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Recent studies of the structure of antigorite by Capitani and Mellini (2004, 2005) and by us (Dodony et al. 2002; Dodony and Buseck 2004a) produced contradictory results. The main point of contention is whether 4- and 8-membered rings of tetrahedra occur at the positions where the tetrahedra in the tetrahedral sheets reverse their orientation. We analyzed electron diffraction patterns and transmission electron microscopy images in the paper of Capitani and Mellini (2005) and found no evidence for 4- and 8-membered tings of tetrahedra. On the contrary, we show that their TEM data confirm our antigorite model (Dodony et al. 2002) for the m = 16 structure. The significance of this debate goes beyond the subtleties of the structure of antigorite and highlights ambiguities in interpretation of HRTEM images as well as problems that can arise during image processing. Keywords: Serpentine, antigorite, crystal structure, HRTEM, electron diffraction, image processing

Several minerals and inorganic compounds contain (X[A.sub.4]) tetrahedra, with anions, X ([O.sup.2-], [N.sup.3-], and [F.sup.-]), as central atoms and cations, A ([Cu.sup.2+], [Zn.sup.2+], [Pb.sup.2+], [Bi.sup.3+], RE[E.sup.3+], etc.), as ligands. These tetrahedra are well defined in these crystal structures because the bond valences between A and X are essentially higher than the bond valences between A and atoms from other structural units. According to their size, anion-centered tetrahedra may be subdivided into large and small tetrahedra, formed from cations with ionic radii near to 1.0 [angstrom] (e.g., [Pb.sup.2+], [Bi.sup.3+], and RE[E.sup.3+]) and 0.5-0.7 [angstrom] (e.g., [Cu.sup.2+] and [Zn.sup.2+]), respectively. The small anion-centered tetrahedra prefer to link through corners, whereas the large tetrahedra prefer to link through edges. When the tetrahedra are built from both 'small' and 'large' cations, 'small' cations prefer to be corners shared between lesser numbers of tetrahedra and not involved in linking tetrahedra via edges. The general crystal-chemical formula of minerals and compounds containing anion-centered tetrahedra may be written as [A[prime].sub.k][[X.sub.n][A.sub.m]][[A.sub.p][X.sub.q]][X[prime].sub.l] where [[X.sub.n] [A.sub.m]] (usually n [less than or equal to] m) is a structural unit based on anion-centered tetrahedra (ACT) and [[A.sub.p][X.sub.q]] (p < q) is a structural unit based on cation-centered polyhedra (CCP). A[prime] is a cation that does not belong to anion- or cation-centered polyhedra; it is usually an interstitial cation such as an alkali metal; X[prime] is an interstitial anion such as halide or [S.sup.2-]. Structures containing both ACT and CCP units may be ordered according to the values of their dimensionality. In structures without CCP units, an important role is played by large interstitial anions that link finite ACT units, chains, or layers into three-dimensional structures or fill cavities in ACT frameworks.

A study was conducted on the influence of temperature on the bond lengths and volumes of silicate tetrahedra at a temperature range of 15 to 1,250 Kelvin. Results showed that the bond lengths and volumes of the tetrahedra with four bridging O atoms increased rapidly with temperature. Meanwhile, bond lengths and volumes for tetrahedra with two or three nonbridging atoms were observed to increase at an intermediate rate.