Potassicarfvedsonite, a new mineral species of the amphibole group, was found in alkaline pegmatites of three agpaitic complexes: IIimaussaq in South Greenland (holotype), and Lovozero and Khibiny, both in Kola Peninsula. Russia (cotypes). It is associated with: analcime, sodalite, aegirine, steenstrupine-(Ce), epistolite (Ilimaussaq); sodalite. ussingite, analcime, aegirine, lomonosovite, vuonnemite, serandite. manaksite, villiaumite, natrosilite. sphalerite. etc. (Lovozero): microcline, nepheline, aegirine, sodalite, lamprophyllite, eudialyte, rinkite, titanite, fluorapatite, etc. (Khibiny). Potassicarfvedsonite forms prismatic to acicular crystals up to 15 cm long. Semitransparent to transparent: black. dark blue-green or blue-grey. The streak is pale greenish-blue and the lustre vitreous. Potassicarfvedsonite shows a {110} perfect cleavage, the fracture is stepped and splintery; it is brittle. Mohs' hardness = 5.5-6. D-meas 3.39(2), D-calc is 3.43 g/cm(3). Optically biaxial (-), 2V(meas) > 60degrees, 2V(calc) = 82degrees, alpha = 1.683(2), beta = 1.692(2), gamma = 1.699(2). The dispersion r > nu is strong. The pleochroism is strong with X dark blueish-green, Y greyish-green, Z pale greenish-grey to pale greenish-brown. Orientation: Z = b; X boolean AND c = 20(10)degrees; Y boolean AND a = 30(10)degrees. IR and Mbssbauer data are given. Electron nucroprobe, Li by atom emission and H2O by modified Penfield method gave (sequence: Ilimaussaq, Lovozero, Khibiny): Li2O 0.45, 0.73, not detected; Na2O 7.01, 7.65, 6.94; K2O 3.29, 3.76, 3.41; CaO 0.30, 0.09, 1.63; MgO 0.04, 4.80, 7.79: MnO 1.37, 3.26, 1.88; ZnO 0.20, 0.11, 0.06; FeO 24.69, 13.82, 17.53; Fe2O3 11.60, 8.50, 4.74; Al2O3 0.58, 0.51, 0.58; SiO2 48.63, 52.82, 51.64; TiO2 0.43, 0.77, 0.74; H2O 1.70, 1.38 (calc.), 1.20 (calc.); F 0.36, 1.21, 1.59; -O = F-2 0.15, 0.51, 0.67; Total 100.50, 98.90, 99.06 wt. %. The empirical formula of the Ilimaussaq holotype based on 46 units of charge is: (K0.67Na0.22)Sigma(0.89)(Na1.95Ca0.05)Sigma(2.00)(Fe2+ Fe-3.29(3+) 1.26Li0.29Mn0.19Ti0.05Zn0.02Mg0.01)(Sigma5.11) (Si7.76Fe3+ Al-0.13(0.11))(Sigma8.00)O-22[(OH)(1.81)F-0.18](Sigma1.99); the ideal formula is: KNa2Fe2+ Fe-4(3+) Si8O22(OH)(2). The strongest lines of the X-ray powder pattern [d in Angstrom, (I)(hkl)] are: 9.02(28)(020), 8.53(100)(110), 3.303(23)(240); 3.184(40)(310), 2.847(17)(330). Potassicarfvedsonite is monoclinic, C2/m; a = 10.002(2), b = 18.054(3), c = 5.319(1) Angstrom, beta = 103.90(3)degrees, V = 932.4(3) Angstrom(3), Z = 2. The crystal structure (Nonius Kappa CCD, lambdaMoK(alpha) radiation) was anisotropically refined to R(F) = 0.0229 for 3296 independent reflections with I-O > 2sigma(I-O). The distribution of the cations over the five independent positions M1, M2, M3, M4 and A (split into three subsites) was obtained on the basis of X-ray diffraction analysis and Mossbauer spectroscopy that indicates Fe2+/Fe3+ = 2.61. The mineral was named as potassium-dominant analogue of artvedsonite. Mineral and name have been approved by the IMA CNMMN. Type specimens are deposited at the Fersman Mineralogical Museum (Moscow).