GUTKOVAITE-Mn, CaK_2Mn(Ti,Nb)_4(Si_4O_(12))_2(O,OH)_4 centre dot 5H_2O, A NEW MINERAL OF THE LABUNTSOVITE GROUP FROM KHIBINY MASSIF, KOLA PENINSULA

摘要

Gutkovaite-Mn, a new mineral of the labuntsovite group, has been found at Maly Mannepakhk Mt., Khibiny alkaline massif, Kola peninsula, Russia. It occurs in pegmatite among nepheline syenite, with microcline, aegirine, arfvedsonite, nepheline, eudialyte, albite, lorenzenite, loparite, aenigmatite, mangan-neptunite, murmanite, analcime, natrolite, stilbite, chabazite, kuzmenkoite-Mn, nontronite, etc. it forms coarse-prismatic crystals up to 0.8mm in lenght. Translucent to transparent; pale yellowish-pink; streak white; luster vitreous. Brittle, cleavage not observed, fracture uneven. Mohs' hardness approx 5. D_(meas) 2.83, D_(calc) 2.70 g/cm~3. Optically biaxial, positive; alpha=1.688, beta=1.700, gamma=1.805; 2V_(meas)=35(10 deg). Chemical composition (microprobe data), wt percent: Na_2O 0.18, K_2O 6.54, CaO 4.81 SrO 0.54, BaO 1.03, MgO 0.17, MnO 4.96, FeO 0.61, ZnO 0.13, Al_2O_3 0.17, SiO_2 41.03, ZrO_2 0.08, TiO_2 24.47, Nb_2O_5 5.14, H_2O (calc.) 8.67. Total 98.53. Empirical formula calculated by [(Si,Al)_(16)O_(48)](O,OH)_8 is: (Ca_(2.00)Na_(0.14) sum_(2.14)(K_(3.24)Ba_(0.16)Sr_(0.12)sum_(3.52)(Mn_(1.63)~(2+)Fe_(0.20)Mg_(0.10)Zn_(0.04)sum_(1.97)(Ti_(7.14)Nb_(0.90)Zr_(0.02)sum_(8.06)[(Si_(15.92)Al_(0.08)sum_(16)O_(48))[O_(4.94)(OH)_(3.06)]sum_8 centre dot 9.7H_2O. Idealized formula is: CaK_2Mn(TiNb)_4(Si_4O_(12)_2(O,OH)_4 centre dot 5H_2O (Z=2). Crystal structure is studied; R=0.058. Monoclinic, space group Cm. Microtwinning by planes (001) and (-401) is observed. Unit cell parameters: a=14.30, b=13.889, c=7.760A, beta=117.51 deg, V=1367 A~3. Structurally related to labuntsovite but A-site occupied with Na in labuntsovite, has been subdivided into two sites in gutkovaite-Mn: A(I)-site is occupied with Ca selectively, A(II)-site is vacant. It's a cause of summetry lowering from C2/m (labuntsovite) to Cm (gutkovaite). Strongest reflexes of X-ray powder pattern are (d, A-I[hkl]): 7.0-7[020,001], 6.33-5[20-1,200], 4.90-4[021], 3.22-9[42-1, 40-2, 400], 3.05-10[022,24-1,240], 2.57-5[24-2,241], 2.48-6[44-1,40-3,401]. IR spectrum is given. Gutkovaite-Mn is named in the memory of Russian mineralogist N.N. Gutkova [1896-1960(?)] who intensively studied Khibiny-Lovozero alkaline complex, and Mn prevailing in D-site. Type specimen is deposited in Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow.