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

Abstract

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 c ₀ · β. However, in contrast to his statements, octahedral alumina was found to reduce c ₀ · sin β 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 b ₀ the combined amounts of octahedral Al and tetrahedral Si cause a reduction of this value, which means indirectly that tetrahedral Al alone would increase b ₀.

Reductions of b ₀ and c ₀ · sin β 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.