Abstract

The crystal structures of lomonosovite, ideally Na10Ti4(Si2O7)2(PO4)2 O4, a = 5.4170(7) Å, b = 7.1190(9) Å, c = 14.487(2) Å, α = 99.957(3)°, β = 96.711(3)°, γ = 90.360(3)°, V = 546.28(4) Å3, Dcalc. = 3.175 g cm-3, and murmanite, ideally Na4Ti4(Si2O7)2O4(H2O)4, a = 5.3875(6) Å, b = 7.0579(7) Å, c = 12.176(1) Å, α = 93.511(2)°, β = 107.943(4)°, γ = 90.093(2)°, V = 439.55(2) Å3, Dcalc. = 2.956 g.cm-3, from the Lovozero alkaline massif, Kola Peninsula, Russia, have been refined in the space group P

\({\bar{1}}\)
(Z = 1) to R values of 2.64 and 4.47%, respectively, using 4572 and 2222 observed |Fo ≥ 4σF| reflections collected with a single-crystal Bruker AXS SMART APEX diffractometer with a CCD detector and Mo-Kα radiation. Electron microprobe analysis gave empirical formulae for lomonosovite
\((\mathrm{Na}_{9.50}\mathrm{Mn}_{0.16}\mathrm{Ca}_{0.11})_{{{\sum}}9.77}(\mathrm{Ti}_{2.83}^{4+}\mathrm{Nb}_{0.51}\mathrm{Mn}_{0.27}^{2+}\mathrm{Zr}_{0.11}\mathrm{Mg}_{0.11}\mathrm{Fe}_{0.10}^{2+}\mathrm{Fe}_{0.06}^{3+}\mathrm{Ta}_{0.01})_{{{\sum}}4.00}(\mathrm{Si}_{2.02}\mathrm{O}_{7})_{2}(\mathrm{P}_{0.98}\mathrm{O}_{4})_{2}(\mathrm{O}_{3.50}\mathrm{F}_{0.50})_{{{\sum}}4},{\ }Z{\ }={\ }1\)
, calculated on the basis of 26(O+F) a.p.f.u., and murmanite
\((\mathrm{Na}_{3.32}\mathrm{Mn}_{0.15}\mathrm{Ca}_{0.21}\mathrm{K}_{0.05})_{{{\sum}}3.73}(\mathrm{Ti}_{3.08}^{4+}\mathrm{Nb}_{0.51}\mathrm{Mn}_{0.18}^{2+}\mathrm{Fe}_{0.15}^{3+}\mathrm{Mg}_{0.07}\mathrm{Zr}_{0.01})_{{{\sum}}4.00}(\mathrm{Si}_{1.98}\mathrm{O}_{7})_{2}(\mathrm{O}_{3.76}\mathrm{F}_{0.24})_{{{\sum}}4}(\mathrm{H}_{2}\mathrm{O})_{4}\)
, Z = 1, calculated on the basis of 22(O+F) a.p.f.u., with H2O determined from structure refinement and Fe3+/(Fe2++Fe3+) ratios obtained by Mössbauer spectroscopy. The crystal structures of lomonosovite and murmanite are a combination of a titanium silicate (TS) block and an intermediate (I) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral), and is characterized by a planar cell based on translation vectors, t1 and t2, with t1 ∼5.5 and t2 ∼7 Å and t1 ^ t2 close to 90°. The TS block exhibits linkage and stereochemistry typical for Group IV (Ti = 4 a.p.f.u.) of the Ti disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to Ti polyhedra of the O sheet adjacent along t1. In murmanite and lomonosovite, the invariant part of the TS block is of composition Na4Ti4(Si2O7)2O4. There is no evidence of vacancy-dominant cation sites or (OH) groups in the O sheet of lomonosovite or murmanite. In lomonosovite, the I block is a framework of Na polyhedra and P tetrahedra which gives 2[Na3 (PO)4] p.f.u. In murmanite, there are four (H2O) groups in the intermediate space between TS blocks. In lomonosovite, TS and I blocks alternate along c. In murmanite, TS blocks are connected via hydrogen bonding. The H atoms were located and details of the hydrogen bonding are discussed.

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