Microstructures and tetrahedral strip-width order and disorder in Fe- rich minnesotaites

Fe-rich minnesotaites with Fe/(Fe + Mg) ratios of 0.80 and 0.92 were studied using high-resolution transmission electron microscopy (HRTEM). Minnesotaite having Fe/(Fe + Mg) = 0.92 exhibits electron-diffraction patterns characteristic of a C-centered cell, by HRTEM images show that tetrahedral strip widths are severely disordered along the a direction. The sample with Fe/(Fe + Mg) = 0.80 contains domains of ordered primitive cells as well as disordered C-centered cells. Strips that are three and four tetrahedra wide predominante, but newly discovered strips that are two tetrahedra wide and others that are more than four tetrahedra wide also occur in both specimens. Wide tetrahedral stips are more abundant in the minnesotaite having Fe(Fe + Mg) = 0.80. The Fe/(Fe + Mg) ratio seems to be important in determining the strip-width disorder as well as the average tetrahedral strip width. Increased strip widths tend toward a talc-like structure, which consists of "infinitely' wide tetrahedral strips. -from Authors