Crystal chemistry, HRTEM analysis and polytypic behaviour of ardennite

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doi: 10.1127/ejm/3/5/0819
Authors:Pasero, Marco; Reinecke, Thomas
Author Affiliations:Primary:
Universita di Pisa, Dipartimento di Scienze della Terra, Pisa, Italy
Ruhr-Universität Bochum, Federal Republic of Germany
Volume Title:European Journal of Mineralogy
Source:European Journal of Mineralogy, 3(5), p.819-830. Publisher: Schweizerbart'sche Verlagsbuchhandlung (Nägele u. Obermiller), Stuttgart, Federal Republic of Germany. ISSN: 0935-1221
Publication Date:1991
Note:In English. 23 refs.; 1 table
Summary:Ardennite is a mixed anion silicate with ideal chemical formula (Mn,Ca,Mg)4[(Al,Fe3+)Mg](AsO4)(SiO4)2(OH)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 [AsO4] tetrahedron -- <= [Si0.9As0.104] -- and a considerable amount of viMn3+. 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. [R.A.H.]
Subjects:Crystal chemistry; Orthosilicates; P-T conditions; Phase equilibria; Polymorphism; Polytypism; Silicates; Sorosilicates; Europe; France; Greece; Southern Europe; Western Europe; Ardennite; Evvia Greece; Haute-Maurienne France
Abstract Numbers:92M/1389
Record ID:1995056214
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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