STRUCTURE AND POLYTYPISM OF BEMENTITE, A MODULATED LAYER SILICATE

High-resolution transmission electron microscope (HRTEM) imaging and e lectron diffraction, combined with powder X-ray diffraction (XRD) data and a partial powder profile analysis by the Rietveld method, have be en used to determine the parent structure and the polytypism of bement ite, Mn7Si6O15 (OH)8, a modulated 1:1 layer silicate from Franklin, Ne w Jersey. The parent structure is monoclinic P2(1)/c with a = 14.838(2 ), b = 17.584(2), c = 14.700(2) angstrom, and beta = 95.54(2)degrees, Z = 4, V = 3814.8 angstrom3, and M = 929 g/mol. It consists of two hex agonal sheets of octahedra, accommodating the Mn, which are alternatel y rotated by 22-degrees in the ab plane. These are interlayered by a c ontinuous tetrahedral sheet containing pairs of six-membered rings int erconnected with five- and seven-membered rings. Inverted tetrahedra f orm strips with like orientation (up or down) parallel to a. Linked pa irs of six-membered rings are rotated relative to pairs across strip b oundaries by 22-degrees to allow coordination with adjacent octahedral sheets. The polytypism, which can be ascribed to an interface modulat ion caused by stacking faults, is best visible in HRTEM images and dif fraction patterns of the [010] projection. The displacement and freque ncy of the faults were determined from the distribution of satellite r eflections and their displacement relative to the parent reflections. Bementite can be interpreted as a combination of two basic stacking va rieties of topologically identical octahedral and tetrahedral sheets. Variety I is considered the parent structure, with beta = 95.5-degrees . Variety II has a stacking error after every Mn layer, which results in beta = 71.3-degrees.