HRTEM evidence for 8-reversals in the m=17 antigorite polysome

Structural imaging of the “odd” m = 17 antigorite polysome is in keeping with the X-ray structure refinement, supplying direct evidence for the presence of 8-reversals. HRTEM also displays the polysomatic and stacking disorder features modeled during X-ray refinement to achieve convergence. Polysomatic faults occur in Mg159 antigorite as (100) lamellae, with wavelength differing from the matrix by only one serpentine module (m = 16 or 18 vs. 17). Mixed ordered polysomes may also occur, such as a regular alternation of m = 16 and m = 17 lamellae, producing a complex polysome with an a repeat distance of 84.5 Å. “Odd” and “even” antigorite polysomes differ in layer topology. “Odd” polysomes (m ≠ 2n, with n = integer) have a primitive cell with Pm space group, m tetrahedral and m-1 octahedral modules, and one 6-reversal and one 8-reversal per unit cell. Polysomes with m = 2n have C-centered cells (C2/m), m tetrahedral and m-2 octahedral modules, and two 6-reversals and two 8-reversals per cell.