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

Abstract Katsarosite, ideally Zn(C 2 O 4 )·2H 2 O, 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 (Fe 2+ ) 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 n α = 1.488(2), n β = 1.550(2), n γ = 1.684(2), with 2 V obs = 71(3)°. Chemical analysis gave on average C 2 O 3 38.32 wt%, ZnO 38.99 wt%, FeO 1.92 wt%, and H 2 O 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 C 2/ 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) Å 3 ( Z = 4). The strongest lines in the Gandolfi X-ray powder pattern [ d obs in Å, I obs / I 100 , ( hkl )] are: 4.6745, 100, (200); 4.7678, 94, (20 $$\overline{2 }$$ 2 ¯ ); 2.9533, 51, (40 $$\overline{2 }$$ 2 ¯ ); 4.7030, 37, (1 $$\overline{1 }\,\overline{1 }$$ 1 ¯ 1 ¯ ); 3.9266, 33, (002); 3.5686, 27, (111); 2.6574, 22, (1 $$\overline{1 }\,\overline{3 }$$ 1 ¯ 3 ¯ ); 3.5992, 8, (1 $$\overline{1 }\,\overline{2 }$$ 1 ¯ 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(C 2 O 4 )·2H 2 O is characterized by chains consisting of isolated ZnO 6 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 H 2 O molecule) donor–acceptor distances of ~ 2.8 Å.