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Although the investigation of micas dates back to the pre-scientific era (see Cipriani, this volume), the idea of polytypism (originally not distinguished from “polymorphism”) in the micas did not ensue until 1934, when Pauling proposed it in a private conversation quoted by Hendricks and Jefferson (1939). The existence of several structural types was however known from goniometric measurements and morphological analysis performed in the 19th century (e.g., Marignac 1847; Baumhauer 1900) and collected in the 4th volume of the Atlas der Krystallformen (Goldschmidt 1918; for a comparative review and later measurements see Peacock and Ferguson 1943) and appears also in the different axial settings introduced to describe the unit cell of micas (e.g., Brooke and Miller 1852; Des Cloizeaux 1862; Koksharov 1875; Tschermak 1878).
The systematic investigation by X-ray diffraction (XRD) started with Mauguin (1927, 1928), who pointed out that the c axis of phlogopite was half that of muscovite. Pauling (1930) was the first to solve the structure of a mica, a fuchsite (now termed “chromian muscovite”, according to Rieder et al. 1998), by visual comparison of a subset of intensities from photographs, and introduced the first model of the structure of phyllosilicates on the basis of the coordination theory. Jackson and West (1931) were the first to perform a complete structure determination, investigating a muscovite-2M1. Hendricks and Jefferson (1939) investigated one hundred samples of micas and discovered several “polymorphs”, many of which were however twins of simpler structural types (shorter-period polytypes). The symmetry of the 2:1 mica layer was not fully recognized until Pabst (1955) showed that the correct space-group type of 1M polytype was C2/m instead than Cm, as previously assumed by Hendricks and Jefferson (1939) and reported also by …