Chemical deviation from ideal form of plagioclase; study using the microprobe
Chemische Abweichungen von der Idealformel bei Plagioklas, eine Studie mit der Mikrosonde
European Journal of Mineralogy (August 1992) 4 (4): 757-766
Measures to decrease the analytical errors of EPMA are discussed. Plagioclases were analysed from different metamorphic rocks of the Central Alps and some volcanic rocks; deviation from the theoretical/stoichiometric compositions were calculated (DELTA Al, DELTA Si). As a result of loss of Na, deviations from the ideal formula of albitic plagioclases were larger than in anorthitic compositions, where the Ca loss is smaller. All the analysed metamorphic anorthites and several pure albites show almost perfect stoichiometry, but oligoclase-labradorite compositions show a distinct excess of Si in relation to Al (DELTA Si: DELTA Al approx 3:1). Plagioclases from volcanic rocks agree well with the theoretical composition, in spite of strong zoning.