Variations in chemical composition and structural properties of antigorites.

Abstract

Chemical, XRD, electron optical, and IR analyses have been made on 20 antigorites from 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 35.4-47.2 A) as well as varying subcell dimensions (a 5.42-5.46, b 9.24-9.26, c 7.24-7.28 A, beta 91.3-91.7o). 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 not=to n/2 types. The minerals with these different types of M aggregate to form common antigorite specimens. The well-known A = 43 A antigorite belongs to the M = n type. Single-crystal X-ray and electron diffraction patterns indicate that the true superstructure periodicity along the a-axis of the antigorites giving M = (2n + 1)/2 type patterns, which may contain odd-numbered octahedra in the one alternating-wave, is 2 A (corresponding with two waves) and the space lattice is C-centred. 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% 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% 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.-T.Y.