Veatchite: structural relationships of the three polytypes

Abstract

Crystal structures of the three polytypes of veatchite, Sr 2B11O16(OH)5.H2O, are determined by X-ray, single-crystal studies. The polytypes are: veatchite-1A, triclinic space group P1, with a = 6.6378(6), b = 6.7387(6), c = 20.982(2) A°, α = 87.860(1), β = 82.696(12), Δ = 60.476(1)°, V = 809.7(2) A°3; veatchite-1M, monoclinic space group P21, with a = 6.7127(4), b = 20.704(1), c = 6.6272(4) A°, β = 119.209(1)°, V = 805.4(2) A°3; and veatchite-2M,monoclinic space group Cc with a = 6.6070(3), b = 11.7125(5), c = 20.6848(9) A°, β = 91.998(1)°, V = 1599.7(2) A°3. The crystal structures have two layer types with similar fundamental building blocks: A layer FBB = 3Δ2o:<Δ2o>-<2Δo> and B layer FBB = 3Δ2o,1Δ:<Δ2o>-<2Δo>, Δ (Grice et al. 1999). Unique in this FBB is the lone polyhedron with triangular coordination, which consists of a neutrally charged [B(OH) 3] group. Layering has a directional component and depending on layer sequence the symmetry may be centrosymmetric or non-centrosymmetric. Related layered borate structures, biringuccite, nasinite, gowerite, and volkovskite, are compared. Observations indicate that veatchite-1A is the low-temperature and low-pressure polytype, veatchite-1M the high-temperature polytype, and veatchite-2M the moderate temperature and higher pressure polytype. © 1997 - 2012 Mineralogical Society of America.