Structures of low gallium albite (NaGaSi₃O₈) and intermediate germanium albite (NaAlGe₃O₈): Tetrahedral-site ordering in sodium feldspar

The structure of low gallium albite (NaGaSi₃O₈), synthesized hydrothermally at 600 °C and 1.3 kbar, has been refined at room temperature with single-crystal X-ray intensities to R = 0.023. Mean T-O bond distances and Ga site occupancies are: T₁0 [1.807(2), 0.935(1)], T₁m [1.610(2), 0.016(1)], T₂0 [1.615(2), 0.025(1)], T₂m [1.619(2) Å, 0.025(1)]. The structure of intermediate germanium albite (NaAlGe₃O₈), grown with a vanadotung- state flux, has been refined similarly to R = 0.026. Mean T-O bond lengths and Al site occupancies are: T₁0 [1.737(2), 0.274(2)], T₁m [1.728(3), 0.375(2)], T₂0[1.736(2), 0.185(2)], T₂m [1.735(2) Å, 0.166(2)]. Tetrahedral-Site ordering in strain-free sodium silicate feldspar structures is promoted by the distinctive stereochemistries of the T₁0 and T₁m sites, which are imposed by spatial accommodation of the Na cation within the feldspar framework. From site occupancies for Si, Al, Ga, Ge, B, and Fe³⁺, it is tentatively concluded that the ordering scheme is determined by the favorable charge or valence electron distribution resulting when the trivalent atom is placed in the T₁0 site, and that this site preference is proportional to the difference in size of the trivalent and quadrivalent atoms. Electrostatic repulsion between the T and Na atoms may be a contributing factor. Intermediate germanium albite has (t₁0 + t₁m) > (t₂0 + t₂m), as in intermediate microcline and low sanidine, but the unusual site preference of t₁m > t₁0 ≈ 0.25 is unexplained.