Polytypism in xonotlite Ca₆Si₆O₁₇(OH)₂

Abstract

Occurrence, chemistry, crystal growth, technical applications, structure and polytypism of xonotlite Ca₆Si₆O₁₇(OH)₂ are reviewed. Atomic coordinates of the three simplest ordered polytypes in modified Gard notation: Ma2bc (space group P2/a, a = 17.032, b = 7.363, c = 7.012 Å, β = 90.36°), Ma2b2c (space group A2/a, a = 17.032, b = 7.363, c = 14.023, β = 90.36°), and M2a2bc (space group P1̅, a = 8.712, b = 7.363, c = 7.012 Å, α = 89.99° β = 90.36°, γ = 102.18°) were modeled from geometric principles based on the known structure of the M2a2b2c polytype (space group A1̅, a = 8.712, b = 7.363, c = 14.023, α = 89.99 β = 90.36, γ = 102.18°). Unique reflection arrangements in the reciprocal lattice, characteristic of each polytype, were defined as criteria to identify xonotlite polytypes on X-ray single-crystal photographs. Precession- and Weissenberg-photographs of a xonotlite from the Kalahari manganese field (Republic of South Africa) indicated the predominance of the (100) twinned M2a2b2c polytype, followed by the Ma2b2c polytype, and very low concentrations of the Ma2bc polytype. The M2a2bc polytype could not be identified which agrees with previous electron diffraction experiments on xonotlites from other localities. Diffuse streaks parallel to a* and less intensive ones parallel to c* on single-crystal photographs suggest the presence of additional disordered polytypes.