Variations in chemical composition and structural properties of antigorites.

Chemical, X-ray, electron optical, and infrared analyses have been made on twenty antigorites form the Nishisonogi and Sasaguri areas, northern Kyushu, Japan, along with two antigorites from other localities. The indexed X-ray powder patterns give various supercell A parameters (A = 35.4–47.2Å) as well as varying subcell dimensions (a = 5.42–5.46, b = 9.24–9.26, c = 7.24–7.28Å, (β = 91.3–91.7°). When the ratio of A⁄a is represented by M (M = 6.5–8.7), the electron diffraction patterns can be classified into M = n (n is integer), M = (2n + 1)/2, and M ≠n⁄2 types. The minerals with these different types of M aggregate to form common antigorite specimens. The well-known A = 43Å antigorite belongs to the M = n type. Single crystal X-ray and electron diffraction patterns indicate that the true superstructure periodicity along the X axis of the antigorites giving M = (2n + 1)/2 type patterns, which may contain odd-numbered octahedra in the one alternating-wave, is 2A (corresponding to two waves) and the space lattice is C-centered. This lattice seems to give more reasonable structures at the inversion points of the alternating-waves than hitherto predicted (Kunze, 1961). The M ≠n⁄2 type patterns may be caused by coherent domains with different A parameters of the former two types. The structural formula of antigorite can be given by Mg6Si4(1 + 1/2M)O10(1+1/2M)(OH)8-2M. Octahedral Mg is substituted by Fe2+, and Al or trivalent cations substitute for both the tetrahedral and octahedral cations, although the trivalent cations may be contained more in the octahedral positions for most materials. Larger Fe2+ content (FeO 5.5% in max.) tends to bring larger a and b, but smaller c and M(A) parameters. The small c is also produced by relatively large Al contents (Al2O3 4.1% in max.), which supports the presence of tetrahedral Al together with the infrared 3570 cm−1 band. The main OH band at 3685–3674 cm−1 tends to decrease in frequency with increasing Fe content and decreasing M parameter. The correlations of the Fe contents to the a and M parameters and to the main OH band are somewhat different between the Nishisonogi and Sasaguri antigorites, which are discussed in relation to their formation conditions.