Serpentines Close-Up and Intimate: An HRTEM View

High-resolution transmission electron microscopy (HRTEM) affords a close look at the complex structures and intergrowths of the serpentine minerals. All contain alternating sheets of cations in tetrahedral and octahedral coordination. Lizardite, the flat species, forms in sufficiently large and well-ordered crystals to permit reliable X-ray structure determinations, and it is the reference mineral for estimates of the structures of antigorite and chrysotile. However, even lizardite forms in a wide variety of polytypes, only some of which have been explored. It also forms polygonal serpentine, a roughly cylindrical variety that typically consists of 15 or 30 sectors, each of which consists of lizardite layers. HRTEM images of the structure at sector boundaries show offsets of fringes that we interpret as the result of inversions of the tetrahedral sheets. Using lizardite as the basis for an estimate of the curled chrysotile structure, we obtained atomic coordinates and used them to calculate fiber-axis X-ray and electron-diffraction patterns for polytypes. HRTEM images obtained viewing down the fiber axis show no ordering between layers. Chisholm (1988) reported 2mm symmetry for fibers when viewed perpendicular to their length, but most of our measured fibers show no symmetry for such orientations. Employing Fourier transforms of HRTEM images, we found a new one-layered orthorhombic chrysotile polytype with mirror symmetry perpendicular to the fiber axis. Antigorite is notable for its conspicuous, modulated structure. We observed the waves to be asymmetrical and infer that the asymmetry results from an inhomogeneous distribution of hydrogen bonding between the layers. This distribution helps with a long-standing problem by explaining some apparently anomalous features of HRTEM images. The abundant (001) faults in antigorite are produced by boundaries of lamellae having different modulation profiles. HRTEM images show the relations of serpentine minerals to each other as well as their host materials. Areas exist where layers of each of the serpentine minerals grade continuously and free of faults from one variety to another. The resulting intermediate or partial structures defy categorization into simple mineral types.