Der Einfluß verschiedener Substitutionen auf die Gitterkonstanten von koexistierenden Biotiten und Muskowiten

Structural formulas were calculated for 8 analysed coexisting biotite-muscovitepairs from granitic rocks. Characteristic components of these structural formulas were plotted against the unit cell dimensions of these micas.In accordance with the results of Gower (1957) the substitution of OH^– by F^– in trioctahedral micas was found to reduce c₀ · . However, in contrast to his statements, octahedral alumina was found to reduce c₀ · sin also and to an even larger extent than fluorine. This observation is in agreement with the commonly encountered compacting of the mica structure through replacement of 3 large cations (Fe²⁺, Mg²⁺, Mn²⁺) by two smaller ones (Al³⁺, Fe³⁺).Concerning b₀ the combined amounts of octahedral Al and tetrahedral Si cause a reduction of this value, which means indirectly that tetrahedral Al alone would increase b₀.Reductions of b₀ and c₀ · sin by Al in synthetic micas were also found by Crowley and Roy (1964) and Seifert (1966) for the substitution Mg + Si by two Al in trioctahedral micas.In an earlier paper the author (Müller, 1966) has already shown with the same mica samples that the substitutions of M²⁺ by Al³⁺ and of OH^– by F^– also have the strongest effects of all substitutions on the refractive indices, densities, and specific refractive energies of these micas. Therefore it is evident that the latter effects as well as the changes of lattice constants described in this paper are interrelated.

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Mein Dank gilt den Herren Dr.W. Harre und Dr. H. Gundlach, Bundesanstalt für Bodenforschung, Hannover, für die Ausführung der chemischen Glimmeranalysen, sowie der Deutschen Forschungsgemeinschaft, Bad Godesberg, für ihre finanzielle Beihilfe. Herrn Dr. F. Seifert, Kiel, danke ich für zahlreiche sachliche Diskussionen und Hinweise und für die Überlassung seines Algol-Rechenprogramms, mit welchem 4 der 16 Strukturformeln elektronisch berechnet wurden. Herrn Prof. Dr. F. Liebau, Kiel, verdanke ich einige freundliche Ratschläge.