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The new mineral agricolaite, a potassium uranyl carbonate with ideal formula K(UO)(CO), occurs in vugs of ankerite gangue in gneisses in the abandoned Giftkiesstollen adit at Jáchymov, Czech Republic. The name is after Georgius Agricola (1494-1555), German scholar and scientist. Agricolaite occurs as isolated equant irregular translucent grains to 0.3 mm with yellow color, pale yellow streak, and vitreous luster. It is brittle with uneven fracture and displays neither cleavage nor parting. Agricolaite is non-fluorescent. Mohs hardness is ~4. It is associated with aragonite, brochantite, posnjakite, malachite, rutherfordine, and 'pseudo-voglite'. Experimental density is higher than 3.3 g.cm, Dcalc is 3.531 g. cm. The mineral is monoclinic, space group C2/ c, with a 10.2380(2), b 9.1930(2), c 12.2110(3) Å, β 95.108(2)°, V 1144.71(4) Å, Z = 4. The strongest lines in the powder X-ray diffraction pattern are d( I)( hkl): 6.061(55)(002), 5.087(57)(200), 3.740(100)(202), 3.393(43)(113), 2.281(52)(402). Average composition based on ten electron microprobe analyses corresponds to (in wt.%) UO 48.53, KO 31.49, CO(calc) 22.04 which gives the empirical formula K(UO)(CO). The crystal structure was solved from single-crystal X-ray diffraction data and refined to R = 0.0184 on the basis of the 1,308 unique reflections with F > 4 σF. The structure of agricolaite is identical to that of synthetic K(UO)(CO) and consists of separate UO(CO) groups organized into layers parallel to (100) and two crystallographically non-equivalent sites occupied by K cations. Both the mineral and its name were approved by the IMA-CNMNC. [ABSTRACT FROM AUTHOR]