Ardennite is a mixed-anion silicate with ideal chemical formula (Mn,Ca,Mg)4[Al,Fe3+)5Mg](AsO4) (SiO4)2(Si3O10)(OH)6). From the structural point of view, it may be regarded as a member of a polytypic family formed by two different kinds of regularly alternating layers. Such a prototypic family is conveniently described in terms of the OD-theory. The results of a combined chemical and HRTEM study are presented for ardennite sampled at two different localities - Western Alps, France and Evvia island, Greece. A plausible structural model for the most common complex polytype of ardennite is suggested with a doubled c period. Contrarily, ardennite from the Western Alps presents neither significant deviations from ideal stoichiometry nor evidence for polytypic behaviour.
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