Przyczynek do znajomości chemizmu biotytów ze skał podłoża krystalicznego Niżu Polskiego

Abstract

A CONTRIBUTION TO THE KNOWLEDGE OF BIOTITE CHEMISM IN ROCKS OF CRYSTALLINE BASEMENT POLISH LOWLANDSummaryThe biotites  here examined originate from the plutonic rocks (granites, syenites, granodiorytes) and metamorphic rocks (gneisses, migmatites) pierced in the north-eastern part of the Polish Lowland area. The biotites were separated in bromoform and certain samples underwent flotation. Part of a sample destined for chemical analysis was crushed in agate mortar. To avoid partial oxidation of iron, determination of divalent iron was made on uncrushed material. Chemical analysis was made partly by means of classical methods (determination of silica Fe2+, total iron, titanium, manganese, phosphorous and water) partly, during the determination of remaining components, also meanof complexometry (determination of aluminum, calcium and magnesium) and of flame-photometry (determination of potassium and sodium). Due to application of pyridine to separate oxides of R2O3 type, manganese was separated quantitatively from iron, aluminum and titanium. Differences in chemical composition of the biotites occurring in the metamorphic and plutonic rocks are small, and can be observed in titanium and aluminum contents. The percentage of potassium corresponding to that of biotites, and a distinct predominance of Fe+2 over Fe+2 allow to assume a well preservation state of the biotites under consideration. The know degree of iron contents (26,44%­–59,25%) of the biotites examined permits to accept that the mother rocks are here not of platform origin. A marked predominance of iron over magnesium and inversely, which can be observed in certain samples, is connected with the conditions of biotite formation. For certain biotitescrystallochemical formulae were calculated in terms of 11 oxygens. Some biotites may be distinguished, in which, except for Si, the total aluminum can be referred to the fourfold co-ordination or those, in which Fe+3comes into the fourfold co-ordination with Si+ Al. Both the Si:Al ratio amounting in the tetrahedral bed to 2,7:1,3 and the content of trivalent cations in octahedral bed (which does not exceed 20 per cent) prove that the examined biotites are of trioctahedral character.