Chemické složení amfibolů v polétavém prachu z vybraných lokalit Brněnska

Abstract

This study aimed to characterise the morphological and chemical properties of amphiboles from airborne dust samples collected from three localities in Brno and its surroundings. We analysed approximately 3 g of airborne dust particles from each sample. The mineralogical and morphological characteristics of the dust particles were examined by polarised-light microscopy and electron microprobe analysis. All studied airborne dust inorganic particles with length ≥ 5 μm show a ratio ≤ 3 : 1 or diameter of > 3 μm and therefore do not be potentially dangerous to human health. We compared the chemical analyses of airborne needle-like dust particles (tremolite-actinolite, magnesiohornblende, pargasite; Si = 6.32–7.90 apfu; Mg/(Mg+Fe2+) = 0.34–0.84) with existing chemical composition amphiboles from possible source areas. The amphibole in airborne dust from the locality in the centre of the Brno town is completely dominated by actinolite and magnesiohornblende (Si = 6.48–7.56 apfu; Mg/(Mg+Fe2+) = 0.50–0.80) derived from basic and ultrabasic rocks of the Central Basic Belt of the Brno Massif. The granitoids of the Brno massif can be also a significant source of dust particles. Anthropogenic activity can explain a substantial proportion of amphibole from a relatively small body of amphibolites near Želešice (quarry in the Central Basic Belt of the Brno Massif) because these rocks are widely used as sanding material in the entire Brno agglomeration. The second locality of Rosice (west of Brno) is dominated by amphiboles (Si = 6.51–7.90 apfu; Mg/(Mg+Fe2+) = 0.55–0.84) from the west and northwest metamorphic units (Moravian and Moldanubicum). However, amphiboles from the rocks of the Brno massif are also presented. Amphiboles in the airborne dust from the Rajhradice (locality situated S from Brno) predominantly originated from the rocks of the Central Basic Belt of the Brno Massif (Si = 6.85–7.63 apfu; Mg/(Mg+Fe2+) = 0.50–0.78). Calcareous microfossils suggest that an essential component of the dust is also the material transported from the southeast, where dominated sediments of the Carpathian Flysch units and their foreland. Our results validate amphiboles as valuable minerals for identifying source areas from which dust has been derived and confirm dominant wind flows in the SE and NW directions.