Short-range constraints on chemical and structural variations in bavenite

Abstract

Bavenite is an orthorhombic calcium beryllium aluminosilicate, a ≈ 23.2, b ≈ 5.0, c ≈ 19.4 Å, V ≈ 2250 Å3, Z = 4, that crystallizes in the space group Cmcm. The crystal structures of 24 bavenite samples from various localities worldwide were refined to R1 values from 2.4 to 7.5% based on ∼1330 unique reflections collected with Mo-Kα X-radiation on a Bruker P4 CCD single-crystal diffractometer. The composition of each crystal was determined by electron microprobe analysis. There is extensive solid-solution in bavenite according to O(2)OH– + T(4)Si4+ + T(3)Be2+ ⇌ O(2)O2– + T(4)Al3+ + T(3)Si4+, such that the general formula may be written Ca4BexSi9Al4–xO28–x(OH)x, where x ranges from 2.00 to ∼3.00 a.p.f.u. Small amounts of additional Be may be incorporated into bavenite via the substitution T(3)Be + O(2)OH– + Na + T(4)Si2 ⇌ T(3)Si + O(2)O2– + Ca + T(4)Al2. Local (short-range) bond-valence considerations indicate that Short-Range Order (SRO) should be extensive in the bavenite structure, and this is confirmed by Fourier Transform Infrared (FTIR) spectroscopy in the principal OH-stretching region and by 27Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy. Short-range bond-valence considerations indicate that the T(3)–T(4)–T(3)–T(4) rings of the framework can incorporate any short-range arrangement of cations consonant with their site populations [T(3) = Be,Si; T(4) = Si,Al], and 27Al MAS NMR spectroscopy confirms this, showing the presence of the local clusters T(3)Be–T(4)Al–T(3)Be, T(3)Si–T(4)Al–T(3)Be and T(3)Si–T(4)Al–T(3)Si. Incorporation of Be at the T(3) site is accompanied by local replacement of O2– by (OH)– at the O(2) site and hydrogen bonding to the adjacent O(3) anion; the latter promotes Be → Si substitution at the T(3) tetrahedron adjacent in the b direction. T(3)–T(4)–T(3)–T(4) rings link in the c direction through a T(3)–(1)–(3) linkage [T(1) = Si]. Local bond-valence considerations show that occupancy of both T(3) tetrahedra by Be violates the valence-sum rule, and that the linkage T(3)Be–T(1)Si–T(3)Si provides the constraint whereby Be does not exceed 3 a.p.f.u. in bavenite when incorporated via the substitution O(2)OH– + T(4)Si4+ + T(3)Be2+ ⇌ O(2)O2– + T(4)Al3+ + T(3)Si4+.

Bohseite, ideally Ca4Be4Si9O24(OH4, from the Piława Górna quarry, the Góry Sowie Block, SW Poland

“…Sitescattering indicates that the T(2) site is occupied by Be, as found by Cannillo et al (1966) longer than the grand <Be-O> distance in minerals of 1.633 Å reported by Hawthorne and Huminicki (2002), but still lies well within the dispersion of their data. It is within the range of <T(2)-O> distances reported for bavenites by Lussier and Hawthorne (2011) and indicates that T(2) is completely occupied by Be in bohseite, as proposed by Cannillo et al (1966) for bavenite. In accord with the substitution of Be for Si at the T(3) site in bavenite (Cannillo et al, 1966;Lussier and Hawthorne, 2011), the site occupancy at the T(3) site was refined as (Be,Si) and gave the following site-populations (Hawthorne et al, 1995): T(3) = 1.488(12) Be + 0.512 Si apfu.…”

“…2 (the other spectrum is similar). There are two relatively sharp absorptions at 3620 and 3550 cm -1 , in accord with the occurrence of (OH) at O(8) and O(2), respectively (Lussier and Hawthorne, 2011).…”

“…The <T-O> bond-lengths at T(1), T(2), T(5) and T(6) ( Table 6) are very similar to those in bavenite (Table 2; Lussier and Hawthorne, 2011), and hence we assign T(1) = T(5) = T(6) = Si. Sitescattering indicates that the T(2) site is occupied by Be, as found by Cannillo et al (1966) longer than the grand <Be-O> distance in minerals of 1.633 Å reported by Hawthorne and Huminicki (2002), but still lies well within the dispersion of their data.…”

“…Initial electron-microprobe analyses of bohseite were performed at the (Lussier and Hawthorne, 2011). Compositions span the range 2.84 < Be < 3.77 apfu ( Table 3) and suggest that there is complete solidsolution from bavenite to close to the end-member composition of bohseite.…”

“…The structure of bohseite was refined in the space group Cmcm using F 2 in the SHELXTL PLUS (PC version) software package; initial positional parameters were those of Lussier and Hawthorne (2011). Full occupancy was assumed for all sites, and there was no subsequent indication of vacancies.…”