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THE CRYSTAL STRUCTURE OF SODIUM TETRA-SILICIC FLUORO-MICA, NaLiMg2Si4O10F2
2020 年 24 巻 1 号 p. 15-21
詳細
2020 年 24 巻 1 号 p. 15-21
The crystal structure of a swellable sodium tetra-silicic fluoro-mica [NaLiMg2Si4O10F2], “Na-TN,” was determined and refined by means of XRD data collected with a single crystal in a dry N2 gas flow. Na-TN is in triclinic space group C1̄, and has unit cell parameters: a=5.2204(10), b=9.0812(17), c=9.652(2) Å, α=91.202(7), β=95.905(7), γ=90.037(7)°, and V=455.06(16) Å3. The structural formula, Na0.874(Mg0.706Li0.294)2(Mg0.715Li0.285)Si4O10F2, suggests an isomorphous substitution of Li++Na+→Mg2++□ forming solid solution series from the ideal composition [NaLiMg2Si4O10F2] toward the dehydrated hectorite composition [Na0.3(Mg,Li)3Si4O10(F,OH)2·nH2O]. The lengths of a and b are slightly shorter than those of tainiolite, and intermediate between those of aspidolite and paragonite. At the same time, the interlayer spacing is markedly collapsed. The 6-membered ring of tetrahedra adopts a roughly hexagonal configuration similar to those in tri-octahedral micas. An interlayer Na+ ion forms an asymmetrically-coordinated polyhedron with 6+4 basal O on the tetrahedral sheet. It is possibly this irregular coordination which makes the sodium tetra-silicic fluoro-mica swellable.
