Abstract

Using a combination of Fourier and direct phase-determining methods for about 1000 data collected by 2θ-scans with Mo X-radiation and a scintillation counter, the structure of gowerite, CaO.3B2O3.5H2O, has been solved and refined by least-squares methods to a residual of 0.09. Gowerite is monoclinic, P21/a, a = 12.882 ± 0.004, b = 16.360 ± 0.007, c 6.558 ± 0.004 Å, β = 121.62 ± 0.05°, Z = 4, density (calc.) 2.003 g/cm3, obs. specific gravity 2.00 ± 0.01. The structural formula is CaB5O8(OH).B(OH)3.3H2O. Polyanions similar to the penta-borate polyanion are cross-linked in two directions to form [B5O8(OH)]2− sheets parallel to (010). Calcium cations fit into spaces within the polyanion sheets, each coordinated by six oxygen anions plus a water molecule on one side of a sheet and two hydroxyl ions of one of the isolated B(OH)3 groups on the opposite side of the sheet. Two other crystallographically distinct water molecules are located between the polyanion sheets. Adjacent sheets are related by the 21 axes and are held together by hydrogen bonds only. Four hydrogen bonds averaging 2.68 Å are between atoms of adjacent sheets; five involving interstitial water molecules average 2.82 Å. The structural role of the water molecules agrees well with previously reported thermal analyses.

Gowerite provides the third example of a borate structure containing isolated B(OH)s groups in the presence of larger polyanions, the other two being the dimorphic pair, veatchite and p-veatchite, Sr2[B5O8(OH)]2.B(OH)3.H2O, recently solved, respectively, by Clark and Christ and by Rumanova and Gandymov. The structure of a synthetic compound, K2B5O8(OH).2HO, solved by Marezio, has polyanion sheets similar to those of gowerite. Comparison of the sheets in all four structures shows remarkable similarities; in particular, the same six oxygen anions of the sheet coordinate the cation in the different structures, with average M-O distances 2.57 Å (Ca), 2.69 Å (Sr), and 2.782 Å (K). In all cases the sheets are linked by hydrogen bonding, which may be entirely direct from one sheet to the next as in the veatchites, or partly direct and partly through interstitial water molecules as in gowerite. In the synthetic potassium polyborate the sheets are linked in both these ways and also by bonding to interstitial potassium cations. These recent studies show that a fifth principle can be added to the four established for formation of complex borate polyanions by C. L. Christ, as follows: the boric acid group, B(OH)3, may exist in isolated form in the presence of more complex polyanions, or such insular groups may themselves polymerize and attach as side chains to more complex polyanions.

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