Polytypism of xonotlite : ( I ) Structure of an AI polytype

Crystals of xonotlite from Heguri, Chiba Prefecture, Japan, which exhibit relatively sharp k odd reflections have been found to be a triclinic variant having dimensions; a 8.712(2)'\, b 7.363(2)'\, c 14.023(4)'\, a=89.99(2)" (3= 90.36(2) °,7=102.18(2) 0, and space group Al; Z=2CCa6Si60,7(OH)2]' The average structure which was determined using only even order reflections in k is similar to the reported average structure. Crystal-chemical consideration permitted to deconvolute the average structure to give the atomic arrangements for the true AI structure which has been refined to R=4.03G for observed reflections including odd order reflections in k. The structure consists of SiG017 double chains, each consisting of a pair of Si30g chains of the wollastonite type and sheets of polyhedra about Ca's. The structural scheme built up of these silicate chains and polyhedral sheets is similar to that of wollastonite though two of the three independent Ca atoms per cell are in trigonal prisms formed by oxygen atoms. Occupancy refinement of Ca appears to show that slight defIciency in Ca occurs only one of the three independent Ca sites. The xonotlite polytypism can be discussed based on a monoclinic subcell, ap 8.516'\, bp 7.363'\, cp 7.012'\, (3p=90.37°, underlying the AI structure. The polytypism may occur, unlike commonly known polytypism, in two directions, c and a: the monoclinic cell may be juxtaposed on (001) either in a continuous position or with a glide b12, and on (100) with a glide b/4 (or -b/4).