sigma(F-o) and 164 refined parameters. It is characterized by Zn(1)-centered tetrahedra, two independent Ca(1)- and Ca(2)-centered polyhedra, and CO3 groups. Heteropolyhedral Ca-Zn-CO3 {001} layers occur in paulisite and are connected along c through CO3 groups and Ca(2)-centered polyhedra, as well as H-bonds. Along with minrecordite, skorpionite, and znucalite, paulisite is the fourth mineral containing Ca, Zn, and (CO3) groups as species-defining elements. Its origin is related to the supergene alteration of ore deposits following the mining activity, probably at low T and basic pH conditions. The mineral and its name, honoring the Czech mineralogist and geologist Petr Paulis (b. 1956), have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (no. 2023-031).