Crystal chemistry, HRTEM analysis and polytypic behaviour of ardennite
Pasero, Marco; Reinecke, Thomas
European Journal of Mineralogy Volume 3 Number 5 (1991), p. 819 - 830
published: Oct 2, 1991
manuscript accepted: Apr 24, 1991
manuscript received: Sep 17, 1990
ArtNo. ESP147050305009, Price: 29.00 €
Abstract 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 polytypic family is conveniently described in terms of the OD-theory. Ardennite, sursassite, pumpellyite and a hypothetical structure type represent the four MDO-polytypes within this polytypic family. The results of a combined chemical and HRTEM study are presented for ardennite sampled at two different localities - Western Alps, France and Ewia island, Greece. With respect to the ideal chemical formula, ardennite from Ewia is characterized by a very high Si/As ratio in the isolated [AsO4] tetrahedron - up to [Si0.9As0.1O4] - and by a considerable amount of VIMn3+. Examination by HRTEM shows the frequent occurrence of complex poly types intergrown within the ardennite matrix, together with disordered sequences as well. A plausible structural model for the most common complex polytype of ardennite is suggested with a doubled c period. It is characterized by a regular alternation of ardennite and the hypothetical structure. Contrarily, ardennite from the Western Alps presents neither significant deviations from ideal stoichiometry nor evidence for polytypic behaviour.