DESYMMETRIZATION OF OD STRUCTURES OF WOLLASTONITE-LIKE CHAIN SILICATES
Jioí Hybler
Institute of Physics, Academy of Sciences of the Czech
Republic, Cukrovarnická 10, 16253 Praha 6, Czech Republic,
E-mail hybler@fzu.cz
Keywords: OD structures, polytypes, chain silicates
Chain silicates wollastonite Ca3[Si3O9], bustamite (Ca,Mn)3[Si3O9] [1, 2, 3], ferrobustamite (Ca,Fe)3[Si3O9] [4, 5] and recently described rare mineral vistepite SnMn4[B2Si4O16](OH)2 [6] are more or less desymmetrized OD structures [7, 8]. The most abundant MDO (maximum degree of order) polytype of these minerals is triclinic (anortic) 1A (mostly rather confusingly reported as 1T ), for which also various primitive and centered unit cells are reported. For the purpose of the OD interpretation, the most convenient cells are these introduced by Peacor & Prewitt [2]:
Wollastonite a 7.94, b 7.32, c 7.07 A, a 90.03, b 95.37, g 103.43o, P1_
Bustamite a 7.736, b 7.157, c 13.824 A, a 90.52, b 94.58, g 103.87o, A1_
Ferrobustamite a 7.862, b 7.253, c 13.967 A, a 89.73, b 95.47, g 103.48o, A1_
Vistepite a 7.365, b 6.973, c 13.651 A, a 90.00, b 96.59, g 103.12o, A1_
These minerals are characterized by following structural features: (1) infinite dreier chains of corner-sharing tetrahedra; (2) bands of edge-sharing octahedra, three octahedra wide. Tetrahedra are occupied by Si or B, octahedra by Ca, Mn, Fe, Sn. In vistepite, one octahedral position is vacant. In all cells reported above, chains of tetrahedra and bands of octahedra are oriented parallel with b axis and planes of octahedra within bands are parallel with 101 and 102 planes in wollastonite and other minerals respectively. OD layers are parallel with 100, so the octahedral bands are stacked diagonally within them.
Symmetry (or pseudosymmetry in the case of desymmetrized structures) of octahedral bands is 21/m in wollastonite, 2/m in remaining minerals. Each band of octahedra is bonded with other ones via six dreier chains, four of them within the same OD layer, other two with adjacent layers. The layer group is thus P(1)21/m1 in wollastonite and A(1)2/m1 in remaining structures. A-centering in all structures except of wollastonite is caused by mutual b/2 shifts of bands within the OD layer. Vicinity condition is preserved for shifts +b/4 or -b/4 of OD layer relative to the preceding one. There are two MDO polytypes, 1A [e.g. 9, 10 ,3] and 2M [11, 12], represented by repetition of +b/4 (or -b/4) shifts and regular alternation of +b/4 and -b/4 shifts respectively. Some rare non-MDO polytypes of wollastonite were also described [13, 14]. Reverting the regular sequence of +b/4 shifts into -b/4 shifts in the 1A polytype can sometimes cause twinning, as it was reported for wollastonite [15] ferrobustamite [4] and vistepite [6].
In reality, however, many atoms are slightly displaced from their ideal positions required by the OD symmetry. In bustamite and ferrobustamite these shifts follow substitution of Ca atoms by various amount of smaller Mn or Fe atoms in octahedra. In vistepite, octahedra are shrinked and unequally deformed. Tetrahedra in chains accommodate deformations and sizes of octahedra by partial rotations and tilting [3, 5], sometimes non-uniform (case of vistepite [6]).
These changes are very complex to be described and evaluated, but some attempts were made to quantify deviations from the OD symmetry: (1) calculation of relative displacements of atoms parallel with and perpendicularl to the b vector. (2) calculation of angular deviations of certain edges of polyhedra from their ideal parallel or perpendicular direction.
Typical values of displacements of atoms varied in range 0-0.13 A. Surprisingly, displacements of certain atoms in wollastonite which is believed to be less desymmetrized were of the same order as in bustamite and ferrobustamite. The degree of desymmetrization, however, can vary even within the same substance depending on the crystallization conditions.
Conclusion. Crystallographers usually benefit from the desymmetrization, because desymmetrized OD structures have much less tendency to be disordered. Therefore it is a good chance to find periodic crystal so that the structure analysis can be performed using ordinary methods. On the other hand, some OD structures can remain unrecognized as such. The signs of their OD character can be e.g. twinning, non-space-group extinctions and higher symmetry of some subsets of diffractions.
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