Crystal chemistry, HRTEM analysis and polytypic behaviour of ardennite

Ardennite is a mixed anion silicate with ideal chemical formula (Mn,Ca,Mg) (sub 4) [(Al,Fe (super 3+) )Mg](AsO (sub 4) )(SiO (sub 4) ) (sub 2) (OH) (sub 6) . It may be regarded as a member of a polytypic family formed by two different kinds of regularly alternating layers. Ardennite, sursassite, pumpellyite and a hypothetical structure type represent the four maximum degree of order (MDO) polytypes. The results of an EPMA and HRTEM study are presented for ardennite from the Western Alps, France, and from Evvia Island (Euboea), Greece. Ardennite from Evvia has a very high Si/As ratio in the isolated [AsO (sub 4) ] tetrahedron -- <= [Si (sub 0.9) As (sub 0.104) ] -- and a considerable amount of (super vi) Mn (super 3+) . HRTEM shows the frequent occurrence of complex polytypes intergrown with the ardennite matrix, together with disordered sequences. A possible structural model for the most common complex is suggested with a doubled c period; it is characterized by a regular alternation of ardennite and the hypothetical structure. The ardennite from the Western Alps, however, shows no significant deviation from ideal stoichiometry or evidence for polytypic behaviour.