Chemische Abweichungen von der Idealformel bei Plagioklas, eine Studie mit der Mikrosonde
Schwander, Hans; Wenk, Eduard
European Journal of Mineralogy Volume 4 Number 4 (1992), p. 757 - 766
published: Aug 11, 1992
manuscript accepted: Feb 11, 1992
manuscript received: Jun 14, 1991
ArtNo. ESP147050404002, Price: 29.00 €
Abstract Precautionary measures are discussed which can decrease the analytical errors (precision and accuracy) of electron microprobe analyses. Plagioclases were analysed from different metamorphic rocks of the Central Alps and some volcanic rocks ; deviations from the theoretical/stoichiometric compositions were calculated (ΔAl, ΔSi). As a result of Na-loss, deviations from the ideal formula of Na-rich plagioclases were larger than in Ca-rich endmembers, where the Ca-loss is smaller. All the analysed metamorphic anorthites and several pure albites show almost perfect stoichiometry. But plagioclases in the range An 20-80 (oligoclase - labradorite), show a distinct excess of Si in relation to Al (ΔSi: ΔA1~3: 1). Plagioclases from volcanic rocks agree well with the theoretical composition, in spite of strong zoning.