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Occurrence, chemistry, crystal growth, technical applications, structure and polytypism of xonotlite Ca 6 Si 6 O 17 (OH) 2  are reviewed. Atomic coordinates of the three simplest ordered polytypes in modified Gard notation:  Ma 2 bc  (space group  P 2/ a ,  a  = 17.032,  b  = 7.363,  c  = 7.012 Å,  β  = 90.36°),  Ma 2 b 2 c  (space group  A 2/ a ,  a  = 17.032,  b  = 7.363,  c  = 14.023,  β  = 90.36°), and  M 2 a 2 bc  (space group  P 1̅,  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  M 2 a 2 b 2 c  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  M 2 a 2 b 2 c  polytype, followed by the  Ma 2 b 2 c  polytype, and very low concentrations of the  Ma 2 bc  polytype. The  M 2 a 2 bc  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.

Polytypism in xonotlite Ca 6 Si 6 O 17 (OH) 2