PARAKUZMENKOITE-Fe, (K,Ba)_2Fe(Ti,Nb)_4[(Si_4O_(12))_2(O,OH)_4 CENTRE DOT 7H_2O - A NEW LABUNTSOVITE GROUP MINERAL

摘要

A new labuntsovite group mineral, parakuzmenkoite-Fe, has been found together with microcline, aegirine, eudialyte, lorenzenite, sodalite, natrolite, elpidite, rancieite, and halloysite in alkaline pegmatite at Kedykverpakhk Mt., Lovozero Massif, Kola Peninsula, Russia. It occurs as prismatic crystals up to 1 mm in size. Parakuzmenkoite-Fe is translucent, orange to reddish-orange. Luster vitreous, streak white, cleavage non-perfect, Mohs' hardness 5. Biaxial, positive, 2V = 22(10) deg, alpha 1.687 (1), beta 1.689 (2), gamma 1.805 (5). The crystral structure has been studied (R=0.047). Monoclinic, space group C2/m, a=14.410 (2), b=13.880 (2), c=15.587 (2) A, beta=117.53 (1) deg, V=2764.6 (8) A~3. The strongest lines of the powder diffraction patter [d, A (I, percent) (hkl)] are: 6.91 (10)(021, 002); 3.19(10)(422, 400, 401); 3.09(10)(042, 024); 1.524(9)(480, 484, 426, 4210); 1.422(8)(482, 486, 446, 864, 4.410). D_(meas)=3.00 (3), D_(calc)=3.07 (3) g/cm~3. Chemical composition is (wt. percent, electron probe, mean over 7 analyses; H_2O content was determined by thermogravimetrical method): Na_2O 0.43; K_2O 2.86; CaO 0.24; SrO 0.45; BaO 8.76; MgO 0.14; MnO 1.79; FeO 4.86; ZnO 0.21; SiO_2 37.35; TiO_2 13.74; Nb_2O_5 17.59 H_2O 10.94; total 99.36. The empirical formula (taking into consideration structural data) is: (K_(1.56)Na_(0.36)Ba_(1.47)Sr_(0.11)Ca_(0.11)Zn_(0.07))_(sum 3.68) (Fe_(1.58)~(2+)Mn_(0.65)Mg_(0.09))_(sum 2.32)(Ti_(4.44)b_(3.41)Fe_(0.16)~(3+)) (sum 8.01)[Si_4O_(12)]_4[O_(6.40)(OH)_(1.60)] centre dot 14.29H_2O. Simplified formula: (K, Ba)_2Fe(Ti,Nb)_4[Si_4O_(12)]_2(O,OH)_4 centre dot 7H_2O (Z=4). IR spectrum is given. Parakuzmenkoite-Fe is named as an analog of kuzmenkoite-Mn with Fe>Mn in the D site and doubled c parameter, also isostructural with organovaite-Mn and organovaite-Zn. Type material is deposited at Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.