Veatchite; structural relationships of the three polytypes

Crystal structures of the three polytypes of veatchite, Sr (sub 2) B (sub 11) O (sub 16) (OH) (sub 5) .H (sub 2) O, 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) Aa, alpha =87.860(1), beta =82.696(12), gamma =60.476(1) degrees , V=809.7(2) Aa (super 3) ; veatchite-1M, monoclinic space group P2 (sub 1) , with a=6.7127(4), b=20.704(1), c=6.6272(4) Aa, beta =119.209(1) degrees , V=805.4(2) Aa (super 3) ; and veatchite-2M, monoclinic space group Cc with a=6.6070(3), b=11.7125(5), c=20.6848(9) Aa, beta =91.998(1) degrees , V=1599.7(2) Aa (super 3) . The crystal structures have two layer types with similar fundamental building blocks: A layer FBB=3Delta 2[]:<Delta 2[]>-<2Delta []> and B layer FBB=3Delta 2[],1Delta :<Delta 2[]>-<2Delta []>,Delta (Grice et al. 1999). Unique in this FBB is the lone polyhedron with triangular coordination, which consists of a neutrally charged [B(OH) (sub 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.