Crystal chemistry of Xonotlite Ca6Si6O17(OH)(2). Part I: Determination of polytypes using X-Ray Powder Diffraction (XRPD)
Neues Jahrbuch Fur Mineralogie-Abhandlungen 186(2): 153-162
The crystal chemistry of xonotlite is mainly controlled by its four different polytypes. Ten natural xonotlites from three different lithologies were studied in order to determine their polytypes by powder methods as X-ray powder diffraction (XRPD). The chemical compositions were obtained by electron microprobe analysis (EMPA) and show constant compositions with minor substitutions of Mn and At. To determine the 4 ordered polytypes known for xonotlite (M2a2b2c, M2a2bc, Ma2b2c, Ma2bc), the theoretical diffraction patterns were calculated based on atom coordinates of HEJNY & ARMBRUSTER (2001). For each polytype characteristic reflections can be chosen. The assignments were conducted by means of pattern matching. The results reveal that xonotlite mainly occurs in nature as intergrowths of two Up to four polytypes. It could be demonstrated I hat X-ray powder diffraction is a useful and fast method to determine the different polytypes of xonotlite.