FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. XXXI. REFINEMENT OF THE CRYSTAL STRUCTURE AND CHEMICAL FORMULA OF SELIVANOVAITE, NaFe3+Ti4(Si2O7)2O4(H2O)4, A MURMANITE-GROUP (SEIDOZERITE SUPERGROUP) TS-BLOCK MINERAL FROM THE LOVOZERO MASSIF, KOLA PENINSULA, RUSSIA

Selivanovaite, ideally NaFe3+Ti4(Si2O7)(2)O-4(H2O)(4), is a murmanite-group (seidozerite supergroup) TS-block mineral from the Lovozero massif, Kola Peninsula, Russia. The crystal structure of selivanovaite was refined in space group C (1) over bar, a 10.524(6), b 13.815(6), c 12.213(14) angstrom, alpha 99.74(6), beta 107.45(8), gamma 90.15(10)degrees, V 1666.8(26) angstrom(3), R-1 = 21.40%. The previously given chemical analysis has been modified to better fit the crystal structure: Nb2O5 8.51, ZrO2 2.94, TiO2 31.96, SiO2 30.62, Al2O3 0.05, Fe2O3 5.08, FeO 3.23, MnO 3.36, CaO 2.14, MgO 0.75, Na2O 2.47, H2O 8.88, sum 99.99 wt.%; H2O was determined from structure-refinement results: H2O = 3.34 pfu, OH = 1.05 pfu. The empirical formula calculated on 22 O apfu is: (Na0.63Ca0.30Mn0.36)(Sigma 1.29)(Fe0.503+Fe0.352+)(Sigma 0.85)(Ti3.14Nb0.50Zr0.19Mg0.15Mn0.01Al0.01)(Sigma 4.00)Si3.99O22.00H7.73, Z = 4. The crystal structure of selivanovaite [basic structure type B1(MG)] is an array of TS blocks (Titanium Silicate) connected via hydrogen bonds. The TS block consists of HOH sheets (H = heteropolyhedral, O = octahedral) parallel to (001). In the O sheet, the Ti-dominant M-O (1,2) sites, Na-dominant M-O (3) site, and square-dominant MO (4) sites give ideally Na square Ti-2 pfu. In the H sheet, the Ti-dominant M-H (1,2) sites, Fe3+ -dominant A(P) (1) site, and vacant A(P) (2) sites give ideally Fe3+square Ti-2 pfu. The M-H and A(P) (1) polyhedra and Si2O7 groups constitute the H sheet. The ideal structural formula of selivanovaite of the form A(2)(P)M(2)(H)M(4)(O)(Si2O7)(2)(X-M,A(O))(4)(X-M,A(P))(4) is Fe3+square Ti2Na square Ti-2(Si2O7)(2)O-4(H2O)(4). Selivanovaite is a Fe3+ -bearing and OH-poor analogue of vigrishinite, ideally Zn square Ti2Na square Ti-2(Si2O7)(2)O2O(OH)(H2O)(4). Vigrishinite and selivanovaite are related by the following substitution: (O)(OH)(vig)(-)+ (H)(Zn2+)(vig) <-> (O)(O2-)(sel) + (H)(Fe3+)(sel). Selivanovaite is a Fe3+ -bearing and Na-poor analogue of murmanite, ideally Na2Ti2Na2Ti2(Si2O7)(2)O-4(H2O)(4). Murmanite and selivanovaite are related by the following substitution: (O)(Na+)(mur)+ (H)(Na-2(+))(mur) <-> (O)(square)(sel) + (H)(Fe3+)(sel)+ (H)(square)(sel). The doubling of the t(1) and t(2) translations of selivanovaite compared to those of murmanite is due to the ordering of Fe3+ and square in the H sheet and Na and square in the O sheet of selivanovaite.