Krupičkaite, Cu6[AsO3(OH)]6·8H2O, a new copper arsenate mineral from Jáchymov (Czech Republic)

Krupičkaite, ideally Cu6[AsO3(OH)]6·8H2O, is a new supergene mineral from the Rovnost I shaft in Jáchymov, Czech Republic. It forms aggregates of pale greenish-blue color and grows along with supergene minerals crystallizing on the strongly altered relics of massive tennantite, Bi-rich tennantite, galena, chalcopyrite, bornite, and chalcocite with disseminated uraninite in quartz. For a long time, krupičkaite has been left out due to its quite inconspicuous ap pearance that can be mistakenly referred to as geminite. At the ambient temperature, krupičkaite is monoclinic, a = 15.504(7) Å, b = 18.144(7) Å, c = 10.563(5) Å, β = 103.30(4)°, V = 2891.5(2) Å3, Z = 4, space group P21/m. Its structure has been solved and refined from 3D electron diffraction and further studied by Raman spectroscopy. The layered structure is built upon the alternation of two different copper-arsenate sheets stacked along b presenting a characteristic wave shape along the a–axis and separated by a thick interlayer with channels containing only H2O. The collapsed chains of copper polyhedra are connected the same way as in geminite through AsO4 tetrahedra. Krupičkaite joins the family of copper arsenate minerals with which it shares structural similarities at the level of the As-Cu layers with the lindackerite supergroup, slavkovite, or yvonite.