ZVYAGINITE NaZnNb <sub>2</sub>Ti[Si <sub>2</sub>O <sub>7</sub>] <sub>2</sub>O(OH,F) <sub>3</sub>(H <sub>2</sub>O) <sub>4+x</sub> (x < 1) - A NEW MINERAL OF EPISTOLITE GROUP FROM LOVOZERO ALKALINE MASSIF (KOLA PENINSULA, RUSSIA)
A new mineral zvyaginite, epistolite-group member, is found at Mt. Malyi Punkaraaiv, Lovozero alkaline complex, Kola Peninsula, Russia. It occurs in a hydrothermally altered peralkaline pegmatite and is associated with ussingite, microcline, aegirine, sphalerite, vigrishinite, sauconite, etc. Zvyaginite forms rectangular or irregularly shaped lamellae up to 0.1 X 1 X 2 cm flattened on [001]. The mineral is translucent to transparent and colourless, pearly-white, yellowish-brownish, pale pink or lilac-pink. The lustre is nacreous on crystal faces and greasy on broken surface. Mohs' hardness is 21/2-3. Zvyaginite is brittle. Cleavage is {001} perfect. D <sub>meas</sub> = 2.88(3), D <sub>calc</sub> = 2.94 g/cm <sup>3</sup>. The mineral is optically biaxial (-), α = 1.626(5), β = 1.714(3), γ = 1.740(5), 2V <sub>meas</sub> = 45(15)°, 2V <sub>calc</sub> = 55°. IR spectrum is given. Chemical composition (wt %; averaged data of 5 electron-microprobe analyses, H <sub>2</sub>O - by modified Penfield method) is: Na <sub>2</sub>O 4.74, K <sub>2</sub>O 0.22, CaO 0.77, MnO 1.36, FeO 0.24, ZnO 9.61, Al <sub>2</sub>O <sub>3</sub> 0.19, SiO <sub>2</sub> 29.42, TiO <sub>2</sub> 12.33, Nb <sub>2</sub>O <sub>5</sub> 27.22, F 1.94, H <sub>2</sub>O 12.65, -O = F <sub>2</sub> -0.82, total 99.87. The empirical formula calculated on the basis of Si+Al = 4 is: Na <sub>1.24</sub>K <sub>0.04</sub>Ca <sub>0.11</sub>Mn <sub>0.16</sub>Fe <sub>0.03</sub>Zn <sub>0.96</sub>Nb <sub>1.66</sub>Ti <sub>1.25</sub>(Si <sub>3.97</sub>Al <sub>0.03</sub>) <sub>Σ4</sub>O <sub>15.07 </sub>(OH) <sub>2.10</sub>F <sub>0.83</sub>(H <sub>2</sub>O) <sub>4.64</sub>. The simplified formula is: NaZnNb <sub>2</sub>Ti[Si <sub>2</sub>O <sub>7</sub>] <sub>2</sub>O(OH,F) <sub>3</sub>(H <sub>2</sub>O) <sub>4+x</sub> (x < 1). Zvyaginite is triclinic, P1, <i>a</i> = 8.975(3), <i>b</i> = 8.979(3), <i>c</i> = 12.135(4) Å, α = 74.328(9)°, β = 80.651(8)°, γ = 73.959(8)°, V = 900.8(6) Å <sup>3</sup>, Z = 2. The strongest reflections of the X-ray powder pattern ( <i>d</i>,Å-I[ <i>hkl</i>]) 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 crystal structure model was obtained on a single crystal, R = 0.159. Zvyaginite and epistolite are close in the structure of the NbTiSiO motif but different from one another in composition of the O layer in the HOH block. Zvyaginite is named in honor of Russian crystallographer, crystal chemist and physicist Boris B. Zvyagin (1921-2002). The type specimen is deposited in the Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow.