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.
All Science Journal Classification (ASJC) codes
- Environmental Science(all)
- Earth and Planetary Sciences(all)