Potassicarfvedsonite, Kna_2Fe~(2+)_ (4)Fe~(3+)Si_8O_(22)(OH)_2, a K-dominant amphibole of the arfvedso-nite series from agpaitic pegmatites - Mineral data, structure refinement and disorder in the A site. With 3 figures and 7 tables

摘要

Potassicarfvedsonite, a new mineral species of the amphibole group, was found in alkaline pegmatites of three agpaitic complexes: Ilimaussaq 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, vuon-nemite, 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) is 3.39(2), D_(calc) is 3.43 g/cm~3. Optically biaxial (-), 2V_(meas) >60 deg, 2V_(calc) = 82 deg, alpha = 1.683(2), beta = 1.692(2), gamma = 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) deg; Y ∧ a = 30(10) deg. IR and Mflssbauer data are given. Electron micro-probe, Li by atom emission and H_2O by modified Penfield method gave (sequence: Ilimaussaq, Lovozero, Khibiny): Li_2O 0.45, 0.73, not detected; Na_2O 7.01, 7.65, 6.94; K_2O 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; Fe_2O_3 11.60, 8.50, 4.74; A1_2O_3 0.58, 0.51, 0.58; SiO_2 48.63, 52.82, 51.64; TiO_2 0.43, 0.77, 0.74; H_2O 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. percent. The empirical formula of the Ilimaussaq holotype based on 46 units of charge is: (K_(0.67)Na_(0.22))_(SIGMA0.89)(Na_(1.95)Ca_(0.05)_( SIGMA2.00)(Fe~(2+)_(3.29)Fe_(3+)_(1.26)Li_(0.29)Mn_(0.19)Ti_(0.05)Zn_(0.02)Mg_(0.01)_( SIGMA5.11) (Si_(7.76)Fe~(3+)_(0.13)Al_( SIGMA8.00))O_(22)[(OH)_(1.81)F_(0.18)]_( SIGMA1.99); the ideal formula is: KNa_2Fe~(2+)_4Fe~(3+) Si_8O_(22)(OH)_2. The strongest lines of the X-ray powder pattern [d in A, (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) A, beta = 103.90(3) de , V = 932.4(3) A~3, Z = 2. The crystal structure (Nonius Kappa CCD, lambde MoK_(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 Ml, M2, M3, M4 and A (split into three subsites) was obtained on the basis of X-ray diffraction analysis and Mossbauer spectroscopy that indicates Fe~(2+)/Fe~(3+) = 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).