[en] Magnetic and crystallographic properties of the mineral langite Cu₄(OH)₆SO${}_4\cdot <!--\; ???\; -->2$H₂O are reported. Thermodynamic measurements combined with a microscopic analysis, based on density-functional bandstructure calculations, identify a quasi-two-dimensional (2D), partially frustrated spin-1/2 lattice resulting in the low Néel temperature of $T_{\mathrm{N}}\simeq <!--\; ???\; -->5.7$ K. This spin lattice splits into two parts with predominant ferro- and antiferromagnetic (AFM) exchange couplings, respectively. The former, ferromagnetic (FM) part is prone to the long-range magnetic order and saturates around 12 T, where the magnetization reaches 0.5 ${\mathrm{\mu <!--\; ??\; -->}}_{\mathrm{B}}$/Cu. The latter, AFM part features a spin-ladder geometry and should evade long-range magnetic order. This representation is corroborated by the peculiar temperature dependence of the specific heat in the magnetically ordered state. We argue that this separation into ferro- and antiferromagnetic sublattices is generic for quantum magnets in Cu²⁺ oxides that combine different flavors of structural chains built of CuO₄ units. To start from reliable structural data, the crystal structure of langite in the 100–280 K temperature range has been determined by single-crystal x-ray diffraction, and the hydrogen positions were refined computationally. (paper)