Abstract

Fe-rich minnesotaites with Fe/(Fe + Mg) ratios of 0.80 and 0.92 were studied using high‐resolution transmission electron microscopy (hrtem). [010] images show the relative positions of tetrahedra and octahedra in the 2:1 layers and the inverted tetrahedra in the interlayer regions. Minnesotaite having Fe/(Fe + Mg) = 0.92 exhibits electron‐diffraction patterns characteristic of a C‐centered cell, but 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 predominate, 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 strips are more abundant in the minnesotaite having Fe/(Fe + Mg) = 0.80, and a strip 10 tetrahedra wide was observed. Various β* values occur; they can be derived by displacement of opposing tetrahedral strips across the octahedral sheet within each layer by (n/10)a along the a axis in the primitive‐cell minnesotaite. Local variations of β* values are also associated with strip‐width disorder. Crystal regions having β* = 64° are most abundant, but regions having β*= 52° are also common.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.

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