IKRANITE, (Na,H_3O)_(15)(Ca,Mn,REE)_6Fe_2~(3+)Zr_3 (square,Zr)(square Si)Si_(24)O_(66)(O,OH)_6Cl centre dot nH_2O AND RASLAKITE Na_(15)Ca_3Fe_3(Na,Zr)_3Zr_3(Si,Nb) (Si_(25)O_(73))(OH,H_2O)_3(Cl,OH) -- THE NEW EUDIALYTE-GROUP MINERALS FROM LOVOZERO MA

摘要

Ikranite and raslakite were discovered in two peralkaline pegmatites at the Karnasurt Mt of Lovozero alkaline massif, in associations with aegirine, microcline, lorenzenite nepheline, lamprophyllite, murmanite, arfvedsonite (for ikranite) and with microcline, aegirine, nepheline, lamprophyllite, kazakovite, terskite, fluorcaphite (for raslakite). The new minerals form brownish-yellow (ikranite) or brownish-red (raslakite) grains up to 2--3 cm in Size. Brittle, cleavage non-perfect, Mohs' hardness 5. Both are optically positive. Ikranite is often weakly biaxial, raslakite is uniaxial: omega = 1.612 (1) and 1.608; epsilon = 1.615 and 1.611;D_(meas) = 2.82 (3) and 2.95 (1) g/cm~3, D_(calc) = 2.83 and 2.945 g/cm~3, respectively. Both minerals are trigonal, space groups R 3 m and R 3, a = 14.167 (2) and 14.229 (7), c = 30.081(2) and c = 30.019 (15) A; V = 5228.5 and 5263.5 A respectively; for both R = 0.037. Ikranite differs from all other eudialyte-group minerals by the prevailing of vacancies in the sites M (3) and M (4) at the centers of the 9-menibered Si-rings and by the strong predominance of ~([6])Fe~(3+) over ~([4])Fe~(2+) in the M (2) position linking 6-mem-bered Ca-rings. In ikranite Na is partly substituted by H_3O; in raslakite M (1) is transformed into two independent sites M (1,1) and M (1,2) occupied mainly by Fe and Ca, respectively.The position M (2) is statistically occupied by ~([5])Na, ~([5])Mn and ~([4])Zr. These structural features are in good agreement with the data of IR and Mossbauer spectroscopic data. The strongest lines of the powder diffraction pattern [d, A (I, percent) (hkl)] are -- ikranite: 6.41(41) (104) 4.30 (91) (205), 3.521 (57) (027), 3.205 (44) (208), 2.963 (92) (315, 13-5), 2.841 (100) (404); raslakite: 4.311 (66) (205), 4.095 (37) (116, 11-6), 3.209 (58) (208), 3.023 (40) (119, 11-9), 2.974 (86) (315, 13-5), 2.853 (100) (404). Chemical compositions of ikranite and raslakite (wt. percent, electron probe, H_2O content -- by thermogravimetric method, for Fe in ikranite -- wet analysis) correspond to the empirical formulae (Z = 3): Na_(7.56)(H_3O)_(6.64)K_(0.27)Ca_(3.31) Sr_(0.46)Ce_(0.27)La_(0.11)Mn_(1.14)~(2+)Fe_(0.16)~(2+)Fe_(1.77)~(3+)Zr_(3.33)Ti_(0.14)Hf_(0.04)Nb_(0.06)Si_(24)O_(72)Cl_(0.74) centre dot 2.64 H_2O (ikranite); Nat_(16.02)K_(0.32)Ca_(3.13) Sr_(0.21)Mg_(0.22) Fe_(2.17)Mn_(0.88)Ce_(0.16)La_(0.08)Ti_(0.14)Zr_(3.80) Hf_(0.06)Nb_(0.17)Al_(0.16)Si_(25.40)Cl_(1.18) H_(4.66)O_(76.465) (raslakite). The minerals were named for Russian akronym IKRAN (Institut Kristallografii Rossiiskoy Adademii Nauky) and from Raslak Cirques near Kedykverpakhk Mt. Their type materials are deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.