Tetrahedral-site occupancies in reedmergnerite and synthetic boron albite (NaBSi₃O₈) Available to Purchase

The structures of three B-substituted albite samples (NaBSi₃O₈), including reedmergnerite from Utah and low and high synthetic boron albite, have been refined at room temperature with single-crystal X-ray intensities. For reedmergnerite, R = 0.019, the structure is ideally ordored with B in the T₁0 site, and the electron density distribution at the Na position is anisotropic as in low albite and low gallium albite. Average T-O distances are T₁0 [1.472(1)], T₁m [1.611(1)], T₂O [1.615(1)], T₂m [1.619(1) Å]. The structure of low boron albite refined to R = 0.027. Average T-O distances and B site occupancies are T₁O [1.497(2), 0.805(1)], T₁m [1.606(1), 0.014(1)], T₂O [1.593(1), 0.135(1)], T₂m [1.610(1) Å, 0.046(1)]. The structure of high boron albite refined similarly to R = 0.037, T₁O [1.571(2), 0.289(3)], T₁m [1.598(2), 0.058(3)], T₂O [1.553(2), 0.384(3)], T₂m [1.574(2) Å, 0.269(3)].

Average T-O distances for reedmergnerite and boron albite exhibit an almost linear correlation with site occupancies of B (t_iB). The tetrahedral-site ordering sequences of t₁O ≫ t₂O ≫ t₂m > t₁m in low boron albite and t₂O > t₁O ≈ t₂m > t₁m in high boron albite are very unusual compared with albite but do correlate with stereochemical features. High boron albite is antiordered with (t₁O + t₁m) < (t₂O + t₂m). The order-disorder transformation in boron albite appears to occur within the temperature range 500–550°C at P_H2O ≈ 1 kbar.