Katsarosite Zn(C2O4)·2H2O, a new humboldtine-group mineral from the Lavrion Mining District, Greece

Abstract

Katsarosite, ideally Zn(C2O4)·2H2O, named for Īraklīs Katsaros, is a new mineral found at the Esperanza Mine in the Kaminiza area of the Lavrion Mining District, Greece. Katsarosite usually occurs directly on sphalerite or embedded in jarosite and/or hydrozincite, often intimately intergrown with gypsum and overgrown by goslarite and/or epsomite. Crystal aggregates are mostly fine granular to earthy, with individual crystals being usually rounded with an average diameter of 30 µm, sometimes prismatic along [001] or platy, exhibiting the indistinct forms {100}, {001}, {110}, and {101}. Katsarosite is malleable with a Mohs hardness of 1½ – 2 and exhibits a perfect cleavage on {110}; the fracture is uneven in all other directions. The colour depends on the iron (Fe2+) content, ranging from pure white in almost Fe-free samples to yellow in Fe-rich specimens. It has a resinous luster and a white streak; no luminescence has been observed under either short- or long-wave ultraviolet radiation. Katsarosite is optically biaxial (+). Refractive indices measured at a wavelength of 589 nm are nα = 1.488(2), nβ = 1.550(2), nγ = 1.684(2), with 2Vobs = 71(3)°. Chemical analysis gave on average C2O3 38.32 wt%, ZnO 38.99 wt%, FeO 1.92 wt%, and H2O 19.04 wt% (the latter was deduced based on the crystal-structure refinement), with traces of MgO and MnO. The new mineral is readily soluble in dilute acids. Katsarosite is monoclinic, space group C2/c, with unit-cell parameters a = 11.768(3), b = 5.3882(12), c = 9.804(2) Å, β = 127.045(8)°, V = 496.2(2) Å3 (Z = 4). The strongest lines in the Gandolfi X-ray powder pattern [dobs in Å, Iobs/I100, (hkl)] are: 4.6745, 100, (200); 4.7678, 94, (20overline{2 }); 2.9533, 51, (40overline{2 }); 4.7030, 37, (1 overline{1 },overline{1 }); 3.9266, 33, (002); 3.5686, 27, (111); 2.6574, 22, (1 overline{1 },overline{3 }); 3.5992, 8, (1 overline{1 },overline{2 }); 2.7032, 4, (020). The crystal structure was refined based on single-crystal X-ray diffraction data to R(F) = 0.08. The observed mass density of 2.50(2) g cm−3 compares well with the calculated value (2.508 g cm−3). Katsarosite belongs to the humboldtine group, whose crystal-structure type is well described for both isotypic minerals and synthetic compounds in the literature. The atomic arrangement in Zn(C2O4)·2H2O is characterized by chains consisting of isolated ZnO6 octahedra which are alternately linked along [010] via oxalate anions. These chains are interconnected through hydrogen bonds only, with Ow···O (with Ow denoting the O atom of the H2O molecule) donor–acceptor distances of ~ 2.8 Å.