Original paper

Potassicarfvedsonite, KNa2 Fe2+4Fe3+Si8O22(OH)2, a K-dominant amphibole of the arfvedsonite series from agpaitic pegmatites - Mineral data, structure refinement and disorder in the A site

Pekov, I.V.; Chukanov, N.V.; Lebedeva, Yu. S.; Pushcharovsky, D. Yu.; Ferraris, G.; Gula, A.; Zadov, A.E.; Novakova, A.A.; Petersen, O.V.


Potassicarfvedsonite, a new mineral species of the amphibole group, was found in alkaline pegmatites of three agpaitic complexes: Ilímaussaq in South Greenland (holotype), and Lovozero and Khibiny, both in Kola Peninsula, Russia (cotypes). It is associated with: analcime, sodalite, aegirine, steenstrupine-(Ce), epistolite (Ilímaussaq); 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. Dmeas is 3.39(2), Dcalc is 3.43 g/cm3. Optically biaxial ( – ), 2Vmeas >60°, 2Vcalc =82°, α =1.683(2), β =1.692(2), γ =1.699(2). The dispersion r >v 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 ∧ c =20(10)°; Y ∧ a =30(10)°. IR and Mössbauer data are given. Electron micro- probe, Li by atom emission and H2O by modified Penfield method gave (sequence: Ilímaussaq, 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; TiO20.43, 0.77, 0.74; H2O 1.70, 1.38 (calc.), 1.20 (calc.); F 0.36, 1.21, 1.59; –O= F20.15, 0.51, 0.67; Total 100.50, 98.90, 99.06 wt. %. The empirical formula of the Ilímaussaq holotype based on 46 units of charge is:

(K0.67 Na0.22 )∑0.89(Na1.95 Ca0.05 )∑2.00(Fe2+ 3.29Fe3+ 1.26Li 0.29Mn 0.19Ti 0.05Zn 0.02Mg 0.01)∑ 5.11(Si7.76 Fe3+ 0.13Al 0.11)∑ 8.00O22[(OH) 1.81F 0.18]∑ 1.99; the ideal formula is: KNa 2Fe2+ 4Fe3+ Si 8O 22(OH) 2. The strongest lines of the X-ray powder pattern [ d in Å, ( 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, C 2/ m ; a= 10.002(2), b= 18.054(3), c= 5.319(1) Å,β = 103.90(3)°, V= 932.4(3) Å3, Z= 2. The crystal structure (Nonius Kappa CCD,λ MoKα radiation) was anisotropically refined to R (F)= 0.0229 for 3296 independent reflections with IO >2 σ(IO). 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 Mössbauer spectroscopy that indicates Fe2+ /Fe3+ =2.61. The mineral was named as potassium-dominant analogue of arfvedsonite. Mineral and name have been approved by the IMA CNMMN. Type specimens are deposited at the Fersman Mineralogical Museum (Moscow).


potassicarfvedsonitenew mineral speciesamphibole groupcrystal structurealkaline pegmatitesilimaussaq complexlovozero massifkhibiny massif