Zvyaginite, NaZnNb2Ti[Si2O7]2O(OH,F)3(H2O)4 + x (x < 1), a new mineral of the epistolite group from the Lovozero Alkaline Pluton, Kola Peninsula, Russia

Abstract

A new mineral, zvyaginite, a member of the epistolite group, has been found at Mt. Malyi Punkaruaiv, Lovozero Alkaline Complex, Kola Peninsula, Russia. It occurs in a hydrothermally altered peralkaline pegmatite and is associated with ussingite, microcline, aegirine, sphalerite, vigrishinite, and sauconite. Zvyaginite forms rectangular or irregular-shaped lamellae up to 0.1 × 1 × 2 cm in size when flattened [001]. The mineral is translucent to transparent and colorless, pearly-white, yellowish brownish, pale pink, or violet pink. The luster is nacreous on crystal faces and greasy on broken surfaces. Its Mohs’ hardness is 2.5–3. Zvyaginite is brittle. The cleavage parallel to {001} is perfect. Dmeas = 2.88(3), Dcalc = 2.94 g/cm3. The mineral is optically biaxial (−), α = 1.626(5), β = 1.714(3), γ = 1.740(5), 2Vmeas = 45(15)°, 2Vcalc = 55°. The IR spectrum is given. Chemical composition is as follows (wt %; average of five point analyses; H2O was determined using the modified Penfield method): 4.74 Na2O, 0.22 K2O, 0.77 CaO, 1.36 MnO, 0.24 FeO, 9.61 ZnO, 0.19 Al2O3, 29.42 SiO2, 12.33 TiO2, 27.22 Nb2O5, 1.94 F, 12.65 H2O, −0.82 −O = F2, for a total of 99.87. The empirical formula calculated on the basis of Si + Al = 4 is: Na1.24K0.04Ca0.11Mn0.16Fe0.03Zn0.96Nb1.66Ti1.25(Si3.97Al0.03)Σ4O15.07(OH)2.10F0.83(H2O)4.64. The simplified formula is: NaZnNb2Ti[Si2O7]2O(OH,F)3(H2O)4 + x (x < 1), Zvyaginite is triclinic, \(P\bar 1\), a = 8.975(3), b = 8.979(3), c = 12.135(4) A, α = 74.328(9)°, β = 80.651(8)°, γ = 73.959(8)°, V = 900.8(6) A3, Z = 2. The strongest reflections in the X-ray powder pattern (d, A-I[hkl]) are: 11.72–100[001], 5.83–40[002], 5.28–53[−1–11, 112], 4.289–86[200, 021], 3.896–36[−1–12, −201, 003, 022, 113], 2.916–57[310, 132, 004], 2.862–72[130, 312]. The model of the crystal structure was obtained on a single crystal, R = 0.159. Zvyaginite and epistolite are similar in the structure of the NbTiSiO motif, but differ from each other in composition of O layer in HOH block. Zvyaginite is named in honor of Boris B. Zvyagin (1921–2002), Russian crystallographer, crystal chemist and physicist. The type specimen is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.